B. Venkateswarlu et al. (eds.), Crop Stress and its Management:
Perspectives and Strategies, DOI 10.1007/978-94-007-2220-0_3,
© Springer Science+Business Media B.V. 2012
Abstract Plantation crops include perennials grown over large areas in monoculture,
excepting fruit trees grown in orchards. These crops face both abiotic and biotic
stresses, incited by factors those coexist in plantations. Although Plantation crops
are well adapted and are grown mostly in the tropics, where innumerable stress
factors operate. Historical signifi cance of few stress havocs in plantation species is
remarkable. A wide range of anatomical, physiological and biochemical features
contribute to various stress adaptation in plantation crops. Notwithstanding,
improvement of stress resistance in plantation species has been mandated to combat
unfriendly factors that jeopardize intensive and extensive cultivation. Conventional
breeding is cumbersome in plantation crops, where in the process has to involve
many generations running for decades, and expensive in terms of time, space and
large volume of individuals handled. Recent developments in molecular genetics
and biotechnology are aiding acceleration of breeding process in plantation species.
Integration of proper crop management strategies with improved cultivars is essen-
tial to meet the goals of stress management. This review presents a comprehensive
coverage of various adaptive mechanisms and mitigation strategies for several biotic
and abiotic stresses affecting major plantation crops like cardamom, cashew, cocoa,
coconut, coffee, date, eucalyptus, oil palm, rubber and tea.
K. K. Vinod (*)
Indian Agricultural Research Institute , Rice Breeding and Genetics Research Centre ,
Aduthurai 612101 , Tamil Nadu , India
e-mail: email@example.com: firstname.lastname@example.org
Stress in Plantation Crops: Adaptation
K. K. Vinod
Although the term “plantation” is loosely defi ned, it generally includes perennial
crops, grown in larger areas in monoculture, producing a wide array of produces
such as spices, condiments, beverages, oilseeds, other allied materials, industrial
and medicinal products, but does not ordinarily include fruit trees grown in larger
orchards. They essentially differ from fi eld crops for size of the holding, size of the
individual plants, perennial nature and vegetative propagation. Plantation crops are
remarkably well adapted plant species grown mostly in tropical regions of the world.
These crops are predominantly cross-pollinated and produce heterozygous and het-
erogeneous population. Inherent genetic variability in the cultivated gene pool of
these crops is generally very narrow (Simmonds 1989 ; Motamayor et al. 2000 ;
Bandyopadhyay 2011 ) . This chapter covers common plantation crops (Table 3.1 )
having various growth forms (trees, palms or shrubs) and grown for various pur-
poses (oil nuts, industrial raw materials, spices, beverages and in timber industry).
The tropical regions harbor oldest ecosystems on earth, which are diverse, highly
evolved, but poorly understood (Ploetz 2007 ) , where plant stress is a common
feature. Stress is a disadvantageous infl uence on a plant, exerted by external factors
and stress adaptation or tolerance is the plant’s fi tness to cope with an unfavorable
environment or an invasion (Taiz and Zeiger 2006 ; Mittler 2006 ; Li et al. 2008 ) .
When exposed to unfavorable environments, plants show departures from its
normal growth and metabolism, which collectively contribute to crop performance
and productivity (Blum 2011 ) . Like any other crops of the domesticated environ-
ment, plantation crops too are affected by various stress factors.
Stresses faced by plantation crops are either abiotic or biotic (Fig 3.1 ), originated
from factors that coexist in the plantation environment. Abiotic stresses are the most
common stress causing physical factors, which occur in all stages of crops’ lifespan.
It is estimated that up to 82% of potential crop yields are lost to abiotic stress
annually (Bray et al. 2000 ) . Among the biotic factors, diseases are predominant than
pests. In some species like cocoa ( Theobroma cacao ), pests and fungal diseases are
responsible for more than 40% yield losses (Argout et al. 2008 ) , of which pod rot
caused by Phytophthora spp. ( P. palmivora and P. megakarya ) claims 30-90% of the
total global crop loss (Bowers et al. 2001 ). A great deal of geographic variation
exists among various forms of stresses that affect plantation crops, as well as in the
adaptation of crops. Stresses can occur from single factor, or from combination of
factors in annual cycles. Furthermore, because of perennial nature, accumulated
stress effects often act detrimental in plantation crops. Predictions for the global
agrarian future warn of rising temperature and humidity fl uctuations, water logging,
salinity and other problems that can adversely affect crop plants. In addition, these
factors may affect diversity and virulence of agricultural pests and diseases leading
to epidemics (Gregory et al. 2009 ) . Fortunately, in plantation crops, very few stress
factors are known to cause serious concerns in the industry and some are histori-
cally documented. During second half of 19 th century, coffee plantations of Ceylon
was devastated by leaf rust disease caused by Hemileia vastatrix , leading to the
3 Stress in Plantation Crops: Adaptation and Management
Table 3.1 Details of major plantation crops
2n a Center of diversity
48 Indian center
Madhusoodanan ( 2002 )
Anacardium occidentale L. Anacardiaceae 42 Tropical Americas
Ohler ( 1979 )
Theobroma cacao L.
Chocolate, cocoa Wood and Lass ( 2001 )
Coconut palm Cocos nucifera L.
32 South America,
Southeast Asia (?)
Thampan ( 1993 )
Coffea arabica L. C. canephora Pierre.
44 East Africa
Wrigley ( 1988 )
22 Central and West Africa
Phoenix dactylifera L.
36 Iran and Arabian region Arid tropical Oil nut
Zaid and Jimenez ( 2002 )
Coppen ( 2002 )
Elaeis guineensis Jacq.
32 West Africa
Hartley ( 1988 )
Hevea brasiliensis Muell.
Euphorbiaceae 36 Amazon basin
Webster and Baulkwill ( 1989 )
Eden ( 1976 )
a 2n Diploid chromosome number
closure of entire coffee plantation industry, paving way to the development of tea
industry (Ward 1882a , b ). Similarly, entire natural rubber plantations were shifted
to South and Southeast Asia from Brazil in the late 19 th century due the epidemics
of South American leaf blight (SALB) disease caused by Microcyclus ulei (Labroy
and Cayla 1913 ; Leiberei 2007 ). In cocoa, two serious diseases (witches’ broom by
Crinipellis perniciosa and frosty pod rot caused by Moniliophthora roreri ) devas-
tated cocoa production in South and Central Americas (Pereira et al. 1996 ; Borrone
et al. 2004 ) . Recently, during 1980s, a leaf spot disease caused by Corynespora cas-
siicola caused total replanting of high yielding rubber clones in Sri Lanka (Silva
et al. 1998 ; Manju et al. 2001 ; Fernando et al. 2009 ) . Among these, serious threat of
SALB to the world economy has led to the decision to include M. ulei as a potential
biological weapon by the U.S. National Research Council (Madden and Wheelis
2003 ) and by the United Nations Offi ce on Drugs and Crime (Lieberei 2007 ).
3.2 Adaptation to Abiotic Stresses
Abiotic stress occurs from the factors of physical environment and plays a major
role in survival and reproduction in plants. Survival under abiotic stress requires
specifi c adaptation that takes place at two levels (a) individual and (b) population.
Fig. 3.1 Environmental stresses in plantation crops
3 Stress in Plantation Crops: Adaptation and Management
Individual phenotypic adaptation (phenotypic plasticity) softens the impact of natu-
ral selection among genotypes (Bradshaw 1965 ; Schlichting 1986 ; Sultan 1987 ) and
by itself forms the chief objective of natural selection (Wright 1931 ) . However, at
population level, adaptive plasticity of a plantation species (Table 3.2 ) will refl ect
on the environmental stresses it had faced repeatedly in its evolutionary history. One
of the best examples for adaptive response is the deciduous nature of rubber trees in
the native evergreen rainforests of Amazon, which remains warm and wet through-
out the year. Apart from the popular understanding that deciduous trees shed leaves
Table 3.2 Natural adaptation and sensitiveness of plantation crops to abiotic stresses
Crop Adaptation Sensitivity
Cardamom Low temperature >10°C High rainfall
Cashew Drought Water logging
Well drained acid soils Low temperature, Frost
Cocoa Shade Drought
Warm humid climate Low temperature
Coconut Warm humid climate Water logging
High rainfall Drought
Salinity Tropical storms
Coffee Cool mountain climate to
Warm humid climate
Murugan et al. ( 2007 )
Paiva et al. ( 2009 )
Herzog ( 1994 )
Amanor ( 1996 )
Nair ( 2010 )
Lakmini et al. ( 2006 )
Batugal et al. ( 2009 )
Nair ( 2010 )
Remison and Iremiren ( 1990 )
Van der Vossen ( 2001 )
Lashermes et al. ( 2008 )
Klein et al. ( 2002 )
Date palm Rain and humidity
Zaid and Jimenez ( 2002 )
Eucalyptus Low temperature Drought
Heavy metal toxicity
Grattapaglia ( 2008 )
Oil palm High rainfall
Jalani et al. ( 1997 )
Henson et al. ( 2005 )
Cha-um et al. ( 2010 )
Nair ( 2010 )
Bolton ( 1964 )
Vinod et al. ( 1996b )
Rubber Long photoperiod
Tea Low temperature Mondal ( 2009 )
Singh ( 1980 )
in order to avoid a winter or a tropical dry season, it is postulated that trees might
also shed leaves to have a break in their parasitism, and to resuscitate their bio-
chemical locks. This possibly explains why the rubber tree became deciduous,
owing to continuous parasitism by M. ulei, the SALB pathogen (Robinson 1995 ) .
Adaptation to environmental stress in plants is determined by their genetic
make up. The difference in regulatory mechanisms of defense related genes decide
whether a plant is resistant or susceptible. These differences are ubiquitous arsenals
that help plants to combat stresses of various kinds.
Drought ranks foremost among the abiotic constraints for production in plantation
crops because most of them are grown in tropical semi-arid conditions. Generally,
plants adapt themselves to survive drought through two different ways viz., drought
escape and drought tolerance. Perennial plantation crops have no scope for drought
escape (Kozlowski and Pallardy 1997 ) , while they withstand water stress by drought
tolerance. Drought tolerance is manifested either through desiccation avoidance or
desiccation tolerance. Desiccation or dehydration avoidance is achieved by maintain-
ing high water potential within plants. In plants, primary mechanisms that maintain
high water potential are reduction in transpiration through stomatal regulation, reduc-
tion in transpiring area, reduction in radiation absorption and maintaining water
uptake by increasing root growth and root hydraulic conductivity. These mechanisms
operate through morphological adaptations involving roots, stem and leaves and
physiological regulation of transpiration; thereby plants increase internal water stor-
age and prevent tissue dehydration during drought. Reduction in tissue hydration is
primarily achieved by maintaining turgor through osmotic adjustment by accumulat-
ing solutes and inducing stomatal closure by increasing abscisic acid (ABA) content
in leaf tissues, while maintaining near normal physiological activities (Repellin et al.
1994 ) . Maintenance of low water potential primarily requires maintenance of turgor
achieved through osmotic adjustments. Desiccation tolerance is rather a limited
choice for majority of woody plants (Kozlowski and Pallardy 1997 ).
Crop adaptation depends on the degree of plant sensitivity to drought. The degree
of drought sensitivity varies among plantation crops at species, genotype, phono-
logical and organ levels. For example, at species level, crops like rubber and cashew
are very sensitive to water logging; and at organ level, leaves are found to be more
vulnerable than stem in evergreen trees (Chen et al. 2009 ) .
Systematic investigations on drought tolerance in plantation crops reveal that a
wide range of anatomical, physiological and biochemical adaptability contributes to
the level of tolerance (Rajagopal et al. 1990 ) . Therefore, many physiological param-
eters such as pre-dawn and midday leaf water potential, net photosynthesis, sto-
matal conductance, transpiration and intercellular CO 2 concentration and biochemical
attributes like ascorbic acid, glutathions, tocopherols, chlorophylls, carotenoids,
free amino acids and soluble carbohydrates have been used to assess drought stress
3 Stress in Plantation Crops: Adaptation and Management
in these crops. Physiological and morphological characteristics, such as stomatal
conductance, crown architecture, root depth and water use effi ciency may be used
as potential traits for selecting coffee genotypes with superior performance under
220.127.116.11 Anatomical Adaptations
Majority of the anatomical adaptations to drought are modifi cations to reduce water
loss from plants during dehydrating conditions. This is mainly achieved by control-
ling transpiration loss through stomatal regulation. Stomatal response is probably
one of the most complex behaviors in plants. External factors like light, air humid-
ity, soil water content, nutrient status etc. and internal factors like ABA concentra-
tion, leaf water status etc. are known to exert direct infl uence on stomatal behavior.
Stomatal behavior is a key indicator in assessment of drought tolerance in plantation
crops. Striking differences in stomatal function during drought has been reported in
rubber (Sangsing et al. 2004a ) , cocoa (Gomes et al. 1987 ; Willson 1999 ) , coconut
(Prado et al. 2001 ; Passos et al. 2005 ; Gomes et al. 2008 ) and many other woody
plant species (Kozlowski and Pallardy 1997 ) .
Under drought situations, xylem cavitation occurs when sap pressure within the ves-
sels fall below a threshold resulting in irreversible collapse of the xylem conduits. In
cavitation resistance, stomatal closure takes place well in advance as a response to
increase in plant hydraulic resistance that prevents water loss, thereby maintaining
sap pressure above the threshold (Sperry and Tyree 1988 ) . Sangsing et al. ( 2004b )
have found (i) relatively high vulnerability of xylem conduits to cavitation, (ii) asso-
ciated responses between stomatal behavior and occurrence of cavitation and
(iii) substantial genotypic variation in vulnerability to cavitation suggesting that
whole plant level drought adaptive mechanisms are in force in rubber trees. Cavitation
resistance therefore is a parameter that pertains more to drought survival. Analysis of
xylem embolism, especially in the petioles, may provide a sound criterion for evalu-
ation of genotype behavior under drought conditions.
As that of rubber tree, coffee can also adapt to drought by regulation of hydraulic
conductance. Hydraulic conductance in coffee is found to be directly associated
with transpiration rate, that may help in regulation of water loss though transpira-
tion and to maintain the sap pressure above threshold to avoid xylem cavitation
(Tausend et al. 2000 ) . A drought tolerant genotype therefore must be able to main-
tain high relative water content (RWC) under moisture stress conditions. It has been
reported that by effi cient stomatal control on transpiration coupled with low cell-
wall elasticity (Pinheiro et al. 2004 ) coffee plants could retain high leaf RWC
(DaMatta et al. 1993 ) under drought. A slight shift in turgor due to the loss of little
water from ensuing drought can signal leaves to maintain a high RWC and retain a
high symplast volume. Further, conservation of a high symplast volume may be
crucial for maintaining gas exchange under drought as seen in arabica coffee
(Meinzer et al. 1990 ) , and hence maintenance of a high RWC is crucial in confer-
ring drought tolerance in coffee (DaMatta 2004 ) .
Leaf abscission occurs in deciduous plantation crops like rubber when they approach
annual period of water defi cit. Leaf abscission may be considered as a whole-plant
mechanism to limit water loss through transpiration in these plants. Leaf shedding was
reported in fi eld grown robusta coffee in response to water defi cit (DaMatta and Rena
( 2001 ) occurring sequentially from older to younger leaves; and more drought-sensitive
the genotype is, greater would be the extent of leaf shedding. However, DaMatta ( 2004 )
suggested that at least in robusta coffee senescence might be merely a consequence of
and not a defense against stress since drought-sensitive clones, which considerably lose
their foliage, also show most impaired water status in the remaining leaves.
Leaf anatomical adaptations such as cell size and number, stomatal frequency, sto-
matal resistance and epicuticular wax content have been described as essential ana-
tomical indicators for assessing moisture stress in coconut (Gomes and Prado 2007 ) .
Waxes are involved in plant’s fi rst defense against abiotic stress of which hydrophobic
waxes play an important role in water retention by limiting non-stomatal water loss
(Kerstiens 1996 ; Riederer and Schreiber 2001 ; Jenks 2002 ) . Drought adaptive ana-
tomical features in coconut includes leafl ets with upper epidermal waxy cuticle two-
times thicker than the lower epidermis, thicker cuticular edges, xylem tracheids with
thick lignifi cation and tracheids with scalariform thickening. Further, water tissue
with thin-walled cells at the upper and lower angles of straightened leafl et margin and
fi brous sheet encircling seven to eight large vascular bundles in a strong midrib are
also seen in leafl et lamina. Presence of two layers of large hypodermal cells below the
upper epidermis and a multi-layered and closely packed palisade tissue also seen in
contrast to scanty spongy parenchyma exist between the upper and lower hypodermis.
Drought tolerant coconut cultivars had more scalariform thickening on tracheids and
large sub-stomatal cavities (Kumar et al. 2000 ) . A negative relationship between the
epicuticular wax content of coconut leaves and transpiration rate has been reported by
Kurup et al. ( 1993 ) substantiating the presence of high epicuticular wax content
among drought tolerant than susceptible genotypes (Riedel et al. 2009 ) .
Stem Vascular Systems
Palms like coconut, oil palm and date have tall singular stems developed from the
apical meristem. Being monocotyledons, vascular bundles of xylem and phloem
3 Stress in Plantation Crops: Adaptation and Management
appear scattered throughout the ground parenchyma, and there is no distinction
between cortex and pith in these plants. Vascular bundles have an anastomosing or
weaving pattern running longitudinally throughout the length of the stem. Stem
anatomy is organized in such a way to minimize isolated functional sectors of mass
fl ux from roots to canopy. The transport capacity of this massive hydrosystem
increases with the stem diameter, and is functional throughout the lifespan of the
palm that can extend well above a century. Hence, the ability to withstand water
stress in palms like coconut is presumably coordinated by stem processes (Tomlinson
2006 ) as well. This complex process may contribute to higher drought adaptation of
tall coconut varieties than the dwarf ones. Passos and Silva ( 1991 ) found that stem
girth of tall coconut decreases between dawn and midday before increasing again
during afternoon suggesting stem organized control of water transport. Variation in
stem diameter as a response to drought stress also has been described in coconut.
