Quality and Quantity Improvement of Citrus: Role of Plant Growth Regulators

Abstract

Citrus is one of the most important fruit tree species in the world, as the fruits are a valuable source of nutrients, vitamins and other antioxidant compounds. The citrus productivity depends on various factors, among these the plant growth regulators holds a prime position. The use of plant growth regulators has become an important component in the field of citriculture because of the wide range of potential roles they play in increasing the productivity of crop per unit area. The plant growth regulating compounds actively regulate the growth and development by regulation of the endogenous processes and there exogenous applications have been exploited for modifying the growth response. Plant growth regulators have been used in citrus fruit production for influencing flowering, fruit set and fruit drop and play a major role in fruit growth and abscission. These regulators have also been used to influence fruit quality factors like peel quality and colour, fruit size, juice quality and to improve total soluble solids in different citrus species. This review may serve as a complete treatise on the possible roles of growth promoting substances on the physiological processes of citrus plant.

Full text

Quality and Quantity Improvement of Citrus: Role of
Plant Growth Regulators
Harsimrat K. Bons1*, Nirmaljit Kaur2 and H.S. Rattanpal1
1Department of Fruit Science, Punjab Agricultural University, Ludhiana, Punjab, India.
2Department of Botany, Punjab Agricultural University, Ludhiana, India.
*Corresponding author: harsimratpau@pau.edu
Paper No. 335 Received: 18 April 2014 Accepted: 22 May 2015 Published: 29 June 2015
Abstract
Citrusis oneof themost importantfruit treespecies inthe world,as thefruits area valuablesource
ofnutrients, vitaminsandotherantioxidantcompounds.Thecitrusproductivitydepends onvarious
factors, among these the plant growth regulators holds a prime position. The use of plant growth
regulatorshas becomean importantcomponent intheeldofcitriculturebecauseofthewiderange
of potential roles they play in increasing the productivity of crop per unit area. The plant growth
regulatingcompoundsactivelyregulatethegrowthanddevelopmentbyregulationoftheendogenous
processesand thereexogenousapplicationshavebeenexploited formodifyingthegrowthresponse.
Plantgrowthregulatorshavebeenusedincitrusfruitproductionforinuencingowering,fruitsetand
fruitdropandplayamajorroleinfruitgrowthandabscission.Theseregulatorshavealsobeenusedto
inuencefruitqualityfactorslikepeelqualityandcolour,fruitsize,juicequalityandtoimprovetotal
solublesolidsindierentcitrusspecies.Thisreviewmayserveasacompletetreatiseonthepossible
rolesofgrowthpromotingsubstancesonthephysiologicalprocessesofcitrusplant.
Highlights
• Impactofplantgrowthregulatorsonfruitqualityandproductivityofcitrus
Keywords : citrus,plantgrowthregulators,owering,fruitgrowth,fruitquality
Thegrowthanddevelopmentincitruslikeallother
fruitcropsisgovernedbyboththeintrinsicas well
asextrinsicfactors.Theintrinsicfactorscompriseof
thegenotypeandtheendogenoushormonebalance,
whereas,theextrinsicfactorsaretheenvironmental
conditions encountered by the plant. There are
other factors, viz., planting density, irrigation and
fertilization, which may aect the growth and
development of the plant. Since the discovery of
the plant growth regulators, they have been used
to manipulate plant growth and development for
the improvement of quality and quantity of the
produceinordertoenablethefruitgrowerstomeet
to pressure of increasing demand for food of high
quality.
In citrus industry, a large number of plant growth
regulatorshavebeenevaluatedfortheircommercial
application. Plant growth regulating chemicals are
usedincitrusproductioninallthecitrusproducing
areas.Theyareusedforproductiontechnologyand
quality improvement. In production technology,
emphasis is primarily on nursery production,
crop regulation by manipulation of owering,
improvement of fruit set and fruit growth and
manipulationofthetimeofharvest.Theapplication
of plant growth regulators for improvement of
HORTICULTURE
International Journal of Agriculture, Environment and Biotechnology
Citation: IJAEB: 8(2): 433-447 June 2015
DOI Number: 10.5958/2230-732X.2015.00051.0
©2015 New Delhi Publishers. All rights reserved

434
Bons et al.
internalfruitqualityandpostharvestfruitstorageis
ofequalimportance.
Plant growth regulators have been used in citrus
fruitproductionforinuencingowering,fruitset
andfruitdrop(Berhow2000).Theseregulatorshave
alsobeen usedtoinuencefruitqualityfactorslike
peelqualityandcolour,fruit size,juicequalityand
to improve total soluble solids in dierent citrus
species.AmongPGRs,theauxinshavedirecteecton
abscissionbycausingadelayofabscissionresulting
inimprovementin fruitqualityandyield incitrus.
Napthylaceticacid,2-4Dichlorophenoxyaceticacid
and Gibberellic acid have been tried for reduction
of physiological drop (Ullah et al. 2014 Salicylic
acidisconsideredapotent plant hormone because
ofitsdiverseregulatoryrolesinplant metabolism.
