The effects of drought on rice cultivation in sub-Saharan Africa and its mitigation: A review

Abstract

Drought is the primary cause of yield loss in agriculture throughout the world, and is currently the most common reason for global food shortages. Three-quarter of the most severe droughts in the last ten
years have been in Africa, the continent which already has the lowest level of crop production and drought adaptive capacity. The increased incidences of drought and erratic rainfall have thrown small holder farmers in Africa into deep poverty, hunger and malnutrition. In this paper, the drought situation in sub-Saharan Africa and its impact on rice production was reviewed. Rice is particularly vulnerable to droughts as it has higher water requirement as compared to other crops. The review has also highlighted physiological and molecular plant responses to drought, with special focus on effects of drought stress on rice grain yield and other related-traits. With climate change predicted to exacerbate the problem of water security in Africa, it is imperative that we develop robust, well-planned and informed strategies to mitigate against drought. Various drought mitigation strategies including breeding for drought tolerance and water harvesting and conservation techniques are also outlined. In order to adapt to drought, there is need for a broad based approach that includes development of appropriate policies, putting in place necessary water related investments and institutions as well as capacity building at various levels.

Full text

Vol. 13(25), pp. 1257-1271, 21 June, 2018
DOI: 10.5897/AJAR2018.12974
Article Number: 23BA03B57525
ISSN: 1991-637X
Copyright ©2018
Author(s) retain the copyright of this article
http://www.academicjournals.org/AJAR
African Journal of Agricultural
Research
Review
The effects of drought on rice cultivation in sub-
Saharan Africa and its mitigation: A review
Ndjiondjop Marie Noelle1*, Wambugu Peterson Weru2, Sangare Jean Rodrigue1
and Gnikoua Karlin1
1Africa Rice Center (Africa Rice), 01 B. P. 2031, Cotonou, Benin Republic.
2Genetic Resources Research Institute, Kenya Agricultural and Livestock Research Organization (KALRO), P. O. Box
30148-00100, Nairobi, Kenya.
Received 4 Janaury, 2018; Accepted 21 February, 2018
Drought is the primary cause of yield loss in agriculture throughout the world, and is currently the most
common reason for global food shortages. Three-quarter of the most severe droughts in the last ten
years have been in Africa, the continent which already has the lowest level of crop production and
drought adaptive capacity. The increased incidences of drought and erratic rainfall have thrown small
holder farmers in Africa into deep poverty, hunger and malnutrition. In this paper, the drought situation
in sub-Saharan Africa and its impact on rice production was reviewed. Rice is particularly vulnerable to
droughts as it has higher water requirement as compared to other crops. The review has also
highlighted physiological and molecular plant responses to drought, with special focus on effects of
drought stress on rice grain yield and other related-traits. With climate change predicted to exacerbate
the problem of water security in Africa, it is imperative that we develop robust, well-planned and
informed strategies to mitigate against drought. Various drought mitigation strategies including
breeding for drought tolerance and water harvesting and conservation techniques are also outlined. In
order to adapt to drought, there is need for a broad based approach that includes development of
appropriate policies, putting in place necessary water related investments and institutions as well as
capacity building at various levels.
Key words: Drought, tolerance, rice, sub-Saharan Africa, quantitative trait loci (QTL), mitigation, adaptation.
INTRODUCTION
Drought is inadequacy of water availability including
periods without significant rainfall, causing a reduction in
available water, thereby affecting crop growth. It can also
occur when atmospheric conditions cause continuous
loss of water by transpiration or evaporation (Singh et al.,
2012), also indicated as a period of dry weather that is
injurious to crops. In this context, drought is related to
changes in soil and meteorological conditions and not
with plant and tissue hydration (Lipiec et al., 2013).
Drought is defined as a situation that lowers plant water
potential and turgor to the extent that plants face
difficulties in executing normal physiological functions
(Lisar et al., 2012). Whatever the definition given to
drought, it remains perhaps the most serious natural
*Corresponding author. E-mail: m.ndjiondjop@cgiar.org. Tel: 225 31 63 25 78.
Author(s) agree that this article remain permanently open access under the terms of the Creative Commons Attribution
License 4.0 International License