Stresses due to drought, fl ooding, mineral defi ciency and some diseases can cause
decreases in stem diameter, which reverses to normal when environmental condi-
tions improve. Therefore, stem girth and form make a good record of palms’ stress
history. Phenological variations in drought sensitivity in oil palm indicate that young
palms are more susceptible since they do not possess extensive root system and
voluminous stem as in adult palms (Villalobos et al. 1992 ) .
18.104.22.168 Physiological Adaptations
Rubber as a deciduous tree has a different phenological adaptation than the ever-
green trees. Annual natural defoliation known as ‘wintering’ occurs at the onset of
winter and extends roughly for 4-6 weeks through the season (Vinod et al. 1996a ) .
Leaf fl ushing occurs at the end of wintering period and well before the arrival of the
rains that coincides with dry months. Flushing process requires large quantity of
water for leaf development and expansion (Elliot et al. 2006 ; Williams et al. 2008 ) .
Root zone activities show signifi cant deep root water uptake during wintering and
fl ushing of rubber trees (Guardiola-Claramonte et al. 2008 ) . Signifi cant reduction in
transpiration (Priyadarshan and Clément-Demange 2004 ) and increase in root water
uptake helps trees to maintain stem water potential required for subsequent leaf
fl ushing. Rubber trees can conserve extracted water without being released to
atmosphere until the new foliage is grown (Guardiola-Claramonte et al. 2010 ) .
Differential adaptation to drought among major sub-types of coffee, arabica and
robusta appears to be governed by changes in rates of water use and/or effi ciency of
extraction of soil water (Meinzer et al. 1990 ; DaMatta et al. 2003 ; Pinheiro et al.
2004 ) . During prolonged drought, reduction in leaf area and alternate assimilate
partitioning new foliage the major adaptive mechanism for survival in arabica coffee
(DaMatta 2003 ) . Behavioral difference among the tolerant and susceptible geno-
types was refl ected in relative drought tolerance, governed by adaptation in root
depth, plant hydraulic conductance and stomatal control of water loss (Pinheiro
et al. 2005 ) . Accordingly, better drought adaptation among genotypes with deeper
root systems is reported among robusta coffee (Pinheiro et al. 2004 ) . Similarly root-
ing depth infl uenced drought tolerance has been reported in tea wherein shallow
rooted clones were found drought susceptible than the deep-rooted clones. Moreover,
drought tolerance was found to increase with rooting depth in shallow rooted clones,
whereas no much variation was found in deep-rooted clones (Nagarajah and
Ratnasuriya 1981 ).
Canopy architecture has been found to play a signifi cant role in drought adaptation
in trees. Canopy structure partially determines the hydraulic architecture of a tree
(Herzog et al. 1998 ) . In coffee, dwarf cultivars with dense crowns are better able to
withstand drought by delaying dehydration than cultivars with open crowns. Canopy
compactness is also achieved by reducing the size of the leaves and altering the
crown shape, that result in better energy dissipation with reduced transpiration
(Kozlowski and Pallardy 1997 ). In rubber trees, adaptation to prolonged drought
has been reported to produce compact canopy. This prevents high light intensity
(which can be photoinhibitory) from reaching the lower canopy leaves and blocks
penetrating radiation from reaching the plantation fl oor that can heat up surface soil,
change vapor pressure defi cit (VPD) and alter microclimate leading to evapotran-
spiration loss. Compact canopy has been implied as an ideal phenotypic feature for
drought tolerance in rubber clones (Devakumar et al. 1999 ) . DaMatta ( 2004 ) has
suggested microclimate associated transpiration loss as one of the probable reasons
of crop failure, in spite of suffi cient supplemental irrigation, at locations with high
evaporative demands planted with coffee cultivars with open crowns.
Stomatal Control – Hydraulic Signaling
Stomatal regulation takes place in plants as a response to signals arising primarily
from roots. These signals trigger a cascade of events such as induction of differential
gene expression, changes in cell metabolism and development of defensive systems
in the above ground organs (Jackson 2002 ; Kholodova et al. 2006 ) . Primarily sys-
temic signals are hydraulic in nature spreading along the xylem that coordinates
physiological responses (Jackson 2002 ) especially in leaves. As available soil mois-
ture falls, a gradual reduction ensues in shoot water status, initiating direct hydraulic
signals to leaves. Several studies suggest ABA accumulation as the primary candi-
date for hydraulic signaling in plants (Davies and Zhang 1991 ; Comstock 2002 ;
Kholodova et al. 2006 ) . Root tissues synthesize ABA as an initial response to dimin-
ishing soil moisture availability. Hydraulic signals are instantaneously spread through
the continuous water phase within plants culminating in foliar stomatal regulation.
As a reliable indicator of stomatal performance, stomatal conductance has been
used in plantation crops for assessing drought sensitivity (Rajagopal and Ramadasan
1999 ; Nainanayake and Morison 2007 ; Lakmini et al. 2006 ; Carr 2011 ) , because
3 Stress in Plantation Crops: Adaptation and Management
stomatal conductance is highly correlated with transpiration rate and photosynthetic
health. Stomatal conductance has been used as an early indicator of stress in arabica
coffee, because decline in stomatal conductance is found to occur even at one-third
depletion of available soil water. Evidences suggest that during short-term water
defi cit, yield reduction in coffee genotypes may be associated with stomatal
conductance and net carbon assimilation (Nunes 1976 ) . Although poor stomatal con-
trol was found during drought (DaMatta et al. 1997 ) the mechanisms leading to
clonal tolerance to drought in robusta coffee still remain largely unknown (Pinheiro
et al. 2005 ) . In coconut, strong evidence of stomatal regulation of plant water status
has been documented at mild to moderate drought (Prado et al. 2001 ; Passos et al.
2005 ; Gomes et al. 2008 ) making it a useful parameter to differentiate drought sensi-
tive and tolerant genotypes in association with tissue water potential (Lakmini et al.
2006 ) . VPD sensitive stomatal regulation was reported in cocoa (Willson 1999 ) ,
wherein leaves exhibit reduced water use effi ciency at high VPD (Gomes et al. 1987 ) ,
leading to tissue water defi cit under conditions of limited soil water supply.
Stomatal Control – Non-hydraulic Signaling
Several studies have shown that signals of moisture depletion reaches leaves from
roots, unassisted by shoot water status, indicating the existence of a non-hydraulic
route of signal transport (Croker et al. 1998 ) . Although perceived as an important
component of plant responses to drought, exact mechanisms involved in non-
hydraulic root to shoot signaling still remain unclear (Davies et al. 1994 ) . Recently,
a non-hydraulic chemical mediated signaling has been reported in coconut, in which
chloride ions are involved in sensing soil water depletion in the root zone and send-
ing stomata-closing signals to the leaves. In coconut, chloride ions are found to play
two important functions in regulating water balance; fi rst, they regulate stomatal
closure by coordinating water fl ow between six neighboring cells (two guard cells
and four subsidiary cells) of the stomatal apparatus, and secondly, chloride ions
improve osmoregulation capacity under water stress (Gomes and Prado 2007 ) .
Variation in photosynthesis and associated systems has also been used as indicators of
drought tolerance in plantation crops. Largely, photosynthetic performance of plants
is determined by environmental variables under fi eld conditions. Photosynthesis is
closely associated with stomatal function since gas exchange in plants takes place
through stomata. Drought-induced stomatal closure limits CO 2 diffusion from the
atmosphere to the intercellular spaces resulting in reduced photosynthetic activity
(Repellin et al. 1994 , 1997 ). Genotypic variations have been reported in coconut in
rates of gas exchange recovery and internal dehydration on exposure to drought. In
dwarf coconut varieties, photosynthetic acclimation was observed after repeated dry-
ing and recovery cycles (Gomes et al. 2008 ) . Use of rate of photosynthesis and
instantaneous water use effi ciency as reference parameters has been reported promising
in coconut (Nainanayake and Morison 2007 ) . Chlorophyll fl uorescence transients
have been implicated in differentiating and screening of coconut seedlings that can
adapt to water stress condition (Bai et al. 2008 )
22.214.171.124 Biochemical Adaptations
Osmotic adjustment (OA) has been found associated with maintenance of gas
exchange under drought (Turner 1997 ) in plants. OA occurs in cells in response to
drought stress signals, where in osmolyte accumulation takes place to prevent cel-
lular dehydration. Osmolytes are low molecular weight metabolites that are able to
compensate high osmotic pressure without interfering with plant metabolism, even
at elevated concentrations. They include sugars, polyols, amino acids and quater-
nary ammonium compounds. Proline is a typical osmolyte, synthesized in plants
under different stress conditions. Although drought sensitive, cocoa plants exhibit
active osmotic adjustments when exposed to dehydrating environments (de Almeida
and Valle 2007 ) , but suffer yield loss under stress (Moser et al. 2010 ) . Osmotic
adjustments and stomatal regulation have been reported as one of the mechanisms
operating in drought tolerant coconut varieties (Rajagopal and Ramadasan 1999 ) .
Other Biochemical Indicators
The plant hormones, ABA and ethylene play signifi cant role in plant adaptation to
environmental stress. Two key multigene families, 9-cis-epoxycarotenoid dioxyge-
nase (NCED) genes (Seo and Koshiba 2002 ) and 1- aminocyclopropane-1-carbox-
ylate synthase (ACC synthase) genes (Yang and Hoffman 1984 ) respectively
regulate the biosynthesis of these hormones. Late embryogenesis abundant (LEA)
proteins are a distinct group of proteins that are induced during dehydration stress
caused by extreme temperatures, drought, salinity and certain developmental events
such as seed maturation (Close 1996, 1997 ) . LEA proteins are believed to be a sub-
group of dehydrins that have extreme hydrophilic nature and are soluble at high
temperature. LEA proteins are believed to act as a novel form of molecular chaper-
one to help prevent the formation of damaging protein aggregates during water
stress (Goyal et al. 2005 ) . Dehydrins in general are structural stabilizers, that protect
nuclear, cytoplasmic, and membrane macromolecules from dehydration-induced
damage, thus maintaining cell structure and integrity. In plantation crops, dehydrin
like proteins have been identifi ed in coffee (CcDH1, CcDH2 and CcDH3; Hinniger
et al. 2006 ) and eucalyptus (Bocca et al. 2005 ) . Another important and highly
diverse set of proteins implicated in dehydration stress are heat shock proteins
(HSPs). HSPs too functions as molecular chaperons minimizing the aggregation of
proteins and targeting aggregated proteins for degradation, while assisting in protein
folding, assembly and transport.
3 Stress in Plantation Crops: Adaptation and Management
Various adaptations to drought include a stress-induced cascade of reactions in
plants, including scavenging of reactive oxygen species (ROS) produced during oxi-
dative stress. To prevent oxidative damage, cells contain antioxidants that scavenge
the free radicals (Yamasaki et al. 1997 ) . Phenolic compounds are cellular compounds
with antioxidant properties (Rice-Evans et al. 1997 ) ; and several studies have shown
that production of compounds with effi cient antioxidant structures, such as addi-
tional hydroxyl groups on ring B of the fl avonoid skeleton is accelerated (Ryan et al.
1998 ) during drought stress. Plant cells also contain enzymes, such as superoxide
dismutase (SOD) and catalase that protect them by scavenging superoxide radicals
and hydrogen peroxide respectively (Takeuchi et al. 1996 ) . Transcriptome profi le of
the rubber tree latex contains many genes related to water stress in abundance, whose
actual role in stress defense is not yet known. Transcripts of two genes encoding for
Hevea brasiliensis ASR (abscisic acid, stress and ripening) like proteins, HbASRLP1
and HbASRLP2 were the most abundant next to those of rubber elongation factor
(REF) and small rubber particle protein (SRPP) in the latex. These genes are homol-
ogous to tomato ASR gene family (Rossi et al. 1996 ) and the putative proteins coded
by these genes have a domain similar to those proteins induced by water defi cit
stress, ABA stress and ripening. Further, a gene family of HbRLPs (REF like pro-
teins) was also found expressed at higher levels in rubber latex. Both HbRLPs and
SRPP are structurally closer to stress related proteins (Ko et al. 2003 ) .
In tea, elevated polyphenol content has been reported as an indicator for drought
tolerance (Hernández et al. 2006 ; Cheruiyot et al. 2007 ) . Drought tolerant tea clones
had higher catalase activity for scavenging hydrogen peroxide formed in the photo-
respiratory pathway (Jeyaramraja et al. 2003a ) . Furthermore, drought induced
reduction in catechin content has been reported in tea clones (Singh et al. 2009a , b ).
This behavior is attributable either to an instability of the catechins under drought
(Jeyaramraja et al. 2003b ) or to possible of loss of catechins due to enhanced cel-
lular injury or to down-regulated pathways leading to limitation in the availability
of precursor molecules (Singh et al. 2008 , 2009a , b ; Jeyaramraja et al. 2003b ;
Sharma and Kumar 2005 ) .
Although mechanisms are many, each plantation crop has its own combination of
adaptive mechanisms to counter drought stress. Overall strategy appears to be to
reduction or cessation of leaf-area development, maintenance of good water use
effi ciency with stomatal regulation, maintenance of effi cient photosynthesis and tol-
erance of additional stress with osmotic adjustment of mature leaves, fruits and
roots. Foliar abscission appears to be the choice during severe stress. Osmotic
adjustment of the functional leaves is effectively carried out to maintain the overall
canopy function and leaf longevity is maintained for a considerably long period.
Most of the plant processes are temperature dependent. In plantation crops, apart
from growth and establishment, signifi cant temperature infl uence may be seen on
reproductive growth such as fl oral initiation, release of bud dormancy, anthesis,
fruit-set and fruit development. Thermal adaptation varies widely among tropical,
subtropical and temperate plantation crops, since the temperature plays a leading
role in limiting plant distribution between tropical and temperate regions. In gen-
eral, fl owers and fruits are injured by extremely low or high temperatures. When
temperature falls, molecular activity gets decreased and essential biochemical proc-
esses involved in sustaining growth are arrested. Low temperatures also decrease
the permeability of membranes and increase protoplasmic viscosity. On the other
hand, when temperatures goes excessively high, molecular activity may get impaired
and the enzymes controlling metabolic processes are denatured or inactivated.
126.96.36.199 Low Temperature
Plant stress occurring due to lowering ambient temperature has been a common
phenomenon in all the crops. Degree of variation in plant responses to low tempera-
ture depends primarily on the basic adaptation pattern of the crops. Temperate spe-
cies can tolerate very low temperature, which are fatal to tropical crops. This is the
reason why tropical and sub-tropical species fail to establish under temperate cli-
mates. In temperate species, low temperature favors shoot growth through its effects
on bud formation, bud dormancy and bud expansion. Low temperature is an essen-
tial requirement for these crops to break bud dormancy, and elevated temperature
during mild winter impairs bud break resulting in crop failures (Weinberger 1950 ) .
However, excessive freezing causes various stress reactions in temperate planta-
tion species. Freezing injury occurs mainly during autumn (winter) season, while
frost injury infl icts damages to fl owers, buds and young fruits during spring season.
Furthermore, rootstocks are more susceptible to freezing injury than scion. Freezing
injury occurs when low temperature spells are followed by a period of mild weather
that permitted growth to start and caused loss of cold resistance. Hence, repeated
mild freezing spells are more deleterious than a single spell of deep freezing. In
addition, a few temperate zone plants are susceptible to chilling injury that occurs
when they are exposed to temperatures a few degrees above freezing. Freezing
injury can be caused either directly by intracellular freezing or indirectly by dehy-
dration of tissues resulting from extracellular freezing. Plants are usually killed by
ice crystal formation within the cells, but formation of ice crystals within the inter-
cellular spaces is not necessarily fatal. In frost hardy plants, cellular freezing is
prevented through a cell sap concentration mechanism. Concentrated cell sap has
lower freezing point than water, and hence ice formation is prevented. Although,
slower cooling causes ice to form initially in the intercellular spaces, and as the
temperature decreases gradually, water moves out of the cells to the intercellular ice
nuclei, making the cell sap thicker. However, rapid cooling is normally fatal because
there is little time for water to move out leading to a sudden intracellular freezing
resulting in mechanical disruption of protoplasm. In tree species, whole plant level
freezing damage occurs when crown and roots are affected; of this, crown damage
is usually irrecoverable and fatal. In Eucalyptus globulus , it was reported that freez-
ing tolerance of provenances occurred due to their ability to evade the formation of
3 Stress in Plantation Crops: Adaptation and Management
ice in their leaves, suggesting that supercooling is the most probable mechanism
conferring resistance in these plants. In addition, accumulation of soluble carbohy-
drates in the leaves provides cryoprotective mechanisms (Moraga et al. 2006 ) as
soluble sugars are known to function as cryoprotectants in plant tissues, especially
at membrane level (Tinus et al. 2000 ) .
Frost cracks can develop on tree trunks when alternating freezing and thawing
occur during the winter and early spring. These cracks occur with a loud bang
and can be of several feet long. The bark at the affected region occasionally peels
off exposing the underlying wood, which becomes site for secondary infection.