It is well established that salicylic acid potentially
generates a wide array of responses in plants and
also aects the photosynthetic parameters which
enhanceyield(Mahdiet al.2012)
Thepoortree healthalsoplaysa signicantrolein
enhancingoffruitdropduetodepletionofnutrient
supplytothedemandingsinks.Citrusishowever,
a relatively high nutrient demanding crop (Wang
et al.2006)anddeciencyorexcessofnutrientscan
leadtoinferiorfruitquality.Thereisaneedtoboost
upyieldthrough propernutritionandmaintaining
internalhormonalbalance.Suitable combination of
micronutrients and growth regulators may control
excessive fruit drop for the improvement of fruit
yieldandquality. Althoughmanyeortshavebeen
made to study (Baldwin 1993) the physiological
and biochemical aspects of citrus, there is still an
enormous unexplored potential in the study of
regulation of metabolites associated with citrus
physiology.
Citrusfruitsuselargeamount ofKas comparedto
othermacronutrients(AlvaandTucker1999)because
Kisinvolvedinseveralbasicphysiologicalfunctions
i.e. formation of sugars and starch, synthesis of
proteins, cell division, growth and neutralization
of organic acids (Liu et al. 2000). It improves fruit
qualitythroughenhancingfruitcolour,sizeandjuice
avor(Tiwari2005).Reportsindicatethedeciencies
of micronutrients like Zn, Cu, Fe and Mn in citrus
orchards of India and among them Zn is more
acute.LiteratureindicatesthattheapplicationofZn
increasesthefruityieldandquality(Rodriguezet al.
2005).Thesuitablecombinationsofmacronutrients,
micronutrientsandgrowthregulatorscouldcontrol
theexcessivefruitdropandimprovethecitrusfruit
yieldanditsquality(DobermanandFairhurst2000).
The literature pertaining the role of plant growth
regulators during dierent periods of growth and
developmentisreviewedunderthefollowingheads:
Role of Plant Growth Regulators
Flowering
Citrus trees, once past the juvenile phase, bloom
everyyear.Annualoweringofadulttreesisaected
by several exogenous and endogenous factors. In
citrus,cooltemperaturecaninduceowering,asin
mosttropicalandsubtropicaltrees(Inoue1990,Lenz
1967,Moss1976,Nishikawaet al.2007,Wilkie et al.
2008).InSatsumamandarin,oralinductionoccurs
intreesexposedto15°Cformorethan1.5months
(Inoue1990b,Nishikawa et al. 2007).Treesgenerally
remaininthevegetativegrowthphaseuntilthetrees
areexposedtotemperaturesoflessthan25°C(Inoue
andHarada1988).Undereld conditions,thetrees
areexposed tocool temperatureduringfall,during
which oral induction proceeds. The plant growth
regulatorsstimulatetheabscissionintheowersthat
causes heavy ower drop. According to Martinez
et al. (2004)oweringofHernandinagetsreducedby
25%andofOrograndeby60%whenGA3(20-50mg/
l)whengivenasa foliar spray (6 L pertree)toall
thecitrustrees.During bud developmentincitrus,
theapplicationof GA3hasshownto inhibitower
production(Guardiola et al. 1982),leadingtogreater
ratio of terminal owers in the leafy shoots thus
higher development of fruits (Iglesias et al. 2007).
Theseresultswerealsoshownbyuseofethychlozate
and GA3 for ower induction in citrus fruits but
GA3causedinhibitoryeect(Takahara et al. 2001).
According to Ben-Cheikh et al. (1997) gibberellins
are the factors responsible for ovary transition. In
vegetativeorgans,gibberellinsactivatetheprocessof

Quality and Quantity Improvement of Citrus: Role of Plant Growth Regulators
435
celldivisionandcellenlargement(Talon et al. 1991)
thus are also associated with initiation of growth
(Talon and Zeevart 1992). Gibberellins reduce the
ower production resulting in higher productivity
of beer quality fruits. It acts like a thinner agent
but it also showed the ability to retain the owers
(Iglesias et al. 2007)whereas2,4-Ddelayorstimulate
the abscission. Talon and Zeevart (1992) reported
thatincitrusthereproductiveprocessesareaected
byplantgrowthregulatorsshowingthatregulatory
mechanism being controlled by critical hormonal
component.
The phenomenon of owering in plants is under
the control of many factors, viz., cultivar, genetic
makeup, the environment and cultural practices.
Mostof thecitrus cultivarsowerprofuselysubject
to optimum environmental conditions and there
aremorethan100,000owerspertree(Agusti et al.
1982).Dependingonthecultivar,mostoftheowers
absciseleadingtolessthan1%fruitset(El-Otmani et
al. 1992).Theapplicationofplantgrowthregulators
is to regulate tree productivity either by reducing
vegetativegrowthandenhancingowering(during
‘o-year‘ )orby enhancingvegetativegrowthand
reducing ower initiation and development ( ‘on-
year ‘). Monselise and Halevy (1964) reported that
gibberellicacidcouldinhibitoweringin‘Shamouti’
orange. Similar inhibitory eect having been
reportedbyDavenport(1990).However,thetriazole
compoundsthatinhibitgibberellicacidbiosynthesis
have been reported to promote inorescence
production (Delgado et al. 1986 1986b; Harty and
van Staden 1988). It is the time of application and
concentration which determines the action of a
particular growth promoter (Guardiola et al. 1982,
Lord and Eckard 1987). In subtropical climates of
the northern hemisphere oral induction occurs
duringtheperiodfromNovemberthroughJanuary.
For sweet oranges and some mandarins induction
occurs early during this period (Guardiola et al.
1982), whereas for ‘Satsuma’ mandarins it occurs
later (lwahori and Oohata 1981). The optimum
concentration for promotion of owering are GA3
( 25 µg/ml ) for ‘Navelate’ sweet orange with the
application in mid-December and for ‘Washington
Navel’ with the appropriate time of application
being mid-November in Spain (Guardiola et al.