1258 Afr. J. Agric. Res.
hazard, affecting a larger proportion of the human
population than any other hazard. It is the most
significant environmental constraint for rice production in
sub-Saharan Africa (SSA) (Reynolds et al., 2015). Its
severity mainly depends on the level of moisture
deficiency and the duration.
The challenge of drought is even greater for crops such
as rice when compared with other crops such as maize
and wheat, as it has relatively higher water needs
(Todaka et al., 2015). Rice is sensitive to deficit in soil
water content because rice cultivars have been
historically grown under flood irrigation conditions where
the soil matric potential is zero. About 3,000 to 5,000 L of
water is required to produce 1 kg of rice seed, with less
than half of that amount needed to produce 1 kg of seed
in other crops such as maize or wheat (Bouman et al.
2002). Moreover, as compared to several other field
crops, rice has relatively weak resistance to drought and
its production systems is more vulnerable to drought than
other cropping systems (O’Toole, 2004). In Africa,
drought has adversely affected agriculture in different
parts of the continent, with production of rice declining in
many parts of West Africa due to increasing water stress
(Bates and Kundzewicz, 2008). Drought has had significant
negative effect on the livelihood of rainfed lowland rice
farmers. The increased occurrence of prolonged droughts
in SSA is a worrying trend as the region is highly
dependent on rainfed agriculture. In order to enhance
sustainable crop production in the face of drought and the
constantly changing climatic conditions around the world,
there is need for constant efforts to adapt our crops and
production systems to the existing and emerging
environmental challenges. In this review, the challenge of
drought and specifically how it impacts rice production in
SSA was discussed. Measures that can be undertaken to
mitigate the effects of drought are also highlighted.
DROUGHT SITUATION IN AFRICA
The greatest challenges to agricultural production and
food security in Africa is drought and climate change.
Agriculture in Africa is mainly dependent on rainfall, with
only about 5% of Africa’s total cultivated land being under
irrigation (You, 2008), meaning the region is highly
vulnerable to drought. In some sort of fate, drought which
continues to degrade some of the most agriculturally
productive environments, is predicted to most severely
affect the most vulnerable populations particularly those
in SSA (FAO/PAR 2011). The recurring droughts in Africa
are negatively impacting the livelihoods of a huge
proportion of the population, with about 25% of the
population facing serious water scarcity (Jarvis et al.,
2009). Drought and climate variability are leading to the
emergence of novel ecosystems where various plant
populations are unable to persist. The proportion of arid
and semi-arid areas continues to increase and it is
projected that by 2080, ASAL areas in Africa will increase
by 6 to 8% (Jarvis et al., 2009). The continued increase in
ASAL areas and the emergence of novel ecosystems
could render large sections of land unproductive thereby
seriously impacting agricultural production in Africa.
Perhaps, the greatest factor contributing to droughts is
the rapidly growing human population, with the latest
World Bank projections indicating that by 2060, about 2.8
billion people will be found on the continent (Canning et
al., 2015). This increase in population puts enormous
pressure on the available resources. It will for example
lead to opening up of agricultural lands and other
productive ecosystems for human settlement, thus
leading to loss of valuable biodiversity. Loss of these
genetic resources will reduce the diversity of plant
responses to biotic and abiotic stresses thereby reducing
the resilience and sustainability of agricultural production
systems.
Three-quarters of the most severe droughts in the last
ten years have been in Africa, the continent which
already has the lowest level of crop production.
Moreover, this region has the lowest drought adaptive
capacity and among the highest levels of poverty, with
about 48% of the total population living on less than
$1.25 a day (Ravallion et al., 2012). This means that this
segment of the human population lacks not only the
technical capacity to deal with drought but their financial
means to address these challenges is also severely
limited. Based on this sad reality and predictions of
climate change models, the drought situation in Africa
does not look promising. The challenge ahead is hugely
enormous but with the concerted efforts of all
stakeholders, it will be manageable. Successful fighting
of droughts is doable.
RICE PRODUCTION AND CONSUMPTION IN AFRICA
Rice is cultivated under a broad range of environmental
conditions in terms of topography, soil type, water regime
(various degrees and duration of drought) and climatic
factors (Khush, 1996). The persistent droughts in SSA
have negatively impacted agricultural production
systems, with rice production being among the worst hit
systems since the crop is more sensitive to droughts than
other crops. The situation is particularly worse in SSA
where rice is largely grown under rainfed conditions that
rely solely on precipitation, making it vulnerable to
droughts. Due to this sensitivity, rice yields reduce
significantly even under mild drought (Guan et al., 2010).
Moreover, rice varieties planted in Africa have only
relatively few adaptations to water-limited conditions and
are extremely sensitive to drought, thereby worsening the
situation. In Africa, the ecosystems under rice cultivation
range from rainfed upland (40% of total area), rainfed
lowland (38%), irrigated lowland (12%), deep
water/floating (6%) to mangrove swamps (4%). Upland