Since these lesions damage phloem, downward translocation of organic solutes
is impeded. Although less hardy, root tissues are relatively less injured than
stems during winter, because soil and snow cover protect them from exposure to
freezing air temperature. However, freeze damage occurs in roots, when dry soil
sucks moisture out of roots during winter, and if the soil is wet, no damage is
found to occur.
Spring frost during growing season often injures cambium, causing the forma-
tion of abnormal stem frost rings. These rings usually have an inner part of frost-
killed cells and an outer layer of abnormal xylem cells produced after the frost. In
forest plantations of eucalypts, frost injury is a common problem in temperate
regions throughout the world making most of the eucalypt species unsuitable for
commercial plantings. E. nitens is a fast-growing species widely adapted for plant-
ing in high altitudes where occurrence of severe frost is a common occurrence
(Turnbull and Eldridge 1984 ) in contrast to E. grandis , use of which is limited due
to its lesser frost tolerance (Eldridge et al. 1994 ) .
On the contrary, tropical species are vulnerable to sub-optimal temperature due
to intrinsic oxidative stress occurring from the exposure to low temperature. Cold
susceptibility occurs from reduction in photosynthetic effi ciency and increased pho-
toinhibition of photosystem (Fryer et al. 1998 ) . Reduction in photosynthetic func-
tions is observed in rubber trees grown under cold climate (Alam et al. 2005 ) .
Moreover, rubber is reported to have potential for acclimation of photosynthesis to
temperature in the growing environs (Kositsup et al. 2009 ) . This photosynthetic
plasticity at a reference temperature is associated with fl exible response in net CO 2
assimilation rate. Although rubber trees grown at 18°C were not able to maintain net
CO 2 assimilation rate, photosynthetic capacity and leaf nitrogen status close to those
of trees grown at 28°C, they had potential to survive low temperature stress.
Metabolic fl exibility of rubber was reported to confer less intrinsic oxidative dam-
age in plants grown under elevated CO 2 in encountering low temperature stress
(Alam et al. 2005 ) . Some rubber varieties were found to have greater resilience to
chilling up to 96 hours without any damage to photosynthetic apparatus (Mai et al.
2009 ) . Moreover, variation in membrane injury caused by low temperature has also
been reported in rubber clones (Ray et al. 2004 ) . Phenological adaptations for low
temperature tolerance in rubber clones include shorter refl ushing time and faster
wintering (Vinod et al. 2010 ) . Abundance of stress responsive proteins, like low
temperature- and salt-responsive protein and low-temperature-induced protein has
been identifi ed in latex transcriptome (Ko et al. 2003 ) .
Most of the palm species are acclimatized to tropical rain forests and are grown
in wide range of habitats outside freezing environments (Gomes and Prado 2007 ) .
An exception is the date palm that is cultivated in arid and semi-arid regions char-
acterized by long, hot and dry summers and winters with sub-freezing temperatures.
Date palm can withstand wide temperature fl uctuations up to its zero vegetation
point of 7°C, below which growth stops and plants enters into a resting stage.
Freezing injury occurs when the temperature fall below 0°C resulting in metabolic
disarrays, leading to partial or total damage of leaves. Infl orescences are also heav-
ily damaged by frost. Pinnae margins turn yellow and dry out at -6° C and exposed
leaves of the canopy dry out from -9 to -15°C. If freeze occurs for a long period (12
hours to 5 days), entire palm looks burnt with all leaves showing frost damage
(Mason 1925 ; Nixon 1937 ) . Even if the entire crown is damaged, palms survive and
grow after normal conditions are returned, because the meristematic area of a date
palm is well protected against frost.
188.8.131.52 High Temperature
Exposure to relatively high temperatures causes heat injury in plantation crops that
reduces growth and diminishes chance of survival. Heat injuries may be direct or
indirect. Direct injury is the immediate plant response during the exposure of ele-
vated temperature or immediately thereafter. Direct heat injury is relatively rare
when compared to indirect injury in planation crops. Indirect heat injury in plants
occurs slowly and may not be apparent for many hours or even days after exposure
to high temperature. High temperature causes rapid decline in apparent photosyn-
thesis beyond a critical high temperature, while continuing to increase respiration
resulting in rapid depletion of carbohydrate reserves and sometimes death. Formation
of toxic compounds, production of biochemical lesions, and breakdown of proto-
plasmic proteins (Levitt 1972 ) are also common.
Most common direct heat injury observed in rubber is severe sun scorching (Vinod
et al. 2003 ) that occurs when the collar region of young plants is damaged by sun
exposed surface soil layer heated well in excess of 55-60°C. In grown-up rubber
plants, excessive heating up of stems facing afternoon sun develops cracks and lesions
called heat canker. Rubber plants are damaged due to sun scorch mostly when they are
very young. After three years of growth expanding crown provides shade protection
from scorching. Generally, sun scorch at the collar region results in girdling and death
of the plants, while bark lesions results in partial drying up of the bark exposing the
stem underneath. Since cambial cells are sensitive to high temperature, they die due to
desiccation and oxidative stress. Exposed bark becomes brittle and cracks develop
resulting in oozing of latex from crack wounds. This enhances internal drying and
secondary pathogen damage ultimately resulting in complete death of the affected
bark region. Among the palms grown in plantation sector, betel-nut palm ( Areca cat-
echu L.) is highly prone to sun scorch (Staples and Bevacqua 2006 ) .
Of the tropical plantation crops, cashew is a hardy tree (Purseglove 1986 ) that is
adapted to a wide range of environmental constraints. It grows well in areas of high
3 Stress in Plantation Crops: Adaptation and Management
temperature, where diurnal temperature can exceed 40°c and can survive seasons of
drought. Cashew has an extensive root system with which it can forage for moisture
and nutrients and can thrive in poor and marginal soils (Kembo and Hornung 1999 ) .
Salinity is a common problem in all parts of the world, especially secondary land
salinization that occurs mainly due to clearing of forests and shrub lands for agricul-
ture and excessive irrigation (Cramer et al. 2004 ). Plant stress due to salinity occurs
when soluble salt concentration in soil and water exceeds limits of plant tolerance.
Salt affected areas can be of two types, sodic and saline. They have excess sodium and
high concentration of carbonate or bicarbonate anions with greater pH ranging from
8.5 to 10.8. They have very poor structure and their sodium absorption ratio (SAR) is
high. Saline soils are rich in sodium ions, but with chloride and sulfate as dominant
anions; pH and SAR are much lower than sodic soils, but have higher electrical con-
ductivity (>4 dS m -1 ). Salt-affected soils contain high concentrations of soluble salts
that reduce growth in most of the plant species (Flowers and Flowers 2005 ) .
Plants that are sensitive to natural salinity are called glycophytes, and those resis-
tant are called halophytes. Some halophytes can tolerate extreme salinity because of
their anatomical, physiological and morphological features or avoidance mecha-
nisms (Flowers et al. 1986 ) .
Salinity induces stress in plants in many ways. It affects numerous physiological
or biochemical processes, many of which are seen at cellular level. Primarily high
salt concentration reduces osmotic potential of the soil solution creating water stress
to root cells even under suffi cient soil water availability. Salts interact with nutrient
ions and make them unavailable to plants. Further, ions such as sodium cause inter-
nal toxicity in plants and they are not readily sequestered into vacuoles in glyco-
phytes (Sairam and Tyagi 2004 ) .
Plant adaptation to salt varies among plantation species in the degree of tolerance
to a range of salt concentration that is achieved through one or more different mecha-
nisms. The most important mechanisms are the exclusion of ions at root zone and
suppression of ionic translocation to the shoot that help plants to keep levels of ions in
growing meristem and young shoots low. Sequestering of salt ions into vacuoles or
deposition into bark, ray cells, tracheid walls and lumens, or older senescent leaves,
also help tolerant species to prevent salts from interfering with normal metabolic
activity in the cells. Some plants show ability to maintain ion exclusion from young
shoots under hypoxic conditions of waterlogging and maintain the ability to uptake
water continuously in presence of high salt concentrations (Niknam and McComb
2000 ) . Leaf and root Na/K ratio as well as stomatal resistance of plants exposed to
salinity proved appropriate indexes of whole plant response to salt stress. Among the
eucalypts, E. grandis tolerates low to medium levels of soil salinity through salt exclu-
sion mechanism, until a threshold concentration, above which the tolerance fails.
Once broken, tissues become very sensitive to salt. On the other hand, E. maculata
shows less tolerance than E. grandis at the lower salt levels, but survives higher salt
levels at which E. grandis becomes susceptible (Sun and Dickinson 1993 ) .
Coconut palms are moderately tolerant to salinity (Remison and Iremiren 1990 ) .
Although grown in wide range of ecological conditions, coconut is considered to
possess halophytic properties (Purseglove 1975 ) . Traditionally planters apply sea
salt in coconut groves in India, Java and Columbia (Child 1964 ; Manciot et al.
1979 ) , and it is experimentally proved that sodium chloride (NaCl) application
improved development of infl orescence, number of female fl owers and nut yield
(Fremond et al. 1966 ; Roperos and Bangoy 1967 ). Coconut is naturally adapted to
maritime shores and is capable of absorbing chloride ions rather easily (Magat et al.
1975 ; Manciot et al. 1979 ; Remison and Iremiren 1990 ) . In coastal areas, chloride
accumulation in coconut leaves is a common phenomenon. Until recently, impor-
tance of chloride ions in coconut physiology was not understood. Evidences indi-
cate that chloride ions play a signifi cant role in non-hydraulic signaling of stomatal
control in palms. It is now found that palms defi cient in chloride had impaired sto-
matal function. The physiological role of chloride in coconut nutrition was earlier
suspected to be associated with the water economy of the plant. Coconut belongs to
a group of plants that lack chloroplasts and starch in their guard cells. For stomatal
movements, such plants require chloride ions (Von Uexkull 1985 ) .
Adaptations of plantation species to local environments include the effi ciency to
harvest radiation for optimal growth and development. Most of the radiation in the
atmosphere is infrared radiation (700-3000 nm, 67% of the photons) and visible
light (400-700 nm, 28%; Nobel 1983 ) . Ultraviolet (UV) radiation (200-400 nm), on
the other hand, reaches the atmosphere in smaller amounts (5% of the photons).
Biologically most hazardous part of UV radiation, i.e. UV-C (200-280 nm) and
UV-B (280-320 nm) are completely absorbed by the stratospheric ozone layer and
by other oxygen molecules in the atmosphere (Frederick 1993 ) . In addition, ozone
layer absorbs some of the longer-wave UV-B and UV-A radiations (320-400 nm).
Therefore, of the photons reaching earth’s surface, only about 2% are in the ultra-
violet range (Nobel 1983 ) . However, of the total solar energy, UV-B comprises
about 1.5% and UV-A radiation about 6.4% (Frederick et al. 1989 ) . Infrared region
is responsible for the thermal stress in plants discussed in the previous section.
Effect of other types of radiations such as X-rays, gamma rays and other ionizing
radiation are negligible in plantation crops.
184.108.40.206 Visible Light
Plants use radiation in the regions approximately from 400 to 700 nm for photosyn-
thesis, the region from 660 to 730nm has important qualitative, photo-morphogenic
3 Stress in Plantation Crops: Adaptation and Management
effects on growth. Physically, effects of light on plant growth depend on irradiant
quality and duration (photoperiod) of which the former depends on the intensity and
quantum of light exposure. Hence, high and low light exposures can make different
effects on plants.
Plantation crops such as cocoa, cardamom, coffee and tea are heliophobes, which
prefer low light. Heliophobes are naturally adapted to shade and does not tolerate
heavy sunlight. Shade tolerance in trees varies with the age of the tree and with
environmental conditions. Trees and shrubs vary widely in their capacity to grow in
shade and this often becomes a decisive factor in their success under low light con-
ditions. Trees tend to show higher degree of shade tolerance in their youth, and
those growing on nutrient rich soils are more tolerant (Daniel et al. 1979 ) . Shade
effect is a complex mechanism in cocoa and is often associated with tree nutrition.
Notwithstanding, young cocoa leaves grow well under shade than under full expo-
sure (Wessel 1985 ) . Furthermore, cocoa trees grown under shade have a bushy
appearance with small leaves, shorter internodes and dense crown, which is desir-
able for commercial cultivation.
Coconut and rubber prefer good light, and low light interception showed adverse
stress effect in growth of rubber (Khasanah et al. 2006 ) . Shade grown young rubber
plants had asynchronous variation in growth between roots and leaves that was
attributed to competition for photosythate between secondary roots and leaves
(Thaler and Pagès 1996 ) . On contrary, in its natural habitats, young rubber plants
grow and establish under the shades of overstorey canopy of rain forests. Rubber
plants are found to show differential adaptation to light intensities at different phe-
nological stages. They show natural adaptation to shade only when they are young,
and lose this trait when the tree crown brushes overstorey canopy. Under commer-
cial planting, rubber seedlings grown on cleared lands are found to show better
growth when put under shades of intercrops (Rodrigo et al. 1997, 2001 ) , possibly
because of reduced thermal and/or photooxidative stresses. Senevirathna et al.
( 2003 ) have concluded that shade adaptation and shade-induced reductions in
dynamic photoinhibition account for the enhanced early growth of rubber under
light shade. Shade provides greater protection against diseases and weeds in coffee,
cocoa and cardamom than under exposed conditions. There are many reports to sug-
gest that shade increases fruit weight and bean size in coffee, but not quality and
fruit-bean ratio (Muschler 2004 ) .
Extensive efforts have been made to understand the factors responsible for shade
tolerance in plants. Stomata of shade tolerant species were found to open more rap-
idly in sun fl ecks than those of intolerant species (Woods and Turner 1971 ; Davies
and Kozlowski 1974 ) , allowing the former to carry out photosynthesis during short
periods of illumination. However, signifi cant relation between stomatal response
and shade tolerance could not be established (Pereira and Kozlowski 1976 ) , because
stomatal response not only varied with fl uctuations in light intensity, but also was
greatly modifi ed by water stress preconditioning, temperature and mineral defi -
ciency (Davies and Kozlowski 1974 ) . This implies that many other factors like
anatomical changes in leaves (Jackson 1967 ) , changes in chlorophyll-protein
ratio, enzyme activity of chloroplasts (Holmgren et al. 1965 ) , rate of respiration
(Loach 1967 ) , rate of photosynthesis (Kozlowski and Keller 1966 ) and various
metabolic changes (Durzan 1971 ) infl uence shade tolerance.
Most of the tropical plantation species are adapted to high light exposure. For
instance, radiation requirement of rubber (Ong et al. 1998 ) and coconut (Ochs 1977 ;
Ohler 1999 ) ranges from 6 to 9 hours per day and exceed 2000 hours annually. Even
though cocoa and coffee are heliophobes naturally, in cultivated conditions high
productivity is recorded in coffee when grown under sunshine duration of 1900-
2200 hours per annum (Descroix and Snoeck 2004 ; Wintgens 2004 ) . In cocoa,
mature trees yield well under full exposure but only in fertile soils. In poor soils,
higher yield is obtained only under shade. In rubber, high irradiance especially in
association with drought, high or low temperature can cause depression of photo-
synthetic productivity due to induction of photooxidative stress (Jacob et al. 1999 ) .
Oxidative stress induced chlorophyll degradation was also reported in rubber under
high irradiance (Ray et al. 2004 ) .
Apart from light intensity, photoperiod variations infl uence both vegetative and
reproductive phases of plant growth (Garner and Allard 1920 ) . At equator, days are
of equal length during the entire year, but as we move away, large seasonal differ-
ences occur in the length of the daylight period. In temperate species, short days
arrest shoot expansion and trigger a dormant state, whereas long days delay or pre-
vent dormancy. Progressive slowing down of shoot elongation takes place under
short-day conditions, successively producing shorter internodes until growth even-
tually ceases. Nevertheless, tropical plantation crops also show varied responses to
day length. Experiments under controlled environments show greatly increased
shoot growth during days longer than normal in coffee, cocoa and many other
woody species (Longman and Jeník 1987 ).
220.127.116.11 UV Radiation
One of the growing concerns of present day is that the quantum of UV-B radiation
reaching earth is increasing. Predictions based on stratospheric chemistry and cli-
mate-change models estimate that maximum springtime UV-B radiation will
increase dramatically by 2020 (Shindell et al. 1998 ; Taalas et al. 2000 ) . The inten-
sity of UV-B radiation, in particular, is regulated by the thickness of the ozone layer
that is rapidly being depleted by alarming concentration of anthropogenic atmos-
pheric contaminants like chlorofl uorocarbons and nitrogen oxides (Crutzen, 1972 ;
Molina and Rowland, 1974 ) .
Plants have several mechanisms for UV-B evasion, which include many UV-B
absorptive pigments, refl ective modifi cations and leaf thickness (Caldwell et al.
1983 ; Beggs et al. 1986 ) . Thicker leaves may have an internal protective infl uence
(Johanson et al. 1995 ; Newsham et al. 1996 ) by slight increase in the thickness of
upper epidermis, spongy parenchyma and spongy intercellular space (Kostina et al.
2001 ) . This feature is common in most of the plantation tree species. In addition,
scattering and refl ection of UV-B radiation is achieved through epidermal (epicu-
ticular) wax and leaf hairs (Karabourniotis et al. 1999 ; Kinnunen et al. 2001 ) . High
level of cuticular wax content is a common feature in plantation crops. Changes in
3 Stress in Plantation Crops: Adaptation and Management
crystalline structure of the epicuticular wax with concomitant changes in light
refl ection may protect plants from damaging effects of UV light or aid them in the
adaptation to different light intensities (Riedel et al. 2009 ) . Protective role of epicu-
ticular wax under abiotic stress is reported in coconut (Kurup et al. 1993 ) .