1977).Optimumconcentrations(100mg/L)andtime
ofapplication(lateJanuary)for‘Satsuma’mandarin
inJapanisreported by lwahoriandOohata (1981).
Agusti et al. (1981) concluded that GA3 (10 mg/l)
applied in late November could reduce owering
to40%in‘Clementine’mandarinirrespectiveofthe
date of harvest (i.e., the presence of the crop was
not inhibiting owering). Low GA3 concentration
reduced owering by 37 and 70% when it was
applied to ‘Satsuma’ mandarin trees in mid-
December(Garcia-Luis et al. 1986)andlateDecember
(Guardiola et al. 1982), respectively. However, the
durationofsensitivityis veryshortandtreatments
must be applied before the developing shoots are
morethan1mm long(Guardiola et al. 1982),which
correspondstothestageoffullsepaldevelopmentof
theapicalower(LordandEckard1987).
Aempts to promote owering using growth
retardants that are reported to inhibit synthesis of
gibberellins viz., CCC and paclobutrazol have not
beenabletoprovideconclusiveresultsasreportedby
HartyandvanStaden(1988)andDavenport(1990).
Greenberg et al. (1993)reportedthattheapplication
of paclobutrazol (foliar spray or soil application)
shifts the balance of shoot types toward the pure
leaess inorescences, which is just the reverse of
GA3, which pushes the balance toward vegetative,
owerlessshoots.
There are reports of role of auxins in the process
of owering by Zeevart (1978). Application of
2,4-D during mid-November to mid-December
to ‘Navelate’ sweet orange reduced owering by
approximately 30% (Guardiola et al. 1977). Higher
concentrations of synthetic auxins did not provide
anyadditionaleect(Agusti1980)andlower rates
didnotshowinhibitoryeect(Garcia-Luis et al. 1986).
Abscisicacidmayalsobeinvolvedintheregulation
ofowering(YoungandCooper1969).Therole of
cytokinins in the regulation of owering in citrus
has received lile aention, although a correlation
betweenendogenouscytokininlevelsandbudbreak
andgrowthhas been reported(Davenport1990). It

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Bons et al.
has been argued by Davenport (1990) that lack of
oweringduring(o-bloom)whentreesarebearing
theheavy‘on-yearcrop’forseverelyalternatebearing
cultivars,islikelyduetolackofcytokinins.Thelow
levelof cytokininsaredue tolackofcarbohydrates
andothernutrientsnecessaryforactiverootgrowth
andfortheproductionofcytokinins.
Gibberellins appear to be the only plant growth
regulators that consistently inhibit owering, but
theuseofgibberellicacidasacommercialregulators
forreducing owernumbertoincreasefruitsizeor
inan expected‘on-year’toreducealternatebearing
is limited. Climatic conditions, i.e., temperature
extremes that might prevail during owering
invocationandinductionmayresultinpoorfruitset
orexcessivefruitdropresultinginlowyield.
Crop Regulation
Consumer preference worldwide is for large sized
andhealthyfruits(Gilllan1987,MillerandHofman
1988). Fruit juice acidity (percent citric acid) and
juice volume are very much aected by fruit size.
Smallfruithaveasubstantiallyhigherpercentacid
than larger fruit and total soluble solids per fruit
is considerably lower in small fruit. Reducing the
proportionofsmallfruitharvestedincreasesgrower
return both for fresh fruit (because there are more
fruit of commercially valuable export size) and for
juice(becauseofincreasedjuiceandTSS).Smallfruit
size results due to many factors, viz., competition
between fruit-lets (Hirose 1981). Usually, after an
extremelyheavycrop,alternatebearingcanresultin
treedeclineandevencollapseinsomeextremecases
as‘Kinnow’mandarin(Jones et al. 1975).Competition
forcarbohydratesreservesamongsubsequentcrops
is probably one of the major cause but results are
inconclusive (Monselise and Goldschmidt 1982).
There is an inverse relationship reported between
fruitsizeandower numberandfruitnumberper
tree (Guardiola 1992). Consequently, ower and
fruitthinning,bothmanually andchemically,have
been used to improve fruit size (Zaragoza et al.
1992). Hand thinning requires a lot of labour and
time.Whereas,chemicalthinninghasbeen desired
strategyforcultivarsthatbearprofuselyinorderto
avoid the eects of alternate bearing (Hirose 1981;
Monselise et al. 1981; Gallasch 1984). A signicant
increaseinfruit sizeoccursonlyif fruitthinningis
considerable and performed suciently early in
fruitdevelopment(Zaragoza et al. 1992).
Themostwidely tested growthregulatorsused for
thinningareethephon(Wheaton1981,Gallasch1988),
NAA (Hirose 1981, Monselise et al. 1981, Wheaton
1981,Gallasch1988, Guardiola et al. 1988).Inorder
toavoidoverthinning,treatmentsshouldbeapplied
earlyinthemorningorlateintheafternoonandonly
during a period in which excessive temperatures
(>30°C)arenotcommon(Hirose1981).Inadditionto
anincreaseinfruitsize,advantagesofthinningalso
include prevention of tree collapse, which ensures
treesurvival.Inthiscase,evenalossinimmediate
protcanbejustied(Monselise et al. 1981).