and lowland rice production which constitute about 80%
of the total rice production area in Africa are projected to
have the greatest vulnerability to drought (Bimpong et al.,
2011a).
Worldwide, more than 3.5 billion people depend on rice
for more than 20% of their daily calorie intake (Ricepedia,
2011; Maclean et al., 2013). Rice production is becoming
increasingly popular in SSA, especially with the recent
release and promotion of new, popular varieties of
NERICA (New Rice for Africa) by the Africa Rice Center
(formerly known as WARDA). An annual increase in rice
consumption of about 6% has been reported (Bernier et
al., 2008). With the high urbanization and increase in
purchasing power, West Africa is experiencing a
significant increase in rice consumption in urban and rural
areas.
This increased consumption has also been followed by
a concomitant increase in rice production in most African
countries. The last 3 decades have recorded a dramatic
increase in rice production in Africa, with the production
more than doubling in the period between 1982 and 2012
(FAO, 2013). However, despite the increased paddy rice
production and the huge potential for rice production in
terms of available land area that exists in the sub-region,
massive rice imports into SSA are still recorded (Nasrin
et al., 2015; AfricaRice, 2009, 2011; Futakuchi et al.,
2011). Rice production in West Africa covers only about
60% of the population’s needs. This has resulted in
increasing rice imports from Asia. With the current trends,
according to FAO estimates (Staatz and Dembele 2007),
rice imports in West Africa will increase from 6.4 Mt in
2008 to 10.1 Mt in 2020. It is imperative that measures
are put in place to boost rice productivity in SSA. These
include use of adapted high yielding rice varieties,
improved husbandry practices and adoption of various
drought and climate change mitigation strategies. Local
rice production, processing and marketing will permit
African citizens to have access to affordable food. This
will contribute to extreme poverty reduction and
elimination of food insecurity within the continent, since
relying on imports is no longer a sustainable strategy.
EFFECT OF DROUGHT ON YIELD AND PHYSIOLOGY
OF RICE
The yield potential of a cultivar under favourable
conditions is important in determining the yielding ability
under water stress. Drought index which provides a
measure of drought related yield loss is an important
criterion that has been used for screening of drought
tolerance genotypes. Evaluation of eighteen rice
genotypes showed reduction in panicle number (72%)
and grain yield (12%) (Swain et al., 2010). Singh et al.
(2010) evaluated six generations (P1, P2, B1, B2, F1 and
F) of six crosses of rice under drought and irrigated
conditions and observed a reduction in several characters
Noelle et al. 1259
including grain yield under drought conditions. The
intensity of drought effect on various traits varied with the
genetic materials. The study indicated strong relationship
between grain yield under drought, leaf rolling and leaf tip
burning for moderately tolerant introgression lines and
also between grain yield and leaf rolling for tolerant Oryza
glaberrima. Similar findings were reported by Ndjiondjop
et al. (2012). This explains the role of leaf rolling and leaf
tip burning potential of a genotype on its development.
Yield decreases are a result of drought effect on
several morphological and agronomic traits, including
plant height, tillering ability and leaf area (Bocco et al.,
2012). Others include various root traits (length,
thickness and depth), spikelet fertility, panicle exertion,
leaf greenness (SPAD), leaf temperature, time to
flowering, time to maturity, leaf tip drying and leaf rolling
(Ndjiondjop et al., 2010a). Ndjiondjop et al. (2010a)
observed 16.9, 13.7, 6.7, 14.1 and 26.7% reduction in the
number of tillers, plant height, number of leaves, leaf
width and grain yield, respectively. Drought-related
reduction in yield and yield components can be attributed
to stomatal closure in response to low soil water content
with a resultant decrease in carbon dioxide intake and
subsequently a reduction in photosynthesis (Chaves,
1991; Cornic, 2000; Flexas et al., 2004). In summary,
prevailing drought reduces plant growth and
development, leading to hampered flower production and
grain filling and thus smaller and fewer grains. A
reduction in grain filling occurs due to a reduction in the
assimilate partitioning and activities of sucrose and starch
synthesis enzymes.
Garrity and O’Toole (1995) observed an increase in
leaf temperature by 9°C due to drought and significant
correlation between midday leaf temperature on the day
of flowering and both grain yield and spikelet fertility. This
increase in leaf temperature under drought is a result of
lower transpiration rate caused by a reduction (closure) in
stomatal aperture. Leaf temperature is, therefore, a very
sensitive indicator of plant water status and is associated
with leaf stomatal conductance (Jones, 1992). Significant
variations among rice cultivars in leaf temperature
increase under drought are reported. Cultivars with high
drought-avoidance potential consistently remained
coolest under drought (Garrity and O’Toole, 1995).
Under drought, flowering time (start, 50 and 100%
flowering) and time to maturity are delayed as a result of
water shortage. The length of the delays is related to the
type of drought, the temperature regimes, the period of
occurrence of drought and the rice genotype (Bocco et
al., 2012; Wopereis et al., 1996). Spikelet fertility is also
influenced by drought. The production of viable pollen,
panicle exertion, pollen shed and germination and
embryo development, which are involved in fertilization
and initiation of grain filling, are all negatively affected by
drought. This causes reduced spikelet fertility and dry
weight of fertile spikelets thereby leading to grain yield
loss (Liu et al., 2006; Rang et al., 2011).