Notwithstanding, most effi cient mechanism of UV-B evasion is the accumula-
tion of UV-B absorbing compounds in the epidermal cells of leaves (Burchard
et al. 2000 ) . Plants contain many phenolic compounds with anti-oxidant proper-
ties. Phenolics like light absorbing fl avonoid compounds have been implicated in
protecting plants from the damaging effects of UV-B radiation (Li et al. 1993 ).
Flavonoid compounds are a group of phenylalanine derived aromatic secondary
products, synthesized via phenylpropanoid pathway. Anthocyanins are the most
common fl avonoid compounds in higher plants. Temporary reddening of mature
leaves seen on exposure to UV-B radiation has been attributed to anthocyanin
accumulation (Steyn et al. 2002 ; Close and Beadle 2003 ; Gould 2004 ; Kytridis
and Manetas 2006 ) . Although not many studies have been reported on the effects
of UV-B radiation in plantation crops, still it is prudent to believe that they have
natural adaptation to this kind of stress. Leaves of tropical plantation crops like
cashew, rubber and cocoa appear copper brown when young, and often turn red-
dish in the event of stress due to the accumulation of fl avonoid pigments. Phenolic
accumulation under photooxidative stress is a common feature in tea (Hernández
et al. 2006 ) and coffee.
3.2.5 Nutrient Stress
Even in the presence of adequate levels of nutrients, crops might not reach optimal
growth and productivity when grown in soils that contain phytotoxic levels of some
metals / nutrients. Some of the micronutrients that can be potentially toxic at higher
concentrations are copper, cobalt, iron, molybdenum, nickel and zinc. Even essen-
tial elements at excess levels can reduce plant growth. Plants therefore experience
two types of stress in presence of soil elements, (i) defi ciency and (ii) toxicity.
18.104.22.168 Nutrient Defi ciency
Most of the plantation crops are adapted to nutrient defi cient situations of marginal
lands, a notable exception being coffee. Annual nutrient requirements of these crops
are lower than many of the annual fi eld crops. Plantation crops are large plants with
extensive root system, which capacitate them to forage for nutrients from a larger
soil volume. However, severe nutrient defi ciency can be deleterious to these crops.
Among the major nutrients, potassium is required for physiological development of
trees, while phosphorus is required for adequate development of root system, mer-
istem growth and wood formation. Magnesium is of particular importance for latex
production in rubber trees. Plants under nutrient defi cient stress exhibit characteris-
tic external symptoms of starvation.
Plantation crops like rubber, cocoa, oil palm (Fageria and Baligar 2008 ) , and
cardamom (Krishnakumar and Potty 2002 ) are suitable for acid soils. Low pH or
acidity favors complex interactions of plant growth-limiting factors involving phys-
ical, chemical and biological properties of soil. Calcium, magnesium and phospho-
rous defi ciencies are considered major nutrient constraints that limit plant growth in
acidic soils. Phosphorus limitation occurs due to fi xation by oxides of iron and alu-
minum found in abundance in acidic soils. Young plants may require addition of
fertilizers when grown in poor soils, but for established trees, there are many reports
of adequate growth and cropping without any additional fertilization. The absolute
phosphorus requirements of trees are small and defi ciency symptoms are seldom
found in fi elds. However, low availability of potassium can be serious and severe
defi ciency can induce compensatory uptake of other positively charged (especially
magnesium and calcium) ions. Therefore, potassium defi cient trees are therefore
often characterized by unusually high leaf magnesium concentrations. Among plan-
tation crops coffee, tea and cardamom are sensitive to low nutrient levels, resulting
in low yields, poor quality and predisposal to pathogen damage.
22.214.171.124 Nutrient Toxicity
Direct nutrient toxicity due to high availability of major nutrients is not common in
plantation crops. However, excess quantities of micronutrients and benefi cial ele-
ments can result in serious toxic stresses limiting crop yield and profi tability. Excess
levels of nitrogen, phosphorus and potassium may boost vegetative growth, favoring
pest and pathogen attacks, and the additional vegetative growth often occurs at the
expense of reproductive phase. Sulfur toxicity occurs when sulfur dioxide adsorbed
by leaves reacts with water to form bisulfate, inhibiting photosynthesis and degrad-
ing chlorophyll (Marschner 1995 ) . Excess application of copper fertilizers or cop-
per-based fungicides can result in copper toxicity, while high zinc fertilization
causes zinc toxicity. Boron toxicity was reported to cause serious leaf damage in
many crops. Aluminum toxicity is a common problem in acid soils. Tea plants show
aluminum tolerance and have high internal tolerance to aluminum which is directly
associated with aluminum accumulation (Rout et al. 2001 ) .
Pollution stress occurs in plants when pollutants such as toxic substances, gases,
particulate matter, acids and radioactive substances contaminate their primary-
growing environments (air, water and soil). Pollutants interfere with photosynthesis
and respiration, enzyme activity and metabolic processes and causes membrane
damage and cell death. Pollution stress is a growing problem in populated and
industrialized regions of the world today. Natural pollution does however occur, due
3 Stress in Plantation Crops: Adaptation and Management
to seismic and volcanic activities and acid rains. Pollution being not a regular natural
process and mostly of anthropogenic origin, natural adaptation to pollutants is sel-
dom found in plantation crops. However, being large plants they do tolerate moder-
ate and short period pollutions without much damaging effects. However, continuous
and heavy exposure may not be tolerated.
126.96.36.199 Air Pollution
Air pollution is a major cause of concern in plantation crops in the recent times.
Major air pollutants are carbon oxides, nitrogen oxides, sulfur oxides, ozone, fl uo-
rides, ammonia and particulate matter. Ozone, nitrogen oxides, sulfur dioxide and
peroxy acetyl nitrates (PANs) can cause direct damage to leaves when they enter
stomata. Chronic exposure of leaves to air pollutants can also break down waxy
layer that protects plants from excessive water loss, diseases, pests, drought and
frost (Miller 1990 ) .
Ozone is produced in the atmosphere during a complex reaction involving nitro-
gen oxides and reactive hydrocarbons in the presence of sunlight. Reactive hydro-
carbons are components of automobile exhausts, fossil fuel combustion and volatile
organic compounds emitted by biosphere. Ozone is a main component in the oxi-
dant smog complex (Thomas 1961 ) and is regarded as a serious air pollutant that
affects crop productivity, climate change, human and animal health (Royal Society
2008 ) . Many species of tropical rain forests and plantation crops like oil palm are
known to emit isoprene (2-methyl-1, 3-butadiene), a potential reactive volatile
organic compound. Emission of isoprene by oil palm is a biological phenomenon
that is believed to be under strong circadian control (Wilkinson et al. 2006 ) . In addi-
tion, oil palm plantations emit oxides of nitrogen from fertilized soils in signifi cant
quantities. Therefore biological emission of volatile organic compound and non-
biological emission of nitrogen oxides aid ground level ozone build-up in and
around oil palm plantations (Hewitt et al. 2009 ) . Regional ozone pollution (Pierce
et al. 1998 ) can cause vegetation injury in plantation surroundings due to oxidant
build-up in the air.
Other major air pollutants are sulfur dioxide and fl uorides. Major sources of
these pollutants are coal-burning operations, burning of petroleum and the smelting
of sulfur containing ores. Sulfur dioxide is converted to bisulfate, which is oxidized
to sulfuric acid by the reaction with water and form acid rain. Fluorides are dis-
solved readily in atmospheric moisture that turns acidic. Sulfur and fl uoride toxicity
occurs on leaves however, species, varietal and phenological level differences in
sensitivity are observed in plants (Griffi ths 2003 ). Nevertheless, scientifi c informa-
tion on sulfur dioxide or fl uoride toxicity in plantation crops is hardly available.
Other air pollutants like ammonia, chlorine, hydrogen chloride, hydrogen cya-
nide etc. does not contaminate atmosphere unless in the event of accidents involving
the storage, transportation or application of these materials. Release large quantities
of these compounds into the atmosphere for brief periods can cause severe injury to
vegetation in the immediate vicinity. In rubber plantations, gaseous ammonia is
used as an anticoagulant for long-term latex preservation, but no adverse effect of
exposure has been reported.
Particulate air pollutants such as cement dust, magnesium-lime dust and carbon
soot is deposited on plantations near such industries. Particle deposits on the leaves
inhibit normal respiration and photosynthesis. Cement dust can cause chlorosis,
necrosis and death of leaves by the combined effect of a thick crust and alkaline
toxicity produced in wet weather (Griffi ths 2003 ). Similar stress effects can also hap-
pen when volcanic ash is deposited on the leaves in the event of volcanic activity.
188.8.131.52 Water and Soil Pollution
Water pollution occurs when industrial and domestic effl uents are mixed with sur-
face and/or ground water or when the effl uents are used directly for irrigation.
Effl uents may contain a wide variety of toxic elements in varying concentrations.
Prolonged usage of contaminated water causes elements to accumulate in the soil to
levels toxic to plants. Common toxic elements are boron, chloride, copper, nickel,
zinc, mercury, arsenic or cadmium. In addition, agricultural soils gets polluted by
heavy metals by the soil addition of industrial and urban wastes, sewage sludge,
fertilizers and pesticides and products from burning of fossil fuels. Studies show
that toxic elements can incite almost all kinds of stress responses in plants, from
altered respiration and photosynthesis, oxidative damage, membrane damage,
impairment of enzyme activity and metabolism, anatomical and ultrastructural
changes, poor growth and quality (Setia et al. 2008 ) .
Limited studies are now available on natural adaptation of plantation crops to
water and soil pollution, especially for heavy metal contamination. Arsenic accu-
mulation in oil palm has been reported when grown in arsenic rich soils (Amonoo-
Neizer and Amekor 1993 ) apparently without any stress symptoms. No direct stress
was found in crops like coffee, rubber, coconut, eucalyptus and tea grown in soils
with high content of lead, chromium, cadmium and nickel, except for the presence
of these ions in elevated quantities (Abreu et al. 2005 ) . There are very few evidences
of heavy metal sequestration in plantation species so far. Metallothionein like genes
(Abdullah et al. 2002 ) and stress inducible metallothionein promotors have been
reported in oil palm, which may play a role in heavy metal detoxifi cation.
Metallothioneins are cysteine-rich low molecular weight proteins capable of seques-
tering metallic ions by binding (Omidvar et al. 2010 ) . Similarly, metallothionein
proteins have been isolated from rubber, which have been induced under oxidative
stress situations (Zhu et al. 2010 ) . Another low molecular weight protein family,
small cysteine rich ligands called phytochelatins, which are involved in accumula-
tion, detoxifi cation and metabolism of heavy metal ions in plants (Maiti et al. 2004 )
have been identifi ed in tea (Yadav and Mohanpuria 2009 ) and oil palm (Teoh et al.
2003 ) . Radical scavenging proteins such as dehydrins that binds metals using a
histidine-rich domain have been suggested to reduce metal toxicity in plant cells
under water-stressed conditions (Hara et al. 2005 ) .
3 Stress in Plantation Crops: Adaptation and Management
Flooding can be accumulation of water at the site of plant growth either by
submerging the plants, fully (inundation) or partially (waterlogging), or for a shorter
(fl ash fl ood) or longer (submergence) period. During waterlogging soil is saturated
and root zone is affected, while during inundation stems are affected in addition to
roots (Mullan and Barrett-Lennard 2010 ) . Generally, inundation is not a problem in
plantation species, but water logging can affect plantations grown at low-lying
areas. Depletion of oxygen from soil (hypoxia) results in reduction in aerobic soil
activities and anaerobic decomposition of organic matter is induced resulting in
accumulation of carbon dioxide. Although rubber tree is sensitive to waterlogging,
genotypic variations in adaptive as well as responsive behavior were recorded in
rubber seedlings subjected to fl ooding (Gomes and Kozlowski 1988 ) ranging from
downward curving of leaves (epinasty), reduced chlorophyll content, accelerated
ethylene production, inhibited growth, induced decay of roots and stimulated pro-
duction of lenticels and adventitious roots with large cells (hypertrophy). These
adventitious roots contain extensive aerenchymatous cells that would facilitate dif-
fusion of atmospheric oxygen to reduce root zone hypoxia, conferring fl ooding tol-
erance. Alternatively, these new roots may restore root to shoot communication and
export essential mineral ions and hormones to the shoot system. Therefore it was
suggested that, in rubber, fl ood adapted genotypes can be used as rootstocks in fl ood
affected areas. In cocoa, although trees can withstand fl ash fl oods of mild intensity,
they are particularly sensitive to water logging, but genotype variation in survival
rate has been observed (Bertolde et al. 2010 ) .
Fire occurrence in plantations is either anthropogenic or natural. Fire damage is a
common occurrence in rubber and oil palm plantations, when people set fi re to clear
the undergrowth or deliberately to settle disputes. Oil palm plantation fl oors of
South East Asia are infested with obnoxious weeds like cogon grass ( Imperata
cylindrica ), which are generally cleared by burning (Yassir et al. 2010 ) occasionally
causing fi re accidents in plantations. In rubber, during wintering, plantation fl oors
are carpeted by dry leaves that aid the fi res to rage through plantations. Oil palms
generally survive fi re damage (Friday et al. 1999 ; Hairiah et al. 2000 ) , but rubber
trees suffer severe damage, rendering affected bark unusable for latex extraction.
Wind and hailstorms are natural phenomena that can affect agricultural crops
adversely. Perhaps plantation crops are most affected by wind among agricultural
crops. Degree of damage depends on the force at which wind blows through the
plantations. Although storms and tornados can destroy entire plantation, mild to
severe winds can result in branch snaps, trunk snaps and uprooting. Steady winds
can give constant strain on the trees. In rubber plantations, when trees are exposed
to steady winds from one direction, trees develop a leaning appearance with trunk
bent along the direction of wind. Constant strain on the wood tissues leads to ten-
sion wood formation on the upper (windward) side (Savill 1983 ; Reghu et al. 1989 ) .
In spite of being susceptible to strong winds, rubber clones show a great degree of
variation in wind tolerance (Vinod et al. 1996b ; Priyadarshan et al. 1998 ) . Some
clones achieve wind fastness by the help of altered crown structure and branching
pattern (Cilas et al. 2004 ) that allow wind to pass through the plantation without
infl icting much damage. Palms are more adapted to wind than the trees. In some
palms as the root system develops, stem base enlarges giving additional stability to
withstand strong winds, storms and hurricanes (Tomlinson 1961 ) . When grown
under optimum environments, coconut palms develop enlarged base in tall as well
as in dwarf genotypes (Satyabalan 1997 ) .
Hails occurs commonly in tropics, at higher altitudes and latitudes with the onset
of summer. There are reports of hailstorm damage to plantation crops like rubber,
coffee, tea, cardamom and coconut. In northeast India where rubber is grown exten-
sively, hail shower is a normal annual occurrence during summer. Although mild hail
showers do not pose any problem to mature rubber trees, they are capable of infl ict-
ing damage in young plants. Notwithstanding, hailstorm of severe intensity can cause
severe damage to rubber trees, by shattering the bark by falling hailstones (Meenattoor
et al. 1995 ) . Trees that survive damage take a longer time to recoup and it is reported
that even after eight years affected bark has not returned to full anatomical features
required for commercial latex collection (Vinod and Thomas 2006 ) .
Lightning is a spectacular natural phenomenon, occurs commonly along with thun-
derstorms, volcanic eruptions and dust wind. A lightning is most likely to hit a tall
green tree, with an upward stroke of current in excess of 100,000 A (Ahrens 2007 )
passing through the tree, causing excessive internal heating, resulting in splitting up
of vascular tissues and cracking of trunk (Thomas et al. 1995 ) . Extensive observa-
tions of lightning damages are reported in crops like coconut and rubber. In rubber,
direct lightning strikes cause trees to wilt within few hours of the incident. In partial
strikes die-back of branches occur after few days, and in few cases, bark on one side
of the trunk is seen damaged on entire length of the tree. Direct lightning strike on
rubber trees are normally fatal, and those survive partial strikes remain unsuitable
for tapping (Steinmann 1925 ) . Direct fatality may occur only on one or more trees
however, many trees around are subsequently killed, from secondary infections of
pathogens and more conspicuously from wood boring ambrosia beetles ( Xyleborus
sp., Xylosandrus sp., Platypus sp). Wood of the affected trees generally shows dark
streaks of vascular burning. However, in coconut palms, lightning leaves no palm to
3 Stress in Plantation Crops: Adaptation and Management
survive. Depending on the degree of electrical discharge, the death may occur
instantly or after few days. In severe direct strikes, the treetop may be burnt, or
decapitated, but such occurrences are relatively rare (Ohler 1999 ) .
3.2.11 Soil Erosion
Soil erosion is a serious problem in plantation crops, since these crops are grown on
cleared lands, and are generally planted in wider spacing. Soil disturbances during
planting activity and removal of natural ground cover enhance steady depletion of
soil layers either by run-off, landslides and wind erosion. Erosion causes loss of
fertility that result in poor development of roots in addition to susceptibility to pest
and diseases. Poor root anchorage affects absorption of soil water and nutrients and
increases susceptibility to wind damage. It is reported that eroded soils under coffee
were more acidic than non-eroded soils, with considerable loss of fertility (Hartemink
2006 ) . Plantation crops like rubber grown in sloppy areas are more susceptible to
run off and landslides during rainy seasons. In mature oil palm plantations, soil ero-
sion depends chiefl y on slope of the site and soil management practices. Intercropping
with soil disturbing crops increases the chance of erosion, while those require mini-
mal soil disturbance may help to prevent erosion. In young oil palm and rubber tree,
the practice of growing cover crops helps to limit erosion. As the trees mature, cover
crop disappears due to canopy or crown closure, resulting in increase of run off and
erosion (Chew et al. 1999 ) . Furthermore, the risk of soil erosion is more in surface
feeders like rubber and oil palm wherein top soil erosion exposes most of the feed-
ing roots that subsequently dry up and die, resulting in poor uptake of water and
nutrients (Ferwerda 1977 ; Howell et al. 2005 ) . Plant stress attributable to soil ero-
sion can be seen in coffee plantation without adequate shade, having low planting
density and grown on slopes. Erosion occurs in tea plantations after pruning.