Fruit-Set
After successful fertilization the early changes in
owerarefollowedbyfruit-set(Petho1993).Theearly
changesoftheowerafterasuccessful fertilization
are the signs of fruit set. In some cases, the mere
fact of pollination may also initiate the growth of
the ovary. Without fertilization, the degeneration
of the ovary is expected, which is followed by the
deathandabscissionoftheowertoo.Flowerdrop
is caused by the appearance of ethylene-produced
autocatalytically.
The pollinated ower develop to a fruit, and the
fertilized ovules grow to seeds due to an intense
synthesis of growth substances. The intense cell
division and growth of the tissues absorbs a lot of
organic maer of the reserves competing with the
vegetative organs, consequently, an interaction
betweenthedierentpartsoftheorgansofthetree
is building up. Not only the fate of the growing
fruit,itssizeand quality butalsothephysiological
potentialofthewholetreeisinuencedbyrelations
ofsourcesandsinks,whichinturn mayimpairthe
maintenanceofthefruitsset.Thegrowthsubstances
induce the growth and thickening of the peduncle
too(Krezdon1973,Saleemetal,2008,Jain et al. 2014)

Quality and Quantity Improvement of Citrus: Role of Plant Growth Regulators
437
Fruit Growth
Fruitsizeisadesirablecharacterforthemarketability
ofcitrus.Smallfruitsizeisverycommoninmandarins.
Thevariationinfruitsizeresultsfromdierencesin
cellnumberorcellsizeoracombinationofboth.The
potentialforincreasingfruit size by enhancingcell
divisionearlyisarelativelyundevelopedapproach.
Treatment with plant growth regulators such as
gibberellins and cytokinins during or shortly after
ower opening enhances early fruit growth (EI-
Otmani et al. 1992; Guardiola et al. 1993)butdoes
not always result in an increase in fruit weight at
harvest.Thisearlyincreaseinfruitsizeisduemainly
to a transient increase in cell division in the ovary
wall.Earlyapplicationofauxinsincreasesnalfruit
sizemoreconsistently(Agusti et al. 1992;1996,1998;
EI-Otmani et al. 1993;Aznar et al. 1995).
Theratesofapplicationoftheplantgrowthregulator
varieswithcultivar,stageoffruitdevelopmentand
climatic region. It is important to ensure optimum
tree nutrition and irrigation as well as to provide
full tree coverage during application. The mode of
action of these plant growth regulators has been
summarizedbyAgustiet al.1996,butitisimportant
to note that they increase vesicle size, not number
(Agusti et al. 1992, 1996; EI-Otmani et al. 1993),
and that their eect on vesicle size is through an
enhancement of cell enlargement, not cell division
(EI-Otmaniet al.1993;Agustiet al.1996).Developing
fruit are important sinks for water and photo
assimilates;theirsinkstrengthisincreasedasaresult
of auxin treatment, with all fruit tissues increasing
in proportion. Auxin application increases fruit
pedunclesizethroughanincreaseinphloemcellsize
(Agusti et al. 1996) consistent with increased sink
strengthandcarbohydratemovementintothefruit.
Reduced fruit growth rate during the early stages
offruit developmentwaspositivelycorrelated with
fruitabscission,particularlywhenfruitweretreated
duringtheJunedropperiod(Agustiet al.1995).
Fruit Drop
AccordingtoSoostandBurne(1961)theexogenous
applicationofGA3improvestheparthenocarpicfruit
set and growth of self-incompatible genotypes like
Clementine that shows negligible parthenocarpic
fruit set in the absence of cross-pollination El-Sese
(2005)conrmedthatthenumber of fruits pertree
andthetotalyieldincreasedwithGA3ascomparedto
control.Thefruitsetwassignicantlyaectedbythe
GA3treatmentindividuallyandalsoincombination
withauxinand gavethemaximum fruitsetof 32.3
per cent (Saleem et al. 2005). The GA3 and 2,4-D
areusedinabscissiontocontroltheoverloadingof
fruitforbeer qualityoffruits(Iglesias et al.2007).
The application of 2,4-D after fruit set results in
accelerationofabscission(Lee2003).Theroleof2,4-D
dependsonexternalandinternalfactors.According
toIglesias et al.(2007), decreasein GAand increase
the ABA level shows that both these plant growth
substancescontroltheinitialfruitsetandJunedrop
along with other essential component. Randhawa
et al.(1961)reportedthat2,4-Dreducedpre-harvest
drop in Jaa oranges. Bajwa et al. (1971) reported
that in sweet orange cv. pineapple when sprayed
with 2,4-D (20ug/ml) or NAA, both plant growth
substancecontrolthepre-harvestdrop.Accordingto
Randhawaet al. (1961)application of2,4-D(15and
20µg/ml) and 2,4,5-T (5 and 10µg/ml) reduced the
fruitdropinLahorelocalandNagpurmandarin.
Theenvironmentalconditionslikehightemperature
andlowhumidityinsoilandinairgreatlyinuence
theJunedrop(Levi1964;Davieset al. 1981).Knapp
(1996)reportedthattheapplicationof2,4-Disopropyl
ester acid at 60 to 70g /ha for during 6 to 8 weeks
afterbloomreducedthesummerdrop.Summerfruit
drop usually occurs from month of mid-Augustto
Octobertillmaturity.
The abscission of fruits is generally coupled with
structural changes in the plant (Baird and Webster
1996), which means that several abscission zones
are formed within the same inorescence. It has
been postulated that there is appearance of two
or three abscission zones around the same fruit.