1260 Afr. J. Agric. Res.
DROUGHT RESISTANCE MECHANISMS
General plant responses to drought
Drought resistance mechanisms include drought escape
via a short life cycle or developmental plasticity, drought
avoidance via enhanced water uptake and reduced water
loss, drought tolerance via osmotic adjustment and
antioxidant capacity.
Escape
The first way for the plant to avoid drought is dodging. It
is an adaptation to the environment allowing the plants to
avoid the critical periods for their good development.
Farmers use this plant strategy to place the crop cycle
when conditions are favourable. For example,
development of varieties with a shorter development
cycle in order to avoid the most stressful periods of the
year for plants or to shift the date of sowing and/or select
varieties to prevent water deficits. This is an important
mechanism for avoiding terminal drought. The shortening
of growth cycle has improved the yield of many varieties
in many annual crop species (Fukai et al., 1999; Turner
et al., 2001). Drought evasion can be achieved through
two mechanisms (i) completing the crop cycle before the
occurrence of a terminal drought; (ii) Avoiding
coincidence between periods of low water availability and
critical or sensitive phases of crop growth where water is
critically required such as flowering and grain filling.
Avoidance
The second way to avoid drought is the ability of the plant
to maintain a satisfactory water state. The reduction in
soil moisture may have led to lower water content in the
leaves causing guard cells to lose turgor pressure and
hence the size of stomatal pores are reduced (Tezara et
al., 2002), causing stomatal closure (Singh et al., 2012).
Avoidance allows plants to limit the effects of stress
through adaptations such as wilting or leaf rolling.
Drought avoidance consists of mechanisms that reduce
water loss from plants due to stomatal control of
transpiration, and also maintain water uptake through an
extensive and prolific root system.
Drought tolerance
From a physiological point of view, drought tolerance is
the ability of the plant to survive and grow under drought.
From an agronomic point of view, a plant is tolerant when
it is able to obtain a higher yield than sensitive plants.
Tolerance allows maintenance of the essential cellular
functions for survival, due to specific and targeted
responses despite the deficiency of water (Passioura,
1996; Tardieu, 2003, 2005). Keeping of turgor in water
deficiency can delay stomatal closure, maintain
chloroplastic volume and reduce leaf wilting which
confers to the plant a better tolerance to internal water
deficit. This tolerance to internal water deficit in turn
allows a prolonged operation of photosynthesis. The
carbon products can then be used for both osmotic
adjustment and root growth. Due to the unpredictability of
water stress, tolerance is the most effective strategy in
severe and prolonged stress situations.
Rice responses to drought stress
Rice responds and adapts to drought stress by induction
of various morphological, physiological and molecular
modifications, with these modifications being made
according to the developmental stage (Figure 1).
Morphological and phenological modifications
In majority of the plant species, water stress is linked to
changes in leaf anatomy and ultrastructure. The first and
foremost effect of drought is impaired germination and
poor stand establishment (Harris et al., 2002). Cell
growth is considered one of the most drought sensitive
physiological processes due to reduction in turgor
pressure. Growth is the result of daughter-cell production
by meristematic cell divisions and subsequent massive
expansion of the young cells (Anjum et al., 2011). Under
drought stress, plants reduce the number of leaves per
plant and individual leaf size as well as leaf longevity by
decreasing the soil’s water potential. Leaf area expansion
depends on leaf turgor, temperature and assimilates
supply for growth.
Rice leaf color plays an important role in leaf
photosynthesis. The reduction in photosynthetic rate in
rice as a result of drought is well documented (Lauteri et
al., 2014). Ndjiondjop et al. (2010a) observed an increase
in leaf greenness value under drought when compared
with full irrigation conditions. However, these
observations contradict those of Zinolabedin et al. (2008)
who reported reduced uptake of water and nutrients by
plant root systems causing reduced chlorophyll
concentration in plant leaves and therefore the yellowing
of the leaves. Under full irrigation conditions, rice leaves
normally do not roll and they do not show tip drying
symptoms either. But under drought, the first response of
the plant is to roll its leaves (Sié et al., 2008) to maintain
a favourable internal water status. Therefore, rice
genotypes with high leaf water maintenance (high leaf
rolling ability) are able to out yield those with lower
ability(Fukai and Cooper, 2002). This explains the
relationship between leaf rolling and grain yield under
drought. Leaf tip drying is also a good indicator of drought

Noelle et al. 1261
Figure 1. Schematic description of rice plant responses under drought stress.
level (Henderson et al., 1995) and just like leaf rolling, is
regarded as a drought avoidance mechanism. The
severity of leaf rolling and leaf tip burning is a function of
the severity of drought especially on very susceptible rice
genotypes. Leaf rolling is reversible but leaf tip drying is
irreversible under drought.
Physiological responses
In response to water deficit, plants are able to establish a
series of physiological responses that allow them to act
on their own water state in order to adapt to
environmental conditions. Some of the physiological
responses to drought include:
Decrease in leaf size: Generally, growth decrease is one
of the first drought manifestations in rice plant. Drought is
manifested in the plant by a slowing down of the initiation
of the new aerial organs (leaves and stems) and a
reduction in the pre-existing organs (Davies and Zhang,
1991; Boyer and Kramer, 1995; Chaves et al., 2002).
Fig. 1: Schematic description of rice plant responses under drought stress.
Morphological responses

Reduction in germination,
leaf size, leaf number,
biomass, cell growth and
elongation

Increase in leaf rolling,
stomata closure, leaf tip
drying and root length
Physiological and biochemical
responses

Reduction in transpiration,
photosynthesis, chlorophyll
content, membrane
stability, stomatal
conductance and
photostem II activity

Increase in osmoprotectant
Molecular responses

Changes in gene expression
(Up/down regulation)

Activation of relevant
transcription factors and
signalling pathways
Rice plant responses to drought
Reduced tillering
Reduced grain filling rate
Delayed flowering
Reduced spikelet fertility
Reduced grain yield