3.2.12 Climate Change
Climate change is a matter of concern of twenty-fi rst century that warns of rising
temperature, unprecedented drought, fl ood, desertifi cation, radiation, cyclones, for-
est fi res, extreme low temperature that can adversely affect agriculture and human
life. Predictions for the fi rst three decades indicate that developing nations will be
affected most because of following three reasons. Firstly, most of the developing
nations are in tropical and sub-tropical regions where the negative impacts of cli-
mate change are expected to be more. Secondly, a large proportion of the global
population growth will be happening in the developing world, and the third, more
than half of the population in developing nations is dependent on agriculture
(Reynolds and Ortiz 2010 ) . This is of particular concern to plantation crops, because
most of them are cultivated in the tropics. Being perennial crops, adverse effect on
these crops will have long-term repercussions - the production decline and
commodity price escalation.
Counting on natural adaptations to climate change in plantation crops is a diffi -
cult task. Being perennials, long-term survival can be the best index of adaptation.
Fortunately, there are no reports of serious infl uence of climate change on plantation
crops so far. However, one recent report from Ethiopia suggests of a shift in crop-
ping pattern attributable to climate change, which reports of conversion some mar-
ginal coffee areas for cultivation of khat ( Catha edulis ), a hardy drought resistant
plant, due to prolonged dry periods (Labouisse et al. 2008 ) . A comprehensive review
on current knowledge and future perspectives on diseases in tropical and plantation
crops as affected by climate changes can be found in Ghini et al. ( 2011 ) .
3.3 Adaptation to Biotic Stresses
Like abiotic stresses, most of the biotic stresses are seasonal in nature. Therefore,
perennial crops are subjected to all kinds of biotic stress irrespective of the seasons.
These crops undergo in situ natural selection against recurring factors, years
after, and become capable of buffering the damage and only the fi ttest is carried for-
ward to future generations. In practice, most of the biotic factors co-exist in cropping
environments in this fashion, but the limit of buffering become apparent when varie-
ties are tested in different environments or when a ‘new’ disease or pest arrives.
3.3.1 Defense Against Pathogenesis
Pathogenesis is associated with onset of favorable weather conditions, and therefore
is limited to certain locations and seasons. Plants have active surveillance mechanisms
either constitutive or induced against pathogens that are genetically programmed into
every species (Bittel and Robatzek 2007 ) . Basal resistance or innate immunity is the
primary level of plant adaptation against diseases, triggered when microbe-associated
molecular patterns (MAMPs) are recognized by pattern recognition receptors (PRRs)
of host plant cells. MAMPs include specifi c proteins, lipopolysaccharides and cell
wall components of pathogens. Activation of PRRs leads to active defense responses,
both in basal and non-host resistance (Jones and Dangl 2006 ) .
184.108.40.206 Constitutive Defense
Constitutive defense in planation crops are manifested through morphological, phys-
iological and biochemical adaptations involving cuticular modifi cations, accumula-
tion of wax and other organic compounds, production of specialized secretions and
stomatal, bark or lenticel adaptations. Most of the morphological modifi cations also
provide structural rigidity and strength to the plants (Freeman and Beattie 2008 ).
3 Stress in Plantation Crops: Adaptation and Management
Cuticle acts as the primary physical barrier of plant’s defense against pathogens.
Recent evidences indicate cuticular modifi cations that confer disease resistance by
release of fungitoxic substances as well as changes in gene expression that form a
multifactorial defense response (Chassot et al. 2008 ) . In plantation crops, cuticular
defense against Colletotrichum kahawae , the coffee berry disease (CBD) pathogen
has been known for a long (Nutman and Roberts 1960 ) in resistant varieties.
Cuticular extracts (Nutman and Roberts 1960 ) , wax extracts (Steiner 1972 ; Lampard
and Carter 1973 ) and hypocotyl extracts (Masaba and Helderman 1985 ) of resistant
varieties have shown clear suppression in conidial germination and mycelial growth
of C. kahawae , however the exact components responsible for resistance have not
been identifi ed yet.
Waxes form a part of the preformed plant defense system against biotic stresses
such as fungi, bacteria and insects (Gülz et al. 1991 ; Yoon et al. 1998 ; Marcell and
Beattie 2002 ) . Epicuticular wax of coconut predominantly contains lupeol methyl
ether, isoskimmiwallin and skimmiwallin (Escalante et al. 2002 ) and other com-
pounds such as lupeol, 3- b -methoxy lupane (lupane methyl ether) and acetates of
lupeol, skimmiwallinol and isoskimmiwallinol. Epicuticular wax constitute 95% of
the total wax content in coconut, while the remaining 5% include triterpenoids,
sterols, primary alcohols, fatty acids and unidentifi ed compounds (Riedel et al.
2009 ) . However, signifi cant genotype variation in epicuticular wax content is found
in coconut as well as in rubber (Rao et al. 1998 ).
Majority of the phytochemicals belongs to three major classes of terpenoids, alka-
loids and phenolics, and all of them are secondary metabolites. The basic terpene is
isoprene, and depending on the number of isoprene units present in the molecules,
terpenoids are classifi ed as mono-, sesqui-, di-, tri-, tetra- and polyterpenes. One of
the most versatile and abstruse compound produced by the rubber tree, the elas-
tomer called rubber is a polyterpene known as cis -poly isoprene. Why rubber tree
and many other species produce this compound expending so much of energy
remains a mystery. Further, rubber is said to be at the metabolic dead-end since no
enzymes are capable of breaking it down (Bonner and Galston 1947 ) . Rubber parti-
cles are produced in latex, the cytoplasm of a specialized anastomosing network of
laticifer cells. Notwithstanding, there seems to be a general agreement on the role of
latex in plant defense against diseases and pests (Dussourd and Eisner 1987 ; Farrell
et al. 1991 ). In support to this, antifungal activity of latex has been demonstrated in
vitro (Moulin-Traffort et al. 1990 ; Giordani et al. 1999 ) .
Alkaloids are nitrogenous compounds, derived from amino acids tryptophan,
tyrosine, lysine and aspartates, possessing antibiotic and antifungal properties.
Caffeine is a predominant alkaloid found in coffee, tea and cocoa. Highly toxic to
fungi and insects, caffeine can even cause allelopathy in other plant species (Freeman
and Beattie 2008 ). The phenolics are another class of defense-related secondary
metabolites commonly found in plantation crops that are produced through shikimic
acid - phenylpropanoid pathways. Shikimate pathway converts carbohydrate pre-
cursors into phenylalanine and tyrosine. Phenylalanine is the precursor molecule for
the phenylpropanoid pathway. The phenylpropanoids commonly found in plants are
anthocyanins, fl avonoids, furanocoumarins, lignins and tannins (Saltveit 2010 ) .
Passive polypeptide defense is achieved in plants through preformed proteins and
enzymes. They include defensins, protease inhibitors, amylase inhibitors and lectins.
Defensins are cysteine-rich cationic proteins showing antipathogenic activity that
are believed to form multimeric pores in the plasma membrane of pathogens leading
to membrane disruption and depolarization by affecting ion-channels resulting in
cellular ion imbalance (Thomma et al. 2002 ; Ganz 2003 ) . Plant defensins are of
three types, hevein type, thionin type and knotting type of which hevein and thionin
types have eight cysteine residues while knotting type has six. Further, hevein and
knotting type have continuous sequence of cysteines at the center while thionone
type has cysteine sequences at the N-terminal end (Fujimura et al. 2005 ) . Among
the defensins, thionines show high affi nity to chitin, the major cell wall component
of pathogenic fungi (Oita et al. 2000 ) .
The rubber tree latex contains hevein, a single-chain serine protease that has
been described as an antifungal protein (Koo et al. 1998 , Ko et al. 2003 ) . It is an
acidic protein with a chitin-binding domain belonging to lectin super family and is
encoded by a multigene family consisting of at least fi ve members (Pujade-Renaud
et al. 2005 ) . 50-70% of the total soluble proteins in rubber tree latex are composed
of hevein (Van Parijs et al. 1991 ) . Hevein contains 18% cysteine in its amino acid
sequence (Lucas et al. 1985 ) and are found in two forms viz., prohevein (Hev b
6.01) and hevein (Hev b 6.02). Prohevien is actually the hevein preprotein that
matures to hevein. Although hevein promoter is constitutively expressed in rubber
(Montoro et al 2008 ) , induced production also has been reported during wound
related stress (Broekaert et al. 1990 ) . Therefore, hevein is believed to play an impor-
tant role in plant defense (d’Auzac et al. 1995 ) . Besides heveins, rubber latex con-
tain many proteins that belong to the class of pathogenesis related (PR) proteins.
They are Hev b 2 (endoglucanase), Hev b 11 ( Hevea endochitinase), Hev b 12 Hevea
Lipid transfer protein (LTP) and hevamine (Hoffmann-Sommergruber 2002 ) . Unlike
that of all known class I chitinases, which are vacuolar proteins Hev b 11 is identi-
fi ed to be a cytosolic (C-serum) protein (Beintema 2007 ) . Hevamine is another chi-
tinase from rubber tree belonging to the family 18-glycosyl hydrolases. This enzyme
has a unique property of cleaving the chitin molecule, and the sugar moiety of pep-
tidoglycan (Bokma et al. 1997 ) . Hevamine was the fi rst enzyme found with chitin
cleavage specifi city (Bokma et al. 2000 ) . Apart from rubber tree, expression and
characterization of a putative defensin (EGAD1) in oil palm infl orescences has also
been reported (Tregear et al. 2002 ) .
3 Stress in Plantation Crops: Adaptation and Management
220.127.116.11 Inducible Defense
Successful pathogens overcome primary defense systems and establish contact with
the host. From this point onwards, inducible defense decides how plants survive the
attack. Induction of defense does not occur until imminence, perhaps to conserve
high energy and nutrient required in production and maintenance of defense
Structural fortifi cation
Pathogen induced structural reinforcement of cell wall has been implicated in
defense, accomplished through accumulation of glucan polymers such as
(1-3)- b -glucan callose (Aist 1976 ) , lignins, suberins (Guest and Brown 1997 ) or
through cross linking of cell wall proteins (Bradley et al. 1992 ) . Induction of callose
and other defense related transcripts has been observed in coconut calli as a suitable
alternative to characterize both biochemical and molecular interactions that occur
between the coconut and its biotic stress factors (Uc et al. 2007 ) . Callose rich haus-
torial encasements are induced in resistant coffee genotypes in incompatible reac-
tion with leaf rust pathogen, Hemileia vastatrix . This encasement is less permeable
thus preventing the fl ow of nutrients to the fungal pegs, thereby arresting the patho-
gen entry (Silva et al. 2006 ) .
Root rot infection by Rigidoporus lignosus and Phellinus noxius in rubber tree
starts with an increase of cell layers below the point of penetration resulting from a
stimulated activity of cork cambium. Cell walls thicken concomitantly due to the
accumulation of suberin or lignin. Callose formation occurs in young cork cells as
well as in the phloem, obstructing pores of sieve tubes. Heterogeneous lignifi cation
takes place in xylem vessels. Tyloses resembling cell islands are formed in the
young phloem in order to slow down the progression of invading hyphae (Nicole
et al. 1985 ) .
Rapid cork formation below the site of infection has been attributed for the resis-
tant reaction in coffee to berry disease pathogen, C. kahawae (Masaba and van der
Vossen 1982 ). Phellogen is formed in few cell layers rapidly, and the fungal inva-
sion is arrested by development of suberized cells (Gichuru 1997 ) .
Many phenolic compounds are induced following pathogenesis. Phytoalexin accu-
mulation in coffee genotypes resistant to Hemileia vastatrix and Pseudomonas
syringae has been implicated in resistance reaction (Rodrigues et al. 1975 ; Guedes
et al. 1994 ) .
A fl uorescent blue phytoalexin identifi ed as scopoletin (7-hydroxy-6-methoxy
coumarin; Giesemann et al. 1986 ) is produced in rubber trees during resistant reac-
tion with Colletotrichum gloeosporioides (Tan and Low 1975 ), Microcyclus ulei
(Garcia et al. 1995 ) and Phytophthora palmivora (Churngchow and Rattarasarn
2001 ) . MAMPs from P. palmivora induced production of scopoletin, o -dianisidine
(a peroxidase) and accumulation of phenolic compounds in rubber seedlings and in
cell suspensions under resistance reaction (Dutsadee and Nunta 2008 ) . Scopolectin
was also isolated from leaves of mature rubber trees uninfected with Corynespora
cassiicola and was found at elevated levels in young leaves post infection. Further,
it was demonstrated that scopoletin could fully suppress conidial germination of
many rubber tree pathogens viz., Corynespora cassiicola , Cylindrocladium quin-
queseptatum , Phytophthora spp., Colletotrichum acutatum and Helminthosporium
heveae (Churngchow and Rattarasarn 2001 ; Silva et al. 2001 ) . On the contrary,
cyanogenesis, the liberation of high amounts of hydrocyanic acid (HCN), is observed
from leaves of susceptible in rubber clones, following infections with M. ulei while
resistant clones released only very little HCN. This indicated that cyanogenesis per
se does not lead to defense against fungal pathogen but impair resistance reaction
(Lieberei 1986 ) .
Lignins are polymers of phenolic monomers commonly found in cell wall,
especially in the secondary cell wall. In woody plants, cell wall lignifi cation
takes place as cells mature. Lignifi cation provides rigidity to the cell, thereby
imparting structural strength to the woody stem. Furthermore, lignins act as ver-
satile defense compounds during pathogen attack. High level of lignin accumula-
tion is a common phenomenon in the renewed bark of the rubber tree, following
tapping and other wound induced reactions (Thomas et al. 1995 ) . Gum exuda-
tions containing phenolic components occur in coconut and cashew following
wounding possibly as a defense against imminent pathogen attack. Apart from
lignins, plantation crops such as tea contain water-soluble polymers of fl avonoids
known as tannins. Tannins are commonly found stored in vacuoles and are toxic
to pests rather than pathogens.
Inducible polypeptide defense
Evidences suggest that stress signaling in plants is modulated by multiple hormone-
signaling pathways of jasmonic acid, salicylic acid, ethylene and ROS (Fujita et al.
2006 ) under the regulatory control of transcription factors (TFs) and cis -acting
elements. TFs are proteins that regulate gene expression by recognizing and binding
to the cis -elements in the promoter regions of the upstream target genes. Major plant
TFs involved in biotic defense belong to fi ve major gene families. Among these,
ethylene responsive element binding factors (ERF) zinc fi nger proteins are involved
in ROS signaling, while MYB (myeloblastosis) and MYC (myelocytomatosis) pro-
teins are involved in jasmonic acid signaling. Further, WRKY (tryptophan, W;
arginine, R; lysine, K; tyrosine, Y) TFs with zinc fi nger motifs are active in salicylic
acid signaling, whereas, NAC (no apical meristem, NAM; Arabidopsis transcription
activation factor, ATAF; cup-shaped cotyledon, CUC) factors activate jasmonic acid
signaling and basic-domain leucine-zipper (bZIP) proteins. However, regulatory
activity of TFs depends on plant specifi c adaptation to stress as well as on the types
3 Stress in Plantation Crops: Adaptation and Management
of stresses plants may experience. Recent developments in functional genomics
have revealed presence of these gene families in some of the plantation crops. In
coffee, most of the isolated genes showed homology to known plant genes suggest-
ing cross-genome conservation of signaling pathways and pathogen resistance
mechanisms (Silva et al. 2006 ) .
The WRKY TFs are now known to play key regulatory functions in plant defense
against salinity, mechanical damage, drought, cold, pest and diseases as well as in
development of seeds, trichome and in the biosynthesis of secondary metabolites.
Gene expression studies in C. arabica plants challenged with the rust fungus
( Hemileia vastatrix ), the root-knot nematode ( Meloidogyne exigua ) and abiotic
treatments showed concomitant expression of CaWRKY1a and CaWRKY1b with
same altered patterns in both biotic and abiotic treatments (Petitot et al. 2008 ) .
WRKY gene based markers have been found useful in delineating genetic diversity
in coconut germplasm, indicating genome wide ubiquity and variation of these
genes (Mauro-Herrera et al. 2007 ) . In cocoa, TcWRKY markers have been devel-
oped for genome scanning for stress resistant genes (Borrone et al. 2004 ) . Recently,
a set of fi ve WRKY TF members have been described in coffee as possible regula-
tors of defense responses (Ramiro et al. 2010 ) .
The rubber tree latex accumulates high levels of induced plant defense gene tran-
scripts and hydrolytic enzymes such as cellulases, polygalacturanses (Kush et al.
1990 ) , b -1,3 glucanase (Chye and Cheung 1995 ) and peroxidases (Dutsadee and
Nunta 2008 ) suggesting a functional role for the latex in defense (Dussourd and
Eisner 1987 ; Farrell et al. 1991 ). A detailed transcriptome profi ling (Ko et al. 2003 )
reveals latex abundance of a gene, HbPI1 ( H. brasiliensis protease inhibitor 1) that
share conserved domain with potato inhibitor 1 family and share high level sequence
homologies with serine proteinase inhibitor, protease inhibitor and PR protein.