Fruitsareabscisedbecausethecellsgetlooseinthe
abscissionzone.Pectin,hemicellulosesandcellulose
are dissolved by the respective enzymes, and the
mechanicalstressdetachthefruits.Sometimes,the

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Bons et al.
deadxylemelementskeepthefruithangingawhile.
Asexternalagents,thewindmayhelpthedrop,but
thedehydrationofsenescentcellscausestensionand
contributestotheprocess.Thescarof thedetached
fruitisgenerallysuberisedorlignied.
Theabscissionzoneisdistinctfromtherestoftissues
notonlyinitsanatomybutalsoinmetabolicterms.
Abscission zone excels by intense cell division,
synthesisofproteinsandRNA,highO2consumption
as well as peroxidase-activity. Important role is
aributed to the middle lamella and the aached
primary cell wall, as being dissolved. The middle
lamella is softened by the enzyme pectin-esterase,
which demethylate the pectin making it soluble
andthecellsareeasilyseparated.Anotherenzyme,
cellulaseisalsoactivatedtogetherwithgalacturonase.
Duringthephaseofabscission,thegenesresponsible
for inducing the synthesis of hydrolytic enzymes
dissolving polysaccharides of the cell wall mainly
as cellulases and pectinases are activated. Those
enzymes dissolve the middle lamella and also the
cellwallsandthecohesionofcellsisweakened.
Followingtheowerdrop,fruitdropensuingbefore
maturityisaributedtothecollapseofthehormonal
balance in the growing fruits, where the growth
substancesbeingactiveinfavourofgrowthlosttheir
inuence against abscisic acid causing abscission.
TheroleofABAbecomesprevalentwhentheyoung
fruit-lets drop, and, when fruits are almost ripe.
Thedierencebetweenvarietiespronetofruitdrop
correspondedtotheirABAcontenttoo.
Theabscissionofyoung fruits seemstodependon
auxin as a correlative dominance signal (Bangerth
1990). The investigations of Luckwill (1953) made
early reports in searching the causal relations
between auxin production of the developing fruit
andfruitdrop. TheyexplainedthatIAAmayspeed
uporalternativelyinhibittheprocessofabscission.
Lackofnutrients,whichdoesnotdependsolelyonthe
meagresoil,butratheronthecompetitionbetween
thevegetativeorgansandthe growingfruitsof the
plant.Thefateofayoungfruitisoftenimpairedby
thedominanceofanotherfruitorvigorousshoots.
The abscission of fruits is generally coupled with
structural changes in the plant (Baird and Webster
1996),whichmeansthatseveralabscissionzonesare
formedwithinthesameinorescence.
Gibberellins are known for their ability to increase
cell enlargement, thus enhancing fruit growth in
certain species such as citrus (Eman et al. 2007, El-
Sese2005),guava(El-SharkawyandMehaisen2005)
and pear. In all species so far studied, gibberellins
had the potential for increasing fruit size. Salicylic
acid is considered to be a potent plant hormone
(Raskin1992)becauseofitsdiverseregulatoryroles
inplantgrowthregulatorand its role isevidentin
fruit yield (Klessig and Malamy 1994). Application
of plant growth regulators like gibberellins and
salicylic acid alone or coupled with micronutrients
mayimprovecroppingpotential.
Early reproductive processes in citrus are strongly
aected by plant growth regulators indicating that
theregulatorymechanismsthatcontrolfruitsetand
abscissionofovariesandfruitletspossessesapivotal
hormonalcomponent(Talonet al.1990).Gibberellins
and cytokinins are generally considered to be
positiveregulatorsoffruitgrowthwhileauxinshave
been reported to act as stimulators of growth and
also as abscission agents (El-Otmani and Oubahou
1996). Abscisic acid (ABA) and ethylene have been
implicatedinseveralwaysinabscission.Gibberellins
arethoughttobepivotaleectorsresponsibleforthe
ovary-fruittransition(Talonet al.1992, Ben-Cheikh
et al. 1997). They activate cell division and cell
enlargement processes in vegetative organs (Talon
et al. 1991) and therefore are generally associated
with the initiation of growth (Talon and Zeevaart
1992). The intensity of abscission during the initial
phasesofgrowthisalsorelatedtothephenologyof
owering.
Salicylicacidandothersalicylatesareknowntoaect
various physiological and biochemical activities of
plants and may play a key role in regulating their
growthandproductivity (Arberg1981).Exogenous
application of salicylic acid may also inuence
seed germination and fruit yield. Sivakumar et al.
(2002) determined that the application of salicylic

Quality and Quantity Improvement of Citrus: Role of Plant Growth Regulators
439
acid increased the content of protein in grains of
pearlmillet.Kalaraniet al.(2002)carriedouta pot
cultureexperimenttodeterminetheeect of foliar
application of dierent concentrations of salicylic
acid on physiological and biochemical constituents
as well as yield and quality of tomato and found
that Salicylic acid at all concentrations showed its
eciencyininducingearlyoweringandincreased
fruitsetpercentage.