Reduction in number and
size of panicles

Reduction in grain size
and weight


1262 Afr. J. Agric. Res.
These modifications, will in the long term limit the
surfaces through which loss of water by transpiration can
take place. Thus growth reduction is not a passive
consequence of the lack of water in the cells, but rather a
controlled and programmed response of the plant, the
result of which is to anticipate the events of drought
stress. Studies have shown that these modifications
result from a decrease in the rate of division of plant cells
(Granier et al., 2000) and a modification of the physico-
chemical properties of the cell walls which become more
rigid thereby inhibiting their growth (Cosgrove, 2005).
Root elongation: Contrary to aerial organs which are
reduced under the effect of water stress, these conditions
promote the development of the root system. Enhancing
the development of the root system traits such as root
length allows the plants to access deep ground water
resources. Plant production is the function of water use
(WU), water use efficiency (WUE) and harvest index (HI).
It is therefore vital to understand its effect during defined
developmental stage in order to design effective selection
methods to improve plant production under dry
environment. WUE provides the means of efficient use of
water and serves as a breeding target in water saving
agriculture. Traditionally, it is defined as the ratio of dry
matter produced per unit of water transpired, and
constitutes one of the key determinants in controlling
plant production. It is also referred to as “transpiration
efficiency” and it is estimated from the measures of leaf
gas exchange or by using carbon isotope discrimination.
Higher WUE in turn lowers photosynthetic rate due to
reduced rate of transpiration and consequently slows the
rate of plant growth (Condon et al., 2004). Currently,
agricultural sectors are slowly moving towards use of
genotypes with increased WUE and improved agronomic
practices ( Pereira et al., 2006).
Leaf water potential (LWP) is a measure of whole plant
water status and has long been recognized as an
indicator of dehydration avoidance (Pantuwan et al.,
2002a). When water deficit in leaf goes beyond a certain
threshold level, the stomata closes as a mechanism of
lowering the rate of transpiration. Stomatas help to
regulate water loss when the tissue water status
becomes too low, thereby minimizing the severity of
water deficiency in plants. Thus, higher LWP is
maintained by stomatal closure and varietal differences in
stomatal response to water status have been reported
(Jongdee et al., 1998). Genotypes possessing stay-green
trait maintain high photosynthetic activity and often
protects the plants from premature senescence during
the onset of stress. It is reported that stay-green plants
assimilate more nitrogen and retain high level of nitrogen
content in the leaf, thereby retaining photosynthetic
capacity under water limited conditions (Borrell et al.,
2001).
Molecular responses to drought stress: As soon as the
stress is detected by plant receptors, a coordinated
series of cellular responses is established. In fact, the
physiological and morphological reactions are based on
these coordinated cellular responses which induce the
expression of a large number of genes. In rice, more than
5,000 genes are up-regulated and more than 6,000 are
down-regulated by drought stress (Maruyama et al.,
2014). Wang et al. (2011) conducted genome-wide gene
expression profiling and detected 5,284 genes which
were differentially expressed under drought stress,
among which were under temporal and spatial regulation.
Recently, it has been shown that a CO-like gene, Ghd2
(grain number, plant height, and heading date2), which
can increase the yield potential under normal growth
condition just like its homologue Ghd7, is involved in the
regulation of leaf senescence and drought resistance.
This gene is down regulated under drought conditions.
Overexpression of Ghd2 resulted in significantly reduced
drought resistance, while its knockout mutant showed the
opposite phenotype (Liu et al., 2016).
Regulatory transcription factors involved in the
response of drought stress have been extensively
investigated. This allowed the discovery of two important
signaling pathways of transcriptional networks under
abiotic stress conditions. One involves a hormone called
abscissic acid (ABA) produced when a plant undergoes
water stress. Abscissic acid will initiate, at the cellular
level, a cascade of signaling involving transcription
factors named ABA Responsive Element Binding (AREB)
(Abe et al., 1997; Uno et al., 2000). The second pathway
is independent of this hormone, and involves other
transcription factors, drought responsive element binding
(DREB) (Yamaguchi-Shinozaki and Shinozaki, 2005).
Many signaling details of ABA have been well elucidated
and reviewed (Jiang and Zhang, 2002; Salazar et al.,
2015; Sah et al., 2016). ABA is an important messenger
that acts as the signaling mediator for regulating the
adaptive response of plants to different environmental
stress conditions (Sah et al., 2016).
DETECTION OF QUANTITATIVE TRAIT LOCI (QTLS)
FOR USEFUL DROUGHT TOLERANCE TRAITS
The recent development of high-density linkage maps
has provided the tools for dissecting the genetic basis
underlying complex traits such as drought resistance into
individual components (Yue et al., 2006). Although,
complex traits such as yield are routinely dissected into
their component traits namely grain size, test weight and
number of productive tillers per plant in rice, sometimes
resulting in the development of functional markers, the
same is not true in drought stress research (Prakash et
al., 2016). Earlier molecular genetic analyses identified
several QTLs of secondary traits important to drought
tolerance such as root architecture, leaf water status,
panicle water potential, osmotic adjustment and relative