Other putative plant defense genes highly expressed in latex included LTI6B (low
temperature Induced 6B - a low temperature and salt responsive protein, Blt101
(barley low temperature induced protein), and RTM1 (restricted TEV movement -
disease resistance protein.
Increased activities of phenylalanine ammonialyase (PAL), NADPH oxidases and
peroxidases were reported in coffee (Silva et al. 2008 ) during incompatible (resis-
tant) reaction with H. vastatrix . Presence of PAL, the key enzyme in the phenylpro-
panoid pathway indicates phenolic production, while rise in lipoxygenase (LOX)
activity (Rojas et al. 1993 ) indicate peroxide synthesis. However, LOX activity
remained constant in a compatible interaction leading to disease. LOX are believed
to provide hydroperoxide substrates that can be metabolized to compounds that play
important roles in plant defense (Baysal and Demirdöven 2007 ) . Peroxidase produc-
tion is also reported in defense reaction against C. kahawae (Gichuru et al. 1997 ) .
Pathogenesis occurs when successful pathogens break down induced defense sys-
tems of the plant and inject a range of effector molecules that suppress host
responses at different defense levels of perception, signaling and suppression.
Notwithstanding, plants have evolved a sophisticated immune system to detect
these effectors using cognate disease resistance proteins, a recognition that is
highly specifi c and often elicits rapid and localized cell death (Xing et al. 2007 ) .
The hypersensitive response (abbreviated as HyR to distinguish from horizontal
resistance, HR) is a deliberate sacrifi ce of cells around the site of infection, locally
confi ning the pathogen and deprives it of water and nutrients to stop it from further
advancing. HyR is, therefore, a type of programmed cell death (PCD) akin to apop-
tosis in mammalian cells (Heath 1998 ) . The most common features of HyR are the
autofl uorescence of dead cells under UV exposure and subsequent browning of
cells caused by oxidized phenolic accumulation. Although ion fl uxes and genera-
tion of ROS commonly precede cell death, direct involvement of ROS may vary
with the plant-pathogen combination (Park 2005 ) . Oxidative burst also occur in
many instances, but it is not necessarily a requirement for PCD (Groover et al.
1997 ). HyR is an effector specifi c reaction and therefore is more pathogen specifi c.
The effector recognition follows signal transduction for the production of specifi c
resistant (R) gene products or R proteins that induce HyR. On successful confi ne-
ment of pathogens, plants set off effector-triggered immunity (ETI) that produces
visible symptoms of HyR. ETI follows classical gene-for-gene theory and a strong
ETI is responsible for an incompatible interaction or specifi c resistance (Göhre and
Robatzek 2008 ) .
Reactions similar to HyR, in which the cells around the penetration hyphae col-
lapse, have been reported from rubber clones that are resistant to Microcyclus ulei
infection. Deposition of autofl uorescent compounds and callose, activation of ROS,
scopoletin accumulation and cell death are found to occur leaving a limited necrotic
patch surrounding the penetrating hyphae (Garcia et al. 1999 ; Lieberei 2007 ).
Responses similar to HyR were also reported from rubber roots infected with root
rot pathogen, Phellinus noxius (Nicole et al. 1992 ) .
Naturally rust resistant coffee varieties too show HyR, (Lam et al. 2001 ; Morel
and Dangl 1997 ) manifested by death of subsidiary and guard cells of stomata
around the site of fungal penetration (Martins and Moraes 1996 : Silva et al. 2002 ) .
Systemic acquired resistance
The activation of ETI enables host tissues to combat a broad range of pathogens
for an extended period, known as systemic acquired resistance (SAR) or ‘whole-
plant’ resistance (Hunt and Ryals 1996 ; Neuenschwander et al. 1996 ; Ryals
et al. 1996 ) . The chemical activation of SAR requires accumulation of endog-
enous salicylic acid (SA) that induce a wide range of genes including PR genes
(Durrant and Dong 2004 ; Zhang et al. 2010a ) leading to production of chiti-
nases, b -(1, 3)-glucanases, lysozyme, permatins and PR proteins (Ryals et al.
1996 ; Schneider et al. 1996 ) .
There are many reports suggesting the presence of SAR in plantation crops, but
conclusive evidences are still at large. If direct measurement of enzyme activities
3 Stress in Plantation Crops: Adaptation and Management
(Heil 1999 ) is to be taken as SAR indicator, rubber tree latex has prominent chitinase
activity (Van Parijs et al. 1991 ) and presence of b -(1,3)-glucanases and many PR
proteins (Hoffmann-Sommergruber 2002 ) . Recently, many artifi cial elicitors such
as benzothiadiazoles and harpin protein are being introduced to mimic host- pathogen
interaction leading to SAR (Gorlach et al. 1996 ; de Capdeville et al. 2001 ) . Chemical
elicitors such as salicylic acid (Mayers et al. 2005 ) , jasmonic acid (Cohen et al.
1993 ) , DL- b -aminobutyric acid (BABA) (Hong et al. 1999 ) , oxalic acid
(Mucharromah and Kuć 1991 ) and acibenzolar-S-methyl benzo-(1,2,3)-thiadiazole-
7-carboxylic acid S-methyl ester (ASM) (Gullino et al. 2000 ) have been success-
fully used as SAR elicitors. Signifi cant increase in PAL, peroxidase and
b -1,3-glucanase activities on elicitor treated tea leaves challenged with pathogen
and 25% decrease in disease intensity have been reported under fi eld conditions.
They are preferred because of relatively lesser risk they pose to non-target organ-
isms and environment than toxic chemical fungicides.
Induced systemic resistance
Tree endophytes are silent colonizers of healthy internal plant tissues (Sieber 2007 ) .
Although some of the colonizers are potentially pathogenic, during symbiotic exist-
ence, disease is not generally incited, perhaps due to the endosymbiotic suppression
of disease induction factors (Giordano et al. 2009 ; Botella et al. 2010 ) . However,
endosymbionts stimulate resistance reaction in host plants similar to SAR known as
induced systemic resistance (ISR). ISR is therefore just another type of adaptation
among crops plants wherein symbiotic relations between the host and colonizing
species has been exploited, as against antagonistic relations as in SAR.
Notwithstanding, ISR and SAR are believed to be in crosstalk in host plants.
Observations during the establishment of successful colonization between the host
(cocoa) and the colonizer ( Trichoderma ) (Bailey et al. 2006 ) indicate high induction
of fungal genes such as glucosyl hydrolase family, serine protease and alcohol oxi-
dase. This suggests of host-colonizer genetic crosstalk that results in endophyte-
stimulated production of many secondary metabolites, including those involved in
induced defense in host (Ryan et al. 2008 ; Gao et al. 2010 ) .
Of different type of resistant mechanisms, ISR has been the least known in plan-
tation crops. The fi rst report of ISR in a woody species was made in citrus, against
Phoma trachetiphila that causes mal secco disease (Solel et al. 1995 ) . Among endo-
phytic fungi, many species of Trichoderma are prominent colonizers in cocoa and
in few other plantation crops (Holmes et al. 2004 ) . Rubini et al. ( 2005 ) demon-
strated that Gliocladium catenulatum , an endophytic fungus, could reduce the inci-
dence of witches’ broom disease in cocoa seedlings up to 70% accentuating their
usefulness against fungal pathogens as biological control agents (Mejía et al. 2008 ) .
Comprehensive reviews on endophytic fungal diversity and use as biocontrol agents
in cocoa and coffee are available (Mejía et al. 2008 ; Vega et al. 2009 ) .
Use of two endophytic bacteria ( Burkholderia cepacia and Pseudomonas aerug-
inosa ) have been reported from oil palm, which could reduce the incidence of basal
stem rot disease caused by Ganoderma boninense to a tune of 76% (Sapak et al.
2008 ) . Plant growth promoting rhizobacterial (PGPR) bioformulations of
Pseudomonas fl uorescens was successfully used in tea to induce ISR to counter
blister blight pathogen under fi eld conditions with treated plants developing high
accumulation of PAL, peroxidases and polyphenol oxidases (Saravanakumar et al.
2007 ). Further, in vitro tea plantlets when treated with two native rhizosphere bacte-
rial isolates of Azospirillum brasilense and Trichoderma harzianum on hardening
process, did not develop root rot and wilt diseases commonly found to infect in vitro
seedlings during hardening process. These inoculated seedlings were found to have
higher activities of defense enzymes like PAL, peroxidase and b -1,3-glucanase
(Thomas et al. 2010 ) .
In rubber trees, induction of systemic tolerance was demonstrated through lesser
incidence of Corynespora cassiicola infection when endophytic bacteria isolated
from different organs were inoculated artifi cially (Philip et al 2005 ) . Rocha et al.
( 2011 ) have isolated and characterized endophytic fungi from cultivated Hevea
brasiliensis , and found that these endophytic fungi exhibit marked inhibitory activ-
ity on M. ulei conidia germination in vitro . Moreover, arbuscular mycorrhizal fungi
(AMF) were also found to improve plant growth and survival during greenhouse
acclimatization of rubber seedlings (Rodríguez et al. 2009 ). AMF are naturally
occurring group of soil fungi that exhibit symbiotic relationships with the roots of
most vascular plant species (Vestberg et al. 2002 ) . Further, AMF are suspected to
induce systemic resistance, apart from improving plant growth by increasing nutrient
uptake, especially phosphorus (Elsen et al. 2008 ) .
Recent evidences show that plants are capable of identifying and degrading patho-
genic viral RNA by sophisticated mechanism of post-transcriptional gene silencing
(PTGS) called RNA silencing, a process similar to RNA interference (RNAi) in
animals. Plants contain small double-stranded RNA of 20 to 26 nucleotides in
length, named short interfering RNA (siRNA) and endogenous small non-coding
single stranded micro RNAs (miRNA) of an average 22 nucleotides. These classes
of RNAs are related to several RNA biological processes such as the defense against
viral invaders, developmental control, cell signaling, transposon silencing and het-
erochromatin formation (Vrettos and Tournier 2007 ) . During viral infection, siR-
NAs bind to a complementary viral RNA and induce its degradation, while miRNAs
are capable of inhibiting messenger RNA (mRNA). On degradation, plants may
retain the digested viral template so that in case future infections they will be able
to initiate quick action (Carbonell et al. 2008 ) .
Successful isolation of two siRNAs from coconut leaves infected with coconut
cadang-cadang viroid (CCCVd) was reported (Vadamalai 2005 ) . Computational
prediction of several miRNA from oil palm expressed sequence tag (EST) sequences
has also been reported (Nasaruddin et al. 2007 ) .
3 Stress in Plantation Crops: Adaptation and Management
3.3.2 Defense to Herbivory
Plantation species in general have well-organized adaptation systems against the
most common herbivores, the insect pests. These crops have inbuilt fortifi cations to
ward off most of the insect feeders, and a few very specifi c pests can survive on
them. Some crops like rubber have no serious pest at all. Pest damage does not pose
serious threat to these crops and damages seldom surpass economic threshold.
However, few sucking pests in cardamom, cashew and tea, borers and spider mites
in coffee and cocoa can sometimes become exceptionally serious. In overall, pest
resistance is of marginal importance in commercial plantation crops.
Similar to that of pathogen defense, plant mechanisms for insect resistance can
be either constitutive or inducible. Among the three mechanisms, antixenosis and
antibiosis are prominent in plantation crops, while the requirement of tolerance is
relatively low. Plantation crops are rich in alkaloids, volatile monoterpenes, polyter-
pene compounds, tannins, lignins, cyanogenic compounds, other phenolic com-
pounds and allergens, through which defense against pests are arbitrated. These
biomolecules, otherwise known as allelochemicals induce a variety of reactions in
pests such as repulsion, toxicity, allergy and indigestion, jeopardizing their normal
metabolism, reproduction and survival. For instance, leaf minor ( Leucoptera cof-
feella ) resistance in coffee species viz., Coffea racemosa, C. setenophylla and
C. kapakata seems to be of antibiosis type wherein phytochemicals interfere with
normal growth of the insects. While in C. canephora , the resistance is exhibited
through antixenosis (Aviles et al. 1983 ) . It has been reported that C. arabica leaves
has larger concentrations of p-cymene, a monoterpene derivative, and lower levels
of beta cymene that greatly enhanced oviposition preference to the leaf miner, which
was not the case in C. canephora and C. racemosa leaves (Filho 2006 ) . Maintaining
higher levels of phenolics in the face of attack was found to be one of the strategies
for the observed tolerance of certain tea varieties to insect attack especially to
Helopeltis (Chakraborty and Chakraborty 2005 ) . This feature is also observed in
rubber tree making it a less preferred host by most of the insects.
Herbivore induced defense gene activation takes place throughout the plants
within hours after suffering the injury. Systemins, a group of plant peptide signal
molecules comprising of short 18 amino acid long peptides (Pearce et al. 1991 ) are
known to play a major role in systemic wound signaling (Ryan and Pearce 2003 ) .
Similar to systemins, short hydroxyproline rich systemin glycopeptides (HypSys)
processed from longer precursor molecules have also been discovered in plants
actively involved in defense reaction. HypSys activates methyl jasmonate, amplify-
ing defense response. Presence of HypSys ( Cc HypSys I, II and II) genes are reported
in robusta coffee (Pearce et al. 2008 ) .
Moreover, rubber tree belongs to euphorbiaceae, a large plant family in which
cyanogenesis is a common occurrence. Cyanogenesis is the ability to synthesize
cyanogenic glycosides, which when enzymatically hydrolyzed by b -glucosidase
(Poulton 1990 ; Francisco and Pinotti 2000 ) produces sugars and a cyanohydrin
compound that spontaneously decomposes to HCN and a ketone or aldehyde.
This step is catalyzed by hydroxynitrile lyase enzyme. Both the enzymes, b -glu-
cosidase and hydroxynitrile lyase are common in cyanogenic plants (Harborne
1993 ; Gruhnert et al. 1994 ) . The cyanogenic glycosides are usually sequestered into
vacuoles, while enzymes are present entirely in the mesophyll tissues (Poulton
1990 ) . On injury, cell disruptions bring both glucoside precursor and the enzyme in
contact resulting in the release of HCN (Gruhnert et al. 1994 ) . Rubber leaves con-
tain a broad substrate specifi c b -glucosidase, linemarase, and cyanogenic gluco-
sides, linamarin and lotaustralin (Lieberei et al. 1985 ) . Since the cyanogenesis is an
obligate feature in rubber trees, all living tissues, including that in seeds are strongly
cyanogenic and contain both accumulated cyanogenic precursor and respective
b -glucosidase. In seeds, endosperm alone contains more than 90% of linamarin
(Poulton 1990 ) . Montoro et al. ( 2008 ) found that cyanogenesis provides very good
defense against herbivores in rubber.
Furthermore, rubber tree is rich in compounds that are potential allergens to her-
bivores. Rubber latex contain chitinases (Type I and III), b -(1,3)-glucanases and
lipid transfer proteins, which are both allergenic and pathogenesis related (Hoffmann-
Sommergruber 2002 ) , besides many other Hev b allergenic proteins. Thirteen types
of allergenic proteins found in latex include rubber elongation factor, small rubber
particle proteins, hevein, prohevein, acid transfer proteins, endochitinases and
superoxide dismutase (Yeang 2004 ) .
Mechanical defense against herbivory is also found in plantation species, espe-
cially in palms. Coconut and other palms protect nuts by providing multiple layers
of thick and strong husk. The chemical makeup and abundance of external protec-
tives like waxes keeps pests away from preferring these crops for colonizing
(Kolattukudy 1987 ) . Structural adaptations for pest resistance are observed in few
crops. Coconut palms with small nuts are more susceptible to eriophyid mites, than
palms with larger nuts. Studies by Mariau ( 1977 ) revealed that in smaller nuts peri-
anth is less fi rmly attached to nut enabling mites to access interspace between nut
and lower perianth lobes for colonization, whereas in larger nuts this gap is mostly
impenetrable. Further, it was observed that on infestation, some palms with smaller
nuts do resist the damage by increasing the perianth-nut gap that makes the mites
uncomfortable to settle, while exposing them to predators (Aratchige et al. 2007 ) .
Allelopathy is a natural adaptation to defend plant-plant competition, which causes
diffi culties in replanting orchards and vineyards and causes poor establishment,
stunted growth and even complete mortality in the subsequent plantings. Autotoxicity
induced by caffeine has been reported in coffee plants (Waller et al. 1989 ) and by
other alkaloids from coffee such as theobromine, theophylline, paraxanthine and
scopolectin (Peneva 2007 ) . Allelopathic effects have also been reported in Eucalyptus
(Sasikumar et al 2001 ; Liu et al. 2008 ) .
3 Stress in Plantation Crops: Adaptation and Management
3.3.4 Physiological Disorders
Sustained physiological stress due to factors other than pathogens and pests brings
out visible symptoms in plantation crops. Although the term physiological disorder
is very broad and can include symptoms of abiotic stresses, in a narrow perspective
it is used to denote disorders of unknown etiology. Physiological disorders due to
known factors can be revert back to normal by stress alleviation, while unknown
factors can bring more severe permanent disturbances. Tapping panel dryness (TPD)
of rubber is one of the widely known physiological disorders among plantation
crops. Hot and cold disease of coffee is a physiological disorder caused in higher
elevations due excessive cooling followed by heating. Other disorders of unknown
etiology in coconut are pencil point disease, bristle top, dry bud rot, fi nschafen dis-
ease, frond rot, leaf scorch decline and Malaysia wilt.