Besides, hormonal induction of fruit growth,
nutrients may have regulatory functions on their
own and/or through the maintenance of adequate
hormonal levels (Gillaspy et al. 1993). The average
number of owers produced in a normal citrus
tree is by far extremely high in comparison to the
number of fruits that the same tree can support
until ripening. Hence, many fruits are abscised
during growth apparently due to competition for
nutrients especially photo-assimilates. During the
initial moments of phase I, citrus fruitlets function
as carbohydrate utilization sinks but over the nal
stagesofthisperiodandduringthetransitionfrom
cell division to cell enlargement, developing fruits
shifttheirmetabolismandstarttobehaveasstorage
sinks (Mehouachi et al. 1995). Defoliation during
phaseIreducescarbohydrateamounts,arrestsfruitlet
growthandpromotesmassiveabscission(Mehouachi
et al. 2000)whereas,defoliation after theJunedrop
also arrests growth but does not induce abscission
(Lenz 1967). The link between carbohydrates and
fruitgrowthiscurrentlysupportedbyawidebody
ofevidenceincludingseveralstudiesonsource-sink
imbalances, defoliation, girdling, shading, sucrose
supplementation, de-fruiting and fruit thinning
(Goldschmidt and Koch 1996, Iglesias et al. 2003,
Syvertsen et al. 2003). First, the enhancement of
carbohydrate availability was associated with an
improvement of fruit set and yield of citrus trees
(Goldschmidt 1999). Later, a strong relationship
was demonstrated between carbohydrate levels
availabletofruitletsandtheprobabilityofabscission
(Gomez-Cadenas et al. 2000, Iglesias et al. 2003).
This phenomenon that has also been described for
other tree species is also supported by studies on
translocationof14CmetabolitesandCO2-enrichment
experiments(Mosset al.1972,Downtonet al.1987).
Hence,photosynthesisactivityhasbeenprovedtobe
crucialsincehighcarbohydraterequirementsduring
fruitsetincreasesphotosyntheticrate (Iglesiaset al.
2002).Thissuggestionalsoimplies thatareduction
in net CO2 assimilation results in lower sugar
production and fruit set. The sugar concentration
inleavesmightbethesignalthatregulatesthefeed-
back mechanism stimulating photosynthesis in
responsetofruit sugar demand.Thus,oncecarbon
demands are fullled, carbohydrate accumulation
may elicit end-product feedback control of
photosynthesis.ThepositiveeectofexogenousGAs
on fruit set and growth may also partially operate
through the induction of a stronger mobilization
of14Cmetabolites toovaries(Powell andKrezdorn
1977). The exogenous GAs have also been shown
to stimulate growth and increase carbon supply in
vegetativetissues (Mehouachi et al.1996).Thus the
sugars are deeply implicated in the regulation of
fruitlet growth and that overall carbon deciency
inducesfruitabscission.
Although the specic mechanism involved in the
response of fruit growth to carbohydrates has not
been studied at the molecular level, observations
suggest that sugars may act not only as essential
nutrientfactorsbutalsoassignalstriggeringspecic
hormonalresponses(Roitsch1999).Theobservation
linkingcarbohydrateandabscissionwasconrmed
withthending thatcarbonshortageduringovary
and fruitlet drop increased ABA and ethylene and
bothareinvolvedintheinductionofearlyabscission
(Gomezet al.2000).
The alterations in the nutrient balance that are
accompanied with increased fruitlet abscission
during the June drop provoke an unambiguous
tendency to both increase nitrogen content and to
reduce carbon shortage. Abscission intensity may
becorrelatedpositivelywithcarbohydrateshortage.
The two main Conclusions that can be extracted
are that the fruit fall that takes place during June
isverylikelydue to thecarbohydrateinsuciency
caused by an increased carbon demand of a huge
population of expanding fruitlets; and second,

440
Bons et al.
carbondeciencyisagainassociatedwithABArise,
ethylene release and massive fruitlet abscission.
Thisideathatcitrusfruit abscissionisconnectedto
carbohydrateavailabilitywasinitiallyanticipatedby
Goldschmidt and Monselise (1977) who suggested
thatcitrusmightpossessaninternalself-regulatory
mechanism that adjusts fruit load to the ability of
the tree to supply metabolites. The above ndings
identify leaf sugar content, ABA and ethylene as
major components of the self-regulatory adjusting
mechanismofabscission. Recently,ithasalso been
proposedthatinadditiontoJunedrop,earliestovary
andfruitletfallsthatoccurthroughabscissionzone,
are also dependent upon nutritional factors such
sugars(Iglesiaset al.2006).
Literatureindicatesthattheapplicationofnutrients
and plant growth regulators increase the fruit
yield and quality, hence the suitable combination
of macronutrients, micronutrients and growth
regulatorscouldcontroltheexcessivefruitdropand
improvethecitrusfruityieldanditsquality.
Fruit Quality
Fruitqualityisaconceptthatvariesaccordingtothe
naluseofthefruitandatwhatpointfromorchard
toconsumerthefruitisevaluated. For thegrower,
anyfruitthatcanbesoldatareasonablygoodprice
is of good quality. For the fruit packing industry,
fruitsthatareofuniformsize,freeofblemishesoer
good market. The harvested fresh fruit requires to
beseedless(lowseednumber),highTSS:Acidratio
and excellent color, shape and rmness. Whereas,
the requirement of the juice industry is fruits with
highjuiceandsugarcontent. Parametersimportant
toboththefreshfruitandjuiceindustrieshavebeen
showntorespondtoplantgrowthregulators.
Thejuicecontentandorganoleptictasteareimportant
parametersanddesirableforfreshfruitconsumption
ofcitrus.Organolepticqualityis theresultofsugar
and acid content, and the presence of volatiles in
the juice. Nutritional quality includes sugar, acid,
vitamins,etc.Forfruitprocessing,internalqualityis
farmoreimportantthantheexternalappearanceof
the fruit. Plant growth regulators that can be used
tomanipulate theseparametersincitrusarealmost
non-existent.Althoughnotaplantgrowthregulator,
leadarsenatewasusedformanyyearsinFloridato
improve fruit internal quality (Knapp 1996). This
compound causes a reduction in total acidity and
consequently,anincreaseinthesugar-to-acidratio.