water content.
Genes/QTL underlying drought secondary traits
In rice, a number of physio-morphological putative traits
have been suggested to confer drought tolerance
(Deivanai et al., 2010). Root system architecture plays a
primary constitutive role in acquisition of water and
nutrient from the soil and maintains appropriate plant
water status (Nguyen et al., 1997; Lafitte et al., 2001;
Kato et al., 2006). Various root architecture traits among
them, rooting depth, root density, root thickness and root
distribution pattern (Pantuwan et al., 1996; Wade et al.,
1996; Lilley and Fukai, 1994; Fukai and Cooper, 1995)
enhance plant water uptake, thereby avoiding
dehydration. QTLs for morphology and the index of root
penetration have been identified in several rice
populations (Champoux et al., 1995; Ray et al., 1996;
Zhang et al., 2001; Kijoji et al., 2014; Henry et al., 2014).
Liu et al. (2009) identified and cloned a gene named
OsDHODH1 which encodes a putative cytosolic
dihydroorotate dehydrogenase (DHODH) in rice.
Overexpression of the OsDHODH1 gene in rice
increased the DHODH activity and enhanced plant
tolerance to salt and drought stresses.
Deep rooting is a very important trait for plants drought
avoidance mechanism and it is usually represented by
the ratio of deep rooting (RDR). The root growth angle
(RGA) is another important trait in drought tolerance,
which determines the direction of root elongation in the
soil and affects the area in which roots capture water and
nutrients. Courtois et al. (2009) conducted a meta-
analysis of QTLs in 12 populations and detected 675 root
trait QTLs. Although, many QTLs for root trait have been
mapped, only 5 major QTLs for deep rooting have been
reported (Kitomi et al., 2015; Uga et al., 2015, 2011) and
only the DRO1 gene has been cloned (Uga et al. 2013a).
DRO1 has been detected on chromosome 9 in
recombinant inbred lines (IK-RILs) derived from a cross
between the shallow-rooting cultivar IR64 and the deep-
rooting cultivar Kinandang Patong (Uga et al., 2011). This
QTL has subsequently been cloned. It has been shown
that the functional allele of DRO1 introduced from
Kinandang Patong (Dro1-NIL) had a significantly larger
RGA and higher grain yield than the parental variety
IR64, which had a non-functional allele of DRO1. The
DRO1 is the first gene associated with root system
architecture (RSA) that has been shown to improve the
ability to avoid drought. Another major QTL for RGA
named DRO2 has been identified on chromosome 4 in
three F2 populations derived from crosses between each
of three shallow-rooting cultivars (ARC5955, Pinulupot1
and Tupa729) and Kinandang Patong (Uga et al., 2013b).
A new QTL for RGA was recently identified on the long
arm of chromosome 7. This QTL named DRO3 is
involved in the DRO1 genetic pathway as its effect on
Noelle et al. 1263
RGA in plants have been detected only with a functional
DRO1 allele (Uga et al., 2015). The Phosphorus Uptake
1 (PUP1) is a QTL that contributes to phosphorus (P)
uptake in low P content soils. The gene underlying the
QTL, later termed Phosphorus-Starvation Tolerance 1
(PSTOL1), was cloned and appeared to encode a
receptor-like cytoplasmic kinase (Gamuyao et al., 2012).
Recently, a novel gene, OsAHL1, was identified through
genome-wide profiling and analysis of mRNAs. Analysis
showed that OsAHL1 has both drought avoidance and
drought tolerance mechanisms and when overexpressed,
it enhances multiple stress tolerances in rice plants
during both seedling and panicle development stages.
Functional studies revealed that OsAHL1 regulates root
development under drought condition to enhance drought
avoidance, participates in oxidative stress response and
also regulates the chlorophyll content in rice leaves (Zhou
et al., 2016). Two QTLs for the root gravitropic response,
and 4 QTLs for seminal root morphology (SRM) have
been reported (Norton and Price, 2009). These 2 traits
are well known to be important components of RGA. The
QTL designed, quantitative trait locus for Soil Surface
Rooting 1 (qSOR1) has been fine-mapped on
chromosome 7, using 124 recombinant inbred lines
(RILs) derived from a cross between Gemdjah Beton, an
Indonesian lowland rice cultivar with soil-surface roots,
and Sasanishiki, a Japanese lowland rice cultivar without
soil-surface roots (Uga et al., 2012).
Liu et al. (2005) identified 2 and 6 main effect QTLs for
canopy temperature and leaf water potential respectively
in RILs (F9) from a cross between Zhenshan97B and
IRAT109. Recently, 6 QTLs for RDR were identified using
1 019 883 single-nucleotide polymorphisms (SNPs) (Lou
et al., 2015). Prince et al. (2015) identified two QTLs for
canopy temperature, 1 QTL for leaf drying and 1 for
SPAD under managed stress and in a rainfed target
drought stress environment, respectively. The
introduction of traits that contribute to drought avoidance
or tolerance should improve resistance of rice to drought
and this strategy therefore has considerable potential to
increase rice production in areas prone to drought (Fukai
and Cooper, 1995; Nguyen et al., 1997). For rice,
considerable research effort has been devoted to
mapping QTL for osmotic adjustment (Lilley et al., 1996),
but only a few loci with major effects have been identified.
QTL for yield and yield related-traits under drought
Several studies using different mapping populations have
identified QTLs for traits related to drought tolerance
(Khowaja and Price, 2008). Bernier et al. (2007) identified
large-effect QTLs for grain yield under drought stress. If
confirmed, these identified QTLs have to be fine mapped
for use in breeding programs. A drought experiment
conducted by Lanceras et al. (2004) using 154 doubled
haploid lines derived from a cross between two rice