18.104.22.168 Tapping Panel Dryness
The TPD, also known as ‘brown bast’ is a serious condition that affects productivity
in rubber plantations. TPD is generally manifested as sudden drying up of latex ves-
sels in tree bark at lower side of the tapping panel, rapidly spreading to wider areas
on the tree trunk. Bark necrosis occurs usually after a period of prolonged fl ow of
diluted latex, but sudden drying up without this symptom is also seen. As necrosis
advances affected bark turn dry, crack and peel off either from the entire trunk or on
from one side. Degeneration of normal bark happens simultaneously with the pro-
duction new bark underneath, which lacks uniform structure and texture. Woody
malformations are commonly found interspersed in the new bark rendering it unsuit-
able for tapping. Apart from the bark degeneration, the affected tree otherwise
remain healthy and grow normally.
Observations suggest that TPD is a disorder resulting from continuous wounding
of latex tapping that may trigger a non-compensated oxidative stress (Faridah et al.
1996 ) . The disorder may be associated with nutrient depletion (Fan and Yang 1994 ) ,
metabolic destabilization leading to bursting of lutoids followed by internal latex
coagulation, followed by oxidative stress (Chrestin 1989 ) . Apparent differences in
metabolic profi le between affected and normal bark are observed (Krishnakumar et al.
2001 ) including putative changes in protein profi les (Sookmark et al. 2002 ) . Genotype
variations in onset of TPD are also observed implying the existance of natural adapta-
tion to the unknown causal factors (Yan and Fan 1995 ; Chen et al. 2003 ) .
Recent evidences indicate association of Myb TFs with TPD (Chen et al. 2003 ) ,
along with many differentially expressed water-stress-related genes (Mongkolsuk and
Schumann 2009 ) . Further, Venkatachalam et al ( 2007 ) reported up-regulation genes
like cysteine protease like mRNA (CP), PR-osmotin precursor gene (PRO), ethylene
biosynthesis related gene (EB), annexin-like protein RJ4 (ALP), phosphatidic acid
phosphatase-related gene (PAP) and ASR (abscisic acid, stress and ripening) like pro-
tein 2 (ASR-2) in TPD affected bark. However, putative MyB TF and translationally
controlled tumor protein (TCTP) were found down regulated. Besides, transcripts of
genes Hb TOM20 ( Hevea brasiliensis translocase of the outer mitochondrial mem-
brane) and Hb TK ( H. brasiliensis thymidine kinase), a putative plant thymidine kinase
were found signifi cantly down regulated in TPD-affected trees when compared to
healthy ones (Venkatachalam et al. 2009, 2010 ) .
22.214.171.124 Stem Tapering
Narrowing of the stem or stem tapering is a common phenomenon in palms. In
coconut plantations, it is commonly known as pencil point disease. Although there
are no specifi c factors implicated to cause stem tapering in coconut, any condition
that deprives water and nutrients to the growing meristem can cause this symptom.
Therefore, associated factors can be drought, disease, pest, mineral defi ciencies,
inadequate drainage and competition from weeds or any combination thereof. Palms
recover in full when the causal factors diminish, but symptoms already produced on
trunks do not revert. Stem tapering is also common in palms that are transplanted or
relocated. Since the tapered portion remains mechanically weak, wind damage is an
associated risk with stem tapering (Ohler 1999 ) .
Wounds are common occurrence in biotic and abiotic stresses. Plant tissues are
damaged in and around the site of wound, exposing underlying healthy tissues to
pest and pathogen invasion. In a controlled experiment in cocoa using conidial sus-
pension of Verticillium dalhiae , it was demonstrated that stem puncture predisposes
quicker pathogenic infection than soil application (Resende et al. 1995 ) . Therefore,
plants need to activate quick expression of defense during wounding. Wound heal-
ing is the primary response triggered, during which damaged cells die, turn necrotic
due to the action of lytic enzymes and form a protective barrier. The superfi cial lay-
ers become lignifi ed and suberized and cambial activity is accelerated. Wound heal-
ing is a stressful and energy expensive process, which falls outside the normal
metabolism of plants. Moreover, the nature and duration of healing process depends
on type and extent of wound (Thomas et al. 1995 ) . Although natural wounding
occurs in varying degrees throughout plants’ lifespan, deliberate and continuous
mechanical wounding causes perpetual stress in plantation crops like rubber and
coconut. Wounding stress occurs in rubber while latex harvesting, whereas it occurs
when palm wine is tapped from coconut palm. Agronomic practices such as pluck-
ing of tea buds and pruning of coffee can also develop wound related stress.
Wounding induces different types of signals in plants, targeted towards defense and
healing process (de Bruxelles and Roberts 2001 ) . Ethylene synthesis is found to occur
in rubber in response to wound healing, wherein ACC oxidase (ACO) production is
upregulated (Kuswanhadi et al. 2010 ) . When Ethephon (2-Chloroethylphosphonic
3 Stress in Plantation Crops: Adaptation and Management
acid), an ethylene releasing stimulant was applied, in a positive feedback mechanism,
ethylene produced was found to enhance expression of genes in the ethylene biosynthe-
sis itself. Further, among three ACO genes, Hb ACO1, Hb ACO2 and Hb ACO3, basal
levels of ethylene production appeared to be under the control of Hb ACO1, while
Hb ACO2 and Hb ACO3 were responsible for the positive feedback mechanism and
wounding response in leaves, but not in the latex. Whereas, a cysteine protease gene,
Hb CP1 ( Hevea brasiliensis cysteine protease) was induced in latex during wound heal-
ing, suggesting that Hb CP1 may be actively involved in protecting rubber plants against
pathogen invasion and environmental stresses that involve ethylene signaling (Peng
et al. 2008 ) .
Studies using an isolated cDNA sequence of isofl avone reductase-like protein,
namely CaIRL , from coffee ( C. arabica ) leaves showed that they encode for a novel
type of phosphoinositide (PIP) family of NADPH-dependent reductases, which are
known to be involved in biosynthesis of defense signaling molecules. Expression
studies showed enhanced accumulation of these gene products in coffee leaves
following mechanical wounding and fungal exposure (Brandalise et al. 2009 ) .
Catechins are synthesized in tea, as an adaptive response to protect against tissue
damage (Jaakola et al. 2002 ; Liu et al. 2006 ) . During wounding, enhanced catechin
production was found to occur mediated through the expressions of Camellia sinen-
sis dihydrofl avonol 4-reductase ( CsDFR ; Singh et al. 2009a ) , Camellia sinensis
CoA ligase ( Cs4CL ; Singh et al. 2008 ; Rani et al. 2009 ), PAL enzymes ( Cs PAL) and
cinnamate 4-hydroxylase ( Cs C4H) (Singh et al 2009b ) . Harvesting of tea is done by
plucking out of terminal bud with two internodes below it, which has meristemati-
cally most active cells. These cells have highest activity of N assimilation enzymes
like cytosolic glutamine synthetase, Cs GS ( Camellia sinensis Glutamine synthetase;
Rana et al. 2010 ) , cell cycle proteins like histones, Cs H3 (Singh et al. 2009c ) and
ribosomal proteins like QM protein, CsQM (Singh et al. 2009d ) . Plucking process
therefore, drains vital nutrients and compounds involved in growth and develop-
ment and put tea plants into tremendous stress resulting in accumulation many stress
Palm wine extraction from infl orescences of coconut and other palms also drains
off vital nutrients, sugars and biochemical compounds. Palm tapping is a common
practice in South and Southeast Asian and African countries. In coconut, although
this practice adversely affects growth and production of palms, no systematic studies
are available on stress effects of this traditional practice.
3.4 Management of Stresses
Stress management is a major imperative in production systems of plantation crops.
In plantations, ultimate adaptability of a crop species may be associated with several
interacting and interrelated factors, as well as their carryover effects. Therefore,
unlike that of annuals, plantation species require counterbalancing mechanisms for
more sustained stress management.
Sustainable adaptation to stress factors needs to be built-in the genotypes to tide
over stress impact. External management can leverage the genetic adaptation poten-
tial so that the crops do not suffer beyond economic threshold. So stress manage-
ment in plantation species needs primarily be genetic and then agronomic.
3.4.1 Crop Improvement
Crop improvement plays a vital role in enhancing stress adaptation potential of
crops. Adaptive fl exibility of genotypes not only can help in intensive and extensive
cultivation but can aid in enduring unforeseen stresses also. For example, increasing
the crop area under irrigation has several limitations such as unreliable rains and
non-availability of perpetual irrigation, therefore, most feasible approach is to
develop varieties that can sustain under limited moisture. These varieties have an
added advantage to withstand unexpected moisture stress conditions in the fi eld.
Being crops of perennial nature, focused attempt for breeding against stresses is
seldom practiced in plantation crops because of limited genetic variability and long
breeding cycle. Conventional breeding is highly cumbersome in plantation crops,
because it involves many generations running for decades and expensive in terms of
time, space and large volume of individuals handled. Except in case of palms, how-
ever, fi xation of a superior individual identifi ed at any stage of breeding is rather
easy by vegetative propagation. Therefore, integrated breeding aimed at improve-
ment of yield, quality and resistance, is the ideal approach in plantation crops.
Notably, improved tolerance to abiotic stress must prove to be stable and inheritable
unlike that against biotic stress wherein the tolerance breaks with the evolution of a
new biotype. It is a challenge to plant breeders to generate crop plants that can
stand, reproduce and set seeds in mild to moderate levels of abiotic stress, if not in
extremes. The concern now is to consolidate these advancements in different crops
and make further in-roads in raising the genetic level of stress tolerance.
Notwithstanding, modern biotechnological tools promise of accelerated breeding to
incorporate resistance through gene transformation, marker assisted introgression
in many of the plantation crops.
126.96.36.199 Diversity and Genetics of Stress Resistance
In recently domesticated plantation crops like rubber, introduced genetic variation
is relatively very limited when compared to wild progenitors. Furthermore, attempts
to create of artifi cial variability by induced mutation and polyploidization has met
only with limited success. Therefore, it is prudent to depend on natural diversity to
target genetic adaptation to environmental constraints. There are several attempts to
identify the genes and their control imparting resistance to various stress factors.
While most of them are turned out to be polygenically controlled, there are few
reports of monogenic and oligogenic inheritance as well.
3 Stress in Plantation Crops: Adaptation and Management
The genus Coffea has about 90 species, with only two cultivated species,
C. arabica (arabica) and C. canephora (robusta). C. arabica is a natural tetraploid,
while C. canephora is a diploid (Chevalier 1948 ) with a lot of inter and intraspecifi c
diversity among them, However, C. arabica has relatively low diversity when com-
pared to C. canephora (Lashermes et al. 2000 ) and lack resistance to major diseases
and pests. Low genetic diversity of C. arabica has been attributed to its allotetra-
ploid origin, reproductive biology and evolution process (Etienne et al. 2002 ) .
C. canephora on the other hand, has wider adaptability to different agro-climatic
conditions and show tolerance to leaf rust pathogen, H. vastatrix . Besides, C. cane-
phora provides the main source of resistance to other disease and pest including
CBD ( C. kahawae ), and root-knot nematode ( Meloidogyne spp.). There are at least
nine dominant genes ( SH1 - SH9 ) conferring resistance against leaf rust in coffee
(Bettencourt and Rodrigues 1988 ) . Of these genes SH1 , SH2 , SH4 and SH5 comes
from C. arabica ; SH3 from C. liberica ; and SH6 - SH9 from Hibrido de Timor (HDT,
a spontaneous natural hybrid between C. arabica x C. canephora ) derivatives
(Rodrigues et al. 1975 ; Bettencourt and Rodrigues 1988 ) . Moreover, C. liberica
also provides resistance against leaf rust (Srinivasan and Narasimhaswamy 1975 ) ,
while C. racemosa provides coffee leaf miner resistance (Filho et al. 1999 ) . CBD
resistance is complete in C. canephora and partial in C. arabica . Genetic studies on
CBD resistance conclude that three major genes viz., R , K and T are responsible for
resistance. Partial resistance observed in arabica and HDT derivatives were due to
recessive nature of these genes at least at any one locus (van der Vossen and Walyaro
1980 ) . Recent reports, however, suggests of oligogenic and quantitative resistance
to CBD (Silva et al. 2006 ) . Recently, a major dominant gene, Ck-1 has been mapped
for CBD resistance in C. arabica (Gichuru et al. 2008 ) , which may perhaps be syn-
onymous to the T locus. Inheritance studies on high-level resistance found in
C. canephora against root-knot nematode, Meloidogyne exigua reveals that a sim-
ply inherited major gene, Mex-1 (Noir et al. 2003 ) , controls resistance.
The tea genus, Camellia , seems to be a genetically obscure one because of incon-
sistent and subtle genetic variation at species level and of genetic instability due to
high out-breeding nature. The cultivated tea, C. sinensis and its progenitors have
originated from Irrawaddy river basin in Myanmar extending between Southeast
China and Assam in Northeast India (Eden 1976 ) . Recorded diversity history shows
that species count increased from 82 species in 1958 (Sealy 1958 ) to more than 325
species in 2000 (Mondal 2002 ) . Presently, more than 600 varieties are under cultiva-
tion world over, of which some are unique in caffeine content and disease tolerance.
Cultivated tea hybridizes well with wild relatives, creating a myriad of variants in
tea genetic pool. One of the particularly interesting wild species is C. irrawadiensis
whose morphological distribution overlaps with that of cultivated tea (Banerjee
1992 ) and a few desirable traits such as anthocyanin pigmentation and special qual-
ity characters of Darjeeling tea might have originated from these species (Wood and
Barua 1958 ) . It is widely accepted that three species i.e. C. assamica , C. sinensis
and C. irrawadiensis have predominantly contributed to the cultivated gene pool of
tea that includes progenies and the hybrids between them (Mondal et al. 2004 ) . The
information available on genetic resistance to various stresses in tea is rather limited.
Resistance to grey blight caused by Pestalotiopsis longiseta is high among C. assamica
and two dominant genes, Pl1 and Pl2 are reported so far. The Pl1 gene, imparts a
higher level of resistance, and has an epistatic action with the Pl2 gene, which has
a moderate level of resistance (Takeda 2002, 2003 ) . Besides, there are partial resis-
tant sources to blister blight (Premkumar et al. 2008 ) , anthracnose and cold damage
(Takeda et al. 1987 ) .
The genus Hevea to which rubber tree belongs has ten recognized species, of
which H. brasiliensis alone is cultivated. Major world production of natural rubber
takes place in its introduced home in the South and Southeast Asian countries
because of a potentially devastating disease, SALB, in its native land. Therefore,
rubber cultivation in Asia is virtually on a vulnerable threat from SALB because of
two reasons, (a) the variability of the introduced genetic pool is very narrow and (b)
no genetic resistance to SALB is available. Further, a little information is available
with respect to genetics of resistance for other diseases of rubber. Various parame-
ters indicative of possible resistance has been described in the case of SALB such
as incubation time, latency, lesion size and number, sporulation, time taken for stro-
mata to appear, and time from inoculation to leaf fall. Simmonds ( 1990 ) suggested
that complete resistance to SALB was likely to be monogenic. However, no conclu-
sive evidence on this claim is available so far. Possibility of combining durable
resistance to SALB looks remote, as the pathogen can adapt so quickly to the host
resistance, much faster than the development of a resistant combination (Rivano
et al. 2010 ) . Notwithstanding, it is reported that resistance to Corynespora cassii-
cola in rubber was articulated by two dominant genes A and B . Gene A is in epistatic
interaction with B in which recessive form of A suppresses expression of B (Hadi
et al. 2004 ) . Le Guen et al. ( 2000 ) used isozyme marker based linkage mapping
approach to map a qualitatively inherited dominant gene, Phr lying at 14.7 cM from
the adh locus, conferring resistance to Phyllachora huberi , which causes black crust
disease. Genotypic variations exist for tolerance to disease as well as environmental
variations in rubber clones (Raj et al. 2005 ) .
Among the palms, coconut is the only species in the genus Cocos . Although
there are many genetic variants in cultivated coconuts, exact evolutionary relations
between them are still obscure. Genetic studies on economic traits show quantitative
inheritance, while there are only a few attempts to study genetics of disease resis-
tance. Studies on complex diseases like lethal yellowing, root wilt etc. are particu-
larly diffi culty because of the diffi culty in effectively inducing diseases under
controlled situations. However, there are many reports confi rming genotype vari-
ability in resistance to most of the stress factors in coconut, with some varieties
carrying exceptionally high level of resistance.
The genus Eucalyptus harbors high inter- and intraspecifi c genetic variability for
resistance to stress factors. Variation in abiotic stress tolerance may extend from
frost susceptibility to extreme frost tolerance as well as tolerance to drought.
Besides, resistance to diseases enables adequate disease management by planting
resistant clones, progenies or species. In E. grandis , a single dominant gene, Ppr-1
(Junghans et al. 2003 ) , governs resistance to rust pathogen, Puccinia psidii , while a
general level of tolerance is exhibited to Cylindrocladium leaf blight.
3 Stress in Plantation Crops: Adaptation and Management
188.8.131.52 Classical Approaches
Breeding for resistance to stress has not been a major breeding objective in plantation
crops until the beginning of 20 th century. However, stress resistance breeding has
become a major objective in breeding most of these crops now, because of following
reasons. Catastrophic events of crop destruction around the world due to various
stress factors has led to better understanding of the biological events related to
stress. Shrinking resources for cultivation and increasing demand for various plan-
tation commodities had led to perpetuation of plantation crops to non-traditional
regions. Further, better understanding of genetics of stress adaptation and modern
tools in crop improvement has opened new avenues for crop improvement.
The utilization of vertical resistance (VR) in breeding for stress resistance espe-
cially for biotic stresses in annual and seasonal crops had been very successful,
occasional disasters notwithstanding. However, in plantation crops that grow in
nearly non-seasonal environments, effective use of VR is not an ideal option. This
is because any attempt to use VR in these crops had resulted only in transient, non-
durable resistance. The experiences shows that pyramiding of VR only helped in
delaying in the development of new pathotypes a little. Further, introgression of VR
genes from wild species would lead a breeder to a very low genetic purity level in
respect of other characters, which would require several backcrosses and many
decades of efforts for improvement.