Granulation of fruit is a physiological condition
in which the juice sacs become gelled with lile
extractable juice It develops pre-harvest (Smoot
et al. 1971) as well as post-harvest (Gilllan and
Stevenson1977).AreductionofthedisorderbyGA3
sprayshasbeenreported,butthiseecthasnotbeen
reproducedconsistently
AccordingtoKauret al.(2000)fruitweightincreased
withincreaseinamountof2,4-DintreesofKinnow
mandarin. The fruit weight and peel thickness has
beenincreasedwiththeapplicationofZnalone or
in combination with GA3 in ‘Washington Navel’
orange(Emenet al.2007).TheapplicationofZnand
Kincreasedfruit weightascompared withcontrol.
According to Sourour (2000) both Zn and EDTA
increasedthenumberandweight offruitspertree.
According to Chundawat and Randhawa (1972)
GA increased the fruit size in Saharanpur special
grapefruittrees.Theapplicationof2,4-Dat20µg/ml
alongwithCuSO4at0.25or 0.50 percent increased
thefruitsize(SinghandMishra1986).
ChundawatandRandhawa(1972)reportedthatGA3
and 2,4-D increased peel thickness in Saharanpur
special variety of grapefruit. Also Dinar et al.
(1977) observed that both gibberellic acid and 2,4-
D increased peel thickness in Marsh grapefruit.
Applicationofnutrients increasedthejuicecontent
(Ram and Bose 2000). Maximum juice from fruits
wasobserved fromthetreessprayed with1%urea
and 0.8% Zinc sulphate (Malik et al. 2000). Trees
treated with gibberillic acid yield juice more than
10%thanthecontrolones (Davieset al.2001).Such
increasewaseconomicallybenecialtoFloridacitrus
growersbecauseprocessedfruitvalueincreasewith
juiceyield(Braddock1999).
The application of Zn alone and with combination
withFeandMnincreasedtheTSSvalueascompared

Quality and Quantity Improvement of Citrus: Role of Plant Growth Regulators
441
tocontrol.AccordingtoMongaandJosan(2000)the
highest TSS was obtained from tree sprayed with
Zn (0.3%) alone. Treatment with 2,4-D at 20µg/ml
andgibberllicacidat20µg/mlincreasedtheTSSin
grapefruitjuice(Llaneset al.1991).
Foliar application of Zn alone and along with Fe
andMnonKinnowmandarinresultedin decrease
acidity as compared to control (Monga and Josan
2000). According to Singh and Mishra (1986) 2,4-D
increased the acidity level in Kinnow fruit. Age of
treeandtype ofcultivaralsoinuencedtheacidity
oforanges(FrometaandEchazabal,1988).
Foliar application of Zn was observed to increase
the ascorbic acid contents of juice in various citrus
varieties (Dawood et al. 2001). Increased content
of ascorbic acid was observed with application of
Znalone,Zn+Mn orZn+B(Tariq et al.2007).High
ascorbic acid was obtained by sprays of Zinc and
gibberellicacid(Emanet al.2007).Chundawat and
Randhawa (1973) reported that vitamin C content
increasedwith sprayof2,4-DinDuncancultivarof
grapefruit. Immature citrus fruits have the highest
amountofascorbicacidwhereas,ripenedfruithave
the least as reported by Nagy (1980). However,
ascorbic acid content increased with ripening of
fruitsinapricot,peachandpapayasbutdecreasedin
applesandmangoes(LeeandKader2000).
The TSS:Acid ratio of juice was increased with
theapplicationofSA,ZnandKalone and also by
combination of both. Abd-Allah (2006) reported
that the application of K in combination with
micronutrientsimprovedTSS:acidratiowhileZn+K
and other dierent combinations was observed by
Ashrafet al.(2012).
Among sugar derivatives; sugar nucleotides, sugar
phosphates, glycosides and polyles are important.
D-galacturonicacid, D-Glucuronic acid,L-ascorbic
acidaresugaracidswerefoundincitrusfruits.Sugar
alcoholssuchasmyo-inositolwasfoundinoranges
and grapefruits in the range of 88-170 mg/100g of
juice Lemons had 56-76 mg myo-inositol/100g of
juice. The reducing sugars, non- reducing sugars
and total sugars increase as the fruit start to ripe.
Fructose,alphaandbeta–glucoseandsmallamount
ofgalactosewerealsofoundinValenciaorangejuice
By using 14C labelled compounds, Sawamura and
Osajima(1973)observedthattranslocationofsugars
fromleafto fruitoccursin theformof glucoseand
fructose which are further changed into sucrose in
thefruit.Kuraokaet al.(1976)foundchangesinsugar
content of avedo tissue of Satsuma mandarins
growninJapan.TingandAltaway (1971) reported
thattheratioofFructose:Glucose:Sucroseas1:1:2
inValenciaoranges.
Production
Optimumyieldisayieldthatissustainableyearafter
yearandthatutilizesthefullpotentialoftheavailable
landalongwiththeenergyandnutritionalresources
ofthetree.This yield is afunctionoftree planting
density,canopydevelopment,intensityofowering,
fruitset,fruitgrowthandnumberoffruitsharvested
at maturity. In young trees, a high proportion of
photosynthateisallocatedtovegetativedevelopment
andgrowth.Usually,only afewowersand fruits
areproducedduring the2to 3yearsafter planting
a nursery tree. Canopy development and growth
continues and reaches an optimum between 5 and
12 years, depending on planting density, at which
timeyieldisatitshighestvalue(Boswellet al.1970
1975). At this point a natural equilibrium between
vegetative growth and reproductive development
isestablishedwithvariationsduetoenvironmental
conditions and genotype of the cultivar, provided
thatnutritionandlightinterceptionareoptimal.