1264 Afr. J. Agric. Res.
cultivars, CT9993-510 and IR62266-42, allowed
identification of 77 QTLs for grain yield and its
components under various drought intensities. Among
them were 7 for grain yield, 8 for biological yield, 6 for
harvest index, 5 for days to flowering, 10 for total spikelet
number, 7 for percent spikelet sterility, 23 for panicle
number and 11 for plant height. A recombinant inbred
population obtained from a cross between high-yielding
lowland rice IR64 and Cabacu was used to identify 10
QTLs for grain yield and component traits under
reproductive-stage drought stress (Trijatmiko et al.,
2014). The qDTY12.1 is the first reported large-effect
QTL for grain yield under severe upland reproductive-
stage drought conditions and was identified in a
population of 436 F3-derived lines from a cross between
Vandana and Way Rarem (Bernier et al., 2007). Two
other large-effect QTLs, qDTY2.1 and qDTY3.1, well
known to affect grain yield under lowland reproductive-
stage drought, were identified in a back cross inbred line
(BIL) population derived from a cross between Swarna
and Apo. Both QTLs showed a very high effect (R2 =
16.3 and 30.7%) under severe lowland reproductive-
stage drought. These QTLs also showed pleiotropic
effects on other traits such as DTF and PHT (Venuprasad
et al., 2009). Another QTL, qDTY6.1 had strong effect on
yield in aerobic drought stress conditions (Venuprasad et
al., 2012b).
A large-effect QTL qDTY1.1 has been identified as
having an effect on grain yield under severe lowland
reproductive-stage drought across F3-derived populations
developed from a cross between N22 and Swarma, N22
and IR64 and N22 and MTU1010 (Vikram et al., 2011).
This QTL has also been reported in CT9993-5-10-1-
M/IR62266- 42-6-2 and Apo/IR64 populations (Kumar et
al., 2007; Venuprasad et al., 2012a).
In the same way, qDTY2.2, qDTY4.1, qDTY9.1 and
qDTY10.1 were identified to have a large effect on grain
yield in BIL population from a cross between Aday Sel
and IR64 (Swamy et al., 2013). Table 1 presents a
summary of large effect QTLs for grain yield reported in
rice.
MITIGATION AGAINST DROUGHT
Mitigating drought and climate change requires robust,
well-planned and informed strategies in order to enhance
agricultural sustainability and ensure that human
livelihood is not negatively affected. Improved rice
technologies that help reduce losses from drought can
play an important role in long-term drought mitigation.
Important scientific progress is being made in
understanding the physiological mechanisms that impart
tolerance to drought (Blum, 2005; Lafitte et al., 2006).
Similarly, progress is being made in developing drought-
tolerant rice germplasm through conventional breeding
and the use of molecular tools (Korres et al., 2017).
Improving the resilience of rice production systems to
climate change requires the development and
dissemination of appropriate combinations of improved
stress-tolerant rice germplasm, natural resource
management strategies and creation of appropriate policy
environments to help increase and stabilize yields in
variable cultivation conditions.
Breeding for drought tolerance and adaptation
One of the main strategies in confronting drought is
breeding for drought tolerance which helps to deliver
adapted genotypes. These breeding efforts will require
characterization and evaluation of diverse germplasm
with the aim of identifying genotypes possessing traits
that are important in enhancing drought tolerance. The
replacement of diverse and adapted traditional rice
varieties with genetically narrow based genotypes has
significantly increased the vulnerability of the agricultural
production systems. The use of a wide range of genetic
resources is critical in the development of varieties that
are adapted to drought. Crop wild relatives are
particularly useful sources of genes for adapting crops to
drought. There exists a variety of physiological traits that
are associated with drought tolerance. Some of these
traits include root traits, early flowering, water use
efficiency, amount of water transpired, transpiration
efficiency, osmotic adjustment and stay green. Breeding
for increased yields under drought tolerance will require
proper understanding of the various traits that are
associated with yield (Pandey et al., 2015). The exact
trait to target in a breeding programme in order to obtain
the best response in terms of drought tolerance may not
always be clear to a breeder.
Africa Rice has been spearheading efforts aimed at
delivering rice varieties that are tolerant to drought. This
has involved screening of a wide range of genetic
resources including indigenous Africa species such as O.
glaberrima and Oryza barthii. A key goal of the breeding
programme has been to develop a rice variety that can
escape terminal drought that frequently occurs at the end
of the wet season through its short growth duration. Short
duration varieties are also preferred to avoid late season
fungus diseases (Jones et al., 1997). Several upland
interspecific O. sativa × O. glaberrima (NERICA) varieties
were evaluated at AfricaRice and it was observed that
they have potential for escaping drought due to their
short growth duration. The capacity of NERICA varieties
to maintain growth under mild drought, their survival
under severe drought, recovery from drought and their
water use efficiency need to be incorporated into
breeding programs (Futakuchi et al., 2011).
Exploitation of drought tolerance traits in African rice
in rice breeding
African rice is one of the two independently domesticated