Nevertheless, the use of horizontal resistance (HR) is promising as the only choice
in plantation crops because HR is reasonably heritable in all these crops. Further,
offspring produced between ideal parents are slightly more resistant than the parents
are. Notwithstanding, directed selection for resistance is not only done individually,
but done also at a particular location in overall, discarding unhealthy and susceptible
genotypes, and selecting only resistant ones. This method of selection has been the
most successful breeding strategy in plantation crops for stress resistance as it accu-
mulates HR, albeit in congruence with yield and quality traits. For effective selec-
tion, however, pre-breeding is an essential necessity for developing best lineages and
for accumulating favorable alleles in the breeding population.
In coffee, one of the main breeding objectives worldwide is to transfer disease
resistance from diploid species such as C. canephora or C. liberica into cultivars of
C. arabica without affecting coffee quality (Etienne et al. 2002 ) . Several stages are
involved in the improvement of C. arabica , of which the fi rst stage is selection and
testing of superior individuals. These superior individuals are seed propagated, and
two cycles of selfi ng is done before testing the characters in each line for stability.
Thereafter, superior lines are intercrossed, in double or multiple crossings, and
selection is done from progenies for improved individuals with respect to resistance
and allied traits. Backcrossing may produce varieties that are more superior. The
best selections emerging out of the selection process are cloned for further multipli-
cation and released for commercial cultivation. Robinson ( 1976 ) employed a crash-
breeding program in Ethiopian coffee, by screening and selection for CBD resistance
among naturally occurring variation in planter’s crop. The seeds collected from the
selections were grown and progeny tested for durability of resistance along with
yield and quality. This intense screening resulted in superior populations with
balanced domestication in seven years. Clonal selection is the most important pro-
cedure followed in rubber breeding. Because of longer selection cycle and possibil-
ity of occurrence of multiple stresses, stress endurance is used as a selection
parameter in rubber along with yield traits. Clones can be evolved at any stage of
breeding. Selective hybridization of promising clones is further done either among
themselves or with wild germplasm lines. The progenies are directly selected from
seedling nurseries and cloned for further evaluation. Natural open pollinated half-
sib populations are also screened for desirable characters including resistance.
Susceptible and poor performing clones are normally discarded while selection.
Polycross gardens comprising of pre-potent clones are also utilized and the selec-
tion is generally exercised in the polyclonal seedling orchards, even at maturity
stage. In India, increased resistance to biotic and abiotic stresses has now been
re-emphasized in rubber breeding and selection (Venkatachalam et al. 2006 ) , in
particular, for low temperature tolerance and resistance to Corynespora leaf fall
disease. In the case of SALB, no durable resistance has been reported so far in
rubber, although source of putative total resistance, characterized by absence of
spores on leaves, are found in some wild clones of H. brasiliensis and in other
species such as H. benthamiana, H. guianensis , H. paucifl ora and H. spruceana
(Simmonds 1990 ) . Introduction of unexploited genetic variability from wild to aug-
ment narrowing genetic base of the cultivated gene pool was carried out by the
International Rubber Research and Development Board (IRRDB) in 1981. An expe-
dition, carried out in the Amazon basin spread across three districts of Acre,
Rondonia and Mato Grosso of Brazil, collected 194 high yielding trees which were
not affected by Phytophthora and SALB along with a total of 63768 seeds,
1413 meters of budwood and 1160 seedlings (Ong et al. 1983 ) . These collections
show continuous variation for SALB resistance in French Guyana (Clement-
Damage et al. 1998 ) and for Phytophthora resistance in India (Mercy et al. 1995 ) .
In tea breeding, mostly conventional approaches are being practiced (Chen et al.
2007 ) , with yield and quality as the prime targets. Nevertheless, breeding for resis-
tance against stress factors has received only little emphasis, because it has not been
largely successful. Natural selection plays an important role in adaptation of tea
clones to a set of stress factors by eliminating susceptible clones during early stages
of multiplication. Since only tolerant clones survive, a well-buffered population
survives under every geographical niche where tea is cultivated. Even if any related
species is identifi ed with a great degree of resistance to a particular stress, such spe-
cies are not included in breeding programs because, most of the wild cross produce
very inferior quality tea that is not acceptable (Bezbaruah 1987 ) and diffi cult to
improve upon. Therefore, a compromise on quality with respect to resistance can be
more economically devastating than the stress factor itself. Since available genetic
base of cultivated tea is narrow, particularly with respect to quality, scope for further
improvement for resistance within the available base remain limited (Willson 1999 ) .
Notwithstanding, breeding attempts for stress resistance have not been without any
success. In Japan, distant hybridization of cultivated and 26 wild species, resulted in
isolation of an interspecifi c hybrid, Chatsubaki ( C. sinensis × C. japonica ), with
3 Stress in Plantation Crops: Adaptation and Management
high resistance to tea anthracnose, grey blight and low temperature besides having
low caffeine content (Takeda et al. 1987 ) . Chatsubaki is now used in regular tea
breeding programs. In India, a high yielding standard tea variety has been developed
by interspecifi c hybridizations involving C. irrawadiensis , C. assamica and
C. sinensis (Bezbaruah 1987 ) . Recently, it has been reported from China that one
excellent new clone with high cup quality, resistant to disease, suitable for fi ne
green tea and very early sprouting in the spring has been selected from the Cobalt-60
g -ray irradiated offspring of Longjing 43 cuttings (Yang et al. 2003 ) . This clone is
undergoing adaptability tests and may become the fi rst clone bred using induced
mutagenesis in plantation crops (Chen et al. 2007 ) .
As a crop of economic importance, at one time, coconut was considered neglected
in terms of breeding and genetics (Harries and de Poerck 1971 ; Williams et al.
1975 ) . Owing to various practical constraints in palm breeding, therefore, genetic
improvement in coconut had made a slow progress. However, declining nut produc-
tion and shrinking cultivation in the major production environments has restored the
efforts for breeding for resistance in coconut recently. Further, widespread inci-
dences of lethal yellowing in the Caribbean and Africa, cadang-cadang in the
Philippines, root wilt disease and recent outbreak of eriophyid mites ( Aceria guer-
reronis ) in India have raised serious concerns, because most of these threats are
beyond control by conventional plant protection measures. Although a long-term
process, development of resistant/tolerant genotypes is the only practical solution to
combat these stress factors.
In coconut, screening of exotic germplasm had resulted in identifi cation of
Malaysian dwarfs (yellow, red and green) tolerant to lethal yellowing in Jamaica in
1950s. These were recommended for planting on a large scale in Jamaica and
Florida, only to be withdrawn sooner due to their susceptibility to many other cli-
matic constraints. Nevertheless, this has led to the development of a resistant hybrid,
Maypan, obtained from the cross Malaysian yellow dwarf (MYD) × Panama tall
(Harries and Romney 1974 ) . Maypan could revive the Jamaican coconut industry
largely, until it was massively destroyed by fresh outbreak of the disease (Broschat
et al. 2002 ) . Recent reports suggest genetic contamination among parents as the
reason for loss of resistance (Lebrun et al. 2008 ) , but other reasons like development
of virulence in pathogen and/or vector and very narrow window of genetic variation
among the cultivars cannot be ruled out (Baudouin et al. 2008 ) .
In root wilt screening program in India, disease escapes are selected from ‘hot
spot’ areas belonging to tall (T) as well as dwarf (D) types and serologically tested for
the presence of phytoplasma. Disease free plants are then intermated in combinations
of T × T, T × D, D × T and D × D. Seedlings raised from these crosses were understory
planted in ‘hot spots’ to subject them for vigorous natural selection. Disease escapes
were further tested serologically to ensure absence of root wilt pathogen. Some D × T
hybrids involving disease-free Chowghat green dwarf (CGD) palms and west coast
tall (WCT) palms planted in 1991 remained disease free a long period (Nair et al.
2006 ) . This program has now resulted in the development and release of two selec-
tions Kalparaksha (Nair et al. 2009 ) and Kalpasree and one hybrid Kalpa Sankara
with fi eld resistance/ tolerance to root (wilt) disease and yield potential.
184.108.40.206 In Vitro Approaches
In plantation crops, several tissue culture techniques such as somatic embryogenesis,
meristem and axillary bud culture, induction of adventitious buds, androgenesis and
protoplast culture are used for plant regeneration and micropropagation. One of the
extreme advantages of a cell culture system is the facility for artifi cial screening of
germplasm and mutant lines to identify stress tolerant genotypes, mutants and
somaclonal variants those can augment accelerated breeding programs. Stress fac-
tors that are introduced in the culture media include increased salinity, pathogenic
toxins, low nutrient content, heavy metal and induced water defi cit. Regeneration of
genetically engineered plants (transgenics) is yet another possibility under in vitro
systems. Besides, micropropagation helps in developing disease free planting mate-
rials in plantation spices like cardamom (Babu et al. 1998 ) .
In vitro selection
Most widely explored stress factor using in vitro systems in crop plants is salt toler-
ance. Cell culture systems offer several unique advantages in studying cellular level
mechanisms and functions of salt tolerance and provide alternative methods for
screening, selecting, and characterizing salt tolerance at the cellular level. Sodium
chloride (NaCl) is the most frequently used salt for salt tolerance screening, although
use of other salts and dilutions of seawater have been reported. Spontaneous varia-
bility generally appears in cell culture, suffi cient to allow effective selection; use of
mutagens such as ethyl methanesulfonate (EMS) and methyl methanesulfonate
(MMS) helps to increase mutation frequencies. Selected salt tolerant cell lines are
further evaluated to see if the tolerance remains stable after the cells had been moved
to salt-free culture systems. There are several reports detailing mechanisms of salt
tolerance as well as successful isolation of tolerant cell lines in many plantation spe-
cies. In Eucalyptus microtheca , nodal segments cultured in vitro , showed varying
degree of ion accumulation, with decreasing K + /Na + ratio with increase in salinity,
implying a simple way of in vitro screening than the cumbersome in vivo screening
(Morabito et al. 1994 ). Pollen germination in vitro in the presence of elevated salt
concentrations has been implicated as a reliable index of pollen tolerance to salinity
in olive, a Mediterranean tree crop similar to tropical plantation crops. Further, a
close correlation between pollen (gametophyte) tolerance and whole plant (sporo-
phyte) responses to salinity was also found in olive (Soleimani et al. 2010 ) .
Induction of artifi cial water defi cit by incorporating polyethylene glycol (PEG-
6000), mannitol and NaCl into in vitro culture of excised coconut embryos revealed
that embryos from putative drought tolerant lines were able to withstand NaCl, but
not PEG and mannitol (Karunaratne et al. 1991 ) . The survival of embryos in high
NaCl concentrations is perhaps indicative of the halophytic nature of coconut and it
is assumed that same mechanism plays a role in drought tolerance as well.
Selection for disease resistance under in vitro systems is reported in coffee, oil
palm, cardamom and date palm. Partially purifi ed culture fi ltrate (PPCFs) containing
3 Stress in Plantation Crops: Adaptation and Management
C. kahawae phytotoxins was used in varying concentrations for screening hypocotyl
explants obtained from CBD resistant (HDT) and susceptible arabica (N39) geno-
types, showed that calli from HDT showed rapid growth and no necrosis, while
N39 calli had varying degree of necrosis and growth suppression. Selections from
the surviving calli of N39 on regeneration had shown increased CBD resistance
(Nyange et al. 1995 ). Dorcas et al. ( 2010 ) isolated basal rot resistant somaclones
using PPCF of Fusarium oxysporum from a series of in vitro screenings in carda-
mom. They used somaclones derived from plants that withstood an initial pathoge-
nicity test in the screen house. These somaclones were then cultured through a
series on PPCF concentrations, selecting only those survived, until the selected
lines survived undiluted PPCF. In date palm, pathotoxins such as fusaric, succinic,
3-phenyl lactic acids and their derivatives, marasmins and peptidic toxins as selec-
tive agents in cell culture are being used for isolating resistant lines against
Fusarium oxysporum f. sp. albedinis that caused fusariosis wilt or bayoud (El
Hadrami et al. 2005 ; El Modafar 2010 ) .
Shoot cultures of Eucalyptus camaldulensis clones with different levels of salt
tolerance were exposed to NaCl and ABA in vitro showed that proline accumulation
increased signifi cantly in all the clones. However, when the cultures were exposed to
NaCl alone, resistant clones had signifi cant high proline accumulation whereas in
susceptible clones proline content remained unchanged (Woodward and Bennett
2005 ) . Substantial accumulation of proline, polyamines (PA) and ABA occurs in
plants during adaptation to various environmental stresses such as salinity, drought
and high and low temperatures. Proline is a known osmoprotectant during drought
stress in plants, and can serve as a nitrogen and carbon source during stress recovery
process (Galiba 1994 ) , and its accumulation is positively correlated to the salinity
level or to the intensity of water stress (Heuer 1999 ) . Elevation of endogenous ABA
in response to cold treatment is hypothesized to induce synthesis of proteins that are
responsible for the increase of frost hardiness (Chen et al. 1983 ) . ABA dramatically
increases freezing tolerance of the cells cultured in vitro , and enables cultured plant
cells to survive freezing temperatures without previous cold treatment (Galiba et al. 1995 ).
One of the remarkable achievements in plant cell culture is the opportunity to trans-
fer genes across organisms through plant transformation employing somatic embry-
ogenesis. Transgenic technology has now been tried in almost all cultivated species
and is one of the remarkable technologies in the area of biotic stress management.
The most successful event of genetic transformation in plants is transferring of cry
(crystalline protein) genes from the soil bacterium, Bacillus thuringiensis (Bt) for
imparting insect resistance. Although herbicide tolerance has also been achieved
through transgenic technology, nevertheless, it is of little importance in plantation
crops, except in the case of cardamom. Herbicide tolerance is incorporated in coffee
and eucalyptus, primarily as selective agents during transgene development or as
model transgenesis system rather than for agronomic use.
94 Download full-text
Agrobacterium tumefaciens mediated genetic transformation is widely used in
plantation crops. Although A. rhizogenes and biolistic methods have been tried, the
popularity of A. tumefaciens system is attributed to its simplicity and ease of gene-
rating transgenic derivatives. Coffee was among the early perennials in which
successful transformation events for stress tolerance were reported. Transgenic cof-
fee plants containing the cry1A(c) gene were produced using both A. rhizogenes
(Leroy et al. 1997 ) and A. tumefaciens (Leroy et al. 2000 ) mediated transformation
systems. The cry1A(c) gene encodes for an insecticidal crystal protein that is toxic
to certain insects including the coffee minor, Perileucoptera coffeela (Filho et al.
1998 ) . Transformed plants showed high degree of fi eld tolerance to leaf miner in
French Guyana (Perthuis et al. 2005 ) . Other than coffee, successful regeneration of
transgenic plants with augmented stress tolerance was reported rubber, oil palm,
eucalyptus, cocoa and tea (Table 3.3 ). Successful incorporation of cowpea trypsin
inhibitor ( cpTI1 ) gene (Abdullah et al. 2003 ; Ismail et al. 2010 ) and Bt gene (Lee
et al. 2006 ) is reported recently towards achieving insect resistance in oil palm.
Transgenic plants obtained are undergoing screening.
Whilst, most of the transformation attempts are targeted against pests, chitinases
and antibody linked small chain variable fragment (scFv) genes are used for gener-
ating disease resistant transgenes. The scFv genes coding for antibodies specifi c to
pathogenic toxins were recently demonstrated to be effi cient against toxin produc-
ing pathogens like Corynespora cassiicola in rubber (Sunderasan et al. 2009 ) . This
provides opportunities to incorporate such genes in tissue transformation systems to
incorporate resistance against pathogens. In oil palm, white rot pathogen, Ganoderma
boninense establishes by destroying lignin fraction of woody tissues and exposing
white cellulose, which fungus utilizes by causing rot. Resistance against G. bonin-
ense is being attempted through transfer of chitinase and ribosome inactivating pro-
tein (RIP) genes (Hashim et al. 2002 ) . Lignin content has been genetically modifi ed
in plantation crops like eucalyptus, opening the possibility of attempting such sys-
tems in oil palm (Price et al. 2007 ; Paterson et al. 2009 ) .
Transgenesis towards abiotic stress tolerance were reported in rubber against
oxidative stress in which superoxide dismutase ( HbSOD ) gene (Jayashree et al.
2003 ) and Mn-superoxide dismutase ( Mn-SOD ) gene (Sobha et al. 2003 ) were suc-
cessfully incorporated. Transgenic development to tackle abiotic stresses has also
been attempted for freezing tolerance in eucalyptus (Zhang et al. 2010b ) and for
salinity tolerance in tea (IHBT 2006 ) . In eucalyptus, cold hardiness is incorporated
through a protein TF called C-repeat binding factor (CBF), such as Eucalyptus gun-
nii derived EguCBF1a and EguCBF1b (El Kayal et al. 2006 ) . CBF are known to
regulate expression of a number of genes conferring frost hardiness. Driven by cold
inducible dehydrin promoters, CBF transgene expression occurred only during cold
stress thereby improving freeze tolerance signifi cantly without negatively infl uenc-
ing other agronomically important traits (Zhang et al. 2010a ) .
Therefore, outlook on transgenic development now focuses on genes that are
expressed only ‘in need’ to tackle stress situations. For instance, if a constitutive pro-
motor such as CaMV35S was used in eucalyptus transformation, CBF mediated nega-
tive impacts such as reduced growth, reduced leaf area and increased thickness