Theproductionperunitareaisthekeyfactorforthe
citriculturists. However, the productivity depends
uponfactorsviz.,lightinterception, carbondioxide
xation, water availability, and mineral nutrient
uptake.Besides, theproductivityisalsoaectedby
competitionamongsimilarordierentplantorgans
for photo assimilates and nutrients. Plant growth
regulatorshavebeen used toimproveproductivity
wherever feasible. Unfavourable environmental
conditions may restrict many of the processes
related to production from ower initiation (Moss
1969; Southwick and Davenport1986; Lova et

442
Bons et al.
al. 1988) to the development of inorescences and
owersormaycauseexcessivedropofowerbuds,
openowers,entireinorescences,anddeveloping
fruitlets.Plantpathogens may causeexcessivefruit
drop and may deteriorate fruit quality (Lima et al.
1980).Imbalanceoffertilizersthatmaysupplyexcess
ofnitrogen leadto excessivevegetativegrowthand
low ower number. On the other hand, nitrogen
deciency leads to low ower initiation and thus
reducedyields(Davenport 1990).
Plant growth regulators are used to reduce the
seasonaluctuationsin yieldandalso tomaximize
energy allocation to harvestable fruit in order to
increase fruit number and fruit size rather than to
enhanceexcessivevegetativegrowth. Plantgrowth
regulators can be used to hasten or delay fruit
coloring and fruit maturation so as to shorten or
extendtheharvestseason to get the best economic
productivity.(Jainet al.2014)
Yield (Kg per tree) includes both fruit number
andfruitsize.Fruit number is afunctionofower
intensityandfruitset.Fruitsizeisafunctionofcell
divisionandcellenlargementprocesses.Thenumber
offruitthatsetandpersisttoharvestinuencesfruit
size.Citrusfruitgrowthisacontinuousprocess,fruit
weightandfruitdiameteris representedbyasingle
sigmoidal growth curve (Guardiola and Lazaro
1987).Thedevelopment oftheovary intoamature
fruitproceedsthroughthreestages:celldivision,cell
enlargement, and maturation. During Stage I, cell
divisionpredominates.Cell divisionisessentialfor
the formation of theovary during owerinitiation
and continues past petal fall to approximately
mid-JunetolateJuly(Lova 1999).Atthis timecell
division,whichoccursinallthetissues,iscompleted
except in the avedo. Thus, during the period
encompassingfruitset andJunedrop, fruitgrowth
isdominatedbycelldivision.StageIIistheperiodof
maximumfruitgrowthoccurringoverthefollowing
3 months for the early maturing cultivars (e.g.,
‘Satsuma’mandarin)andthenext7to10monthsfor
thelatematuringcultivars (e.g., ‘Valencia’ orange).
This stage of fruit development is dominated by
cell dierentiation and cell enlargement. The juice
vesiclesandloculesincreaseinsizewiththeuptake
of water, and the avedo continues cell division,
whereas the cells of the albedo must expand and
stretch to accommodate the growth of the locules.
Drymaerandwateraccumulateinthevacuolesat
ahighrate(Guardiola1992).Fruitthatpersistonthe
treethroughtheJune drop period arenotlikelyto
absciseintheabsenceofpestdamage,physiological
disorders such as spliing, or stress such as water
decit and excess temperatures. Stage III is the
periodoffruitmaturation.Fruitcontinuetoincrease
in size predominately by cell expansion, but at a
slower rate. Peel thickness may increase in some
cultivarsandpunessmayoccur,particularlyinthe
mandarintypes.
According to Bengal et al. (1982), the combined
application of NAA (25ug/ml) and urea (1%)
increasedtheseedyield.FoliarapplicationofNAA
onpaddy(Oryza sativaL.)under low level of N (0
and60kg/ha)gavethebenecialeectsasreported
by Grewal and Gill (1986). The benecial eect
wasobserved byincreasein numberofear-bearing
shoots/plant,grainweight, numberoflledgrains/
panicle.Thesucientnitrogensupplywasessential
forincreasingtheyieldof rye and barleybymono
ethanolamine(BergmannandEckert1990).
Conclusion
Plantgrowthregulatorsarecommerciallyexploited
intheeldofhorticulturebecauseofthewiderange
ofpotentialrolestheyplayinincreaseofproductivity
per unit area. Their application in citrus industry
duringdierentphasesofgrowthanddevelopment
iscurrentlyinpracticeasisevidentfromthepresent
literature.Theadvantageofplantgrowthregulators
is their use at very low concentration because of
whichtheydonotlayanyhealthhazards.However,
itisimportanttounderstandthebasic mechanisms
underlying the citrus growth and development in
ordertomanipulatethekeyphysiologicalprocesses
and make use of the plant growth regulators at
the appropriate stage of development and at the
optimum dose. Therefore it is desirable to persue
researchonthemechanismofactionofplantgrowth

Quality and Quantity Improvement of Citrus: Role of Plant Growth Regulators
443
regulators on the physiological, biochemical and
geneticregulationofgrowthanddevelopment.
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