Noelle et al. 1265
Table 1. Large effect QTLs reported for grain yield under drought stress conditions.
QTL
name
Chrom
Interval
Population
Ecosystem
R2
References
qDTY1.1
1
RM11943–RM12091
N22/Swarna
Lowland
13
Vikram et al. (2011)
qDTY1.1
1
RM11943–RM12091
N22/IR64
Lowland
17
Vikram et al. (2011)
qDTY1.1
1
RM11943–RM12091
N22/MTU1010
Lowland
13
Vikram et al. (2011)
qDTY1.1
1
RM486–RM472
Apo/IR64
Upland
58
Venuprasad et al. (2012a)
qDTY1.2
1
RM259–RM315
Kali Aus/MTU1010
Upland
7
Sandhu et al. (2014)
qDTY 1.3
1
RM488–RM315
Kali Aus /IR64
Upland
5
Verma et al. (2014)
qDTY2.1
2
RM327-RM262
Apo/Swarna
Lowland
16
Venuprasad et al. (2009)
qDTY2.2
2
RM236–RM279
Aday Sel./ IR64
Lowland
11
Swamy et al. (2013)
qDTY2.2
2
RM236–RM555
Aday Sel./ IR64
Lowland
3
Swamy et al. (2013)
qDTY2.2
2
RM236–RM555
Aday Sel./ IR64
Lowland
9
Swamy et al. (2013)
qDTY2.2
2
RM211–RM263
Kali Aus/ MTU1010
Upland
6
Sandhu et al. (2014)
qDTY2.2
2
RM211–233A
Kali Aus/ MTU1010
Lowland
16
Palanog et al. (2014)
qDTY3.1
3
RM520–RM16030
Apo/Swarna
Lowland
31
Venuprasad et al. (2009)
qDTY3.1
3
RM168–RM468
IR55419-04/TDK1
Lowland
8
Dixit et al. (2014)
qDTY3.1
3
RM168–RM468
IR55419-04/TDK1
Upland
15
Dixit et al. (2014)
qDTY 3.2
3
RM569–RM517
Aday Sel./ Sabitri
Lowland
23
Yadaw et al. (2013)
qDTY 3.2
3
RM60–RM22
N22/Swarna
Lowland
19
Vikram et al. (2011)
qDTY 3.2
3
id3000019–id3000946
Moroberekan/Swarna
Lowland
8
Dixit et al. (2014b)
qDTY 3.2
3
id3000019–id3000946
Moroberekan /Swarna
Upland
19
Dixit et al. (2014b)
qDTY 4.1
4
RM551–RM16368
Aday Sel./IR64
Lowland
11
Swamy et al. (2013)
qDTY6.1
6
RM589–RM204
Vandana/IR72
Upland
40
Venuprasad et al. (2012b)
qDTY6.1
6
RM589-RM204
Apo/IR72
Upland
63
Venuprasad et al. (2012b)
qDTY6.1
6
RM586-RM217
IR55419-04/TDK1
Lowland
9
Dixit et al. (2014)
qDTY6.1
6
RM586-RM217
IR55419-04/TDK1
Upland
36
Dixit et al. (2014)
qDTY6.2
6
RM121-RM541
IR55419-04/TDK1
Lowland
9
Dixit et al. (2014)
qDTY6.2
6
RM121-RM541
IR55419-04/TDK1
Upland
20
Dixit et al. (2014)
qGY8.1
8
RM38–RM331
MASARB25 / Pusa
Basmati; HKR47/ MAS26
Upland
34
Sandhu et al. (2014)
qDTY10.1
10
MTU1010/N22
RM216–RM304
Lowland
5
Vikram et al. (2011)
qDTY10.2
10
Aday Sed./IR64
RM269–G2155
Lowland
17
Swamy et al. (2013)
qDTY11.1
11
id11002304-id11006765
Moroberekan- Swarna
Upland
25
Dixit et al. (2014b)
qDTY12.1
12
RM28166–RM28199
IR74371-46-1-1 / Sabitri
Lowland
24
Mishra et al. (2013)
qDTY12.1
12
RM28048 -RM511
Way Rarem/ Vandana
Upland
33
Bernier et al. (2007)
rice species, with its distribution being limited to West
Africa. Its genetic potential in terms of resistance to both
biotic and abiotic stresses has been well documented
and deployed in rice improvement (Wambugu et al.,
2013). Its tolerance to drought is a particularly valuable
trait during these periods that are characterized by
increased occurrences of drought and erratic rainfall.
Some alien introgression lines derived from an
interspecific cross between O. sativa and O. glaberrima
under drought conditions had higher yield than the
parents (Bimpong et al., 2011b). This demonstrates the
potential of transferring drought related traits from African
rice to Asian rice. In this study, novel QTLs for drought
related traits such as yield and yield components were
identified with about 50% of the beneficial alleles being
contributed by African rice.
A total of 2000 African rice accessions conserved at
AfricaRice genebank were evaluated by Shaibu et al.
(2018) for drought tolerance in three locations in West
Africa over a period of 3 years. Results of this screening
showed that four O. glaberrima genotypes had
significantly higher yields under both drought and rainfed
conditions than the O. glaberrima check, CG14, which is
considered a drought tolerant variety. Though, these
genotypes were not significantly different from the O.
sativa checks (Table 2), they will serve to widen the
African rice genepool that can be used for breeding for
drought tolerance. African rice has several drought
avoidance mechanisms such as early flowering. It has
also been reported to have thin leaves which easily roll

1266 Afr. J. Agric. Res.
Table 2. Grain yield (g/m2) of selected Oryza glaberrima accessions and standard checks under drought, rainfed and control conditions during
2013-14 at three locations in West Africa.
Entries
Drought
Rainfed
Control
Ibadan
Ibadan
Ibadan
Badeggi
Cotonou
Ibadan
Ibadan
Cotonou
DS 2013
DS 2013
DS 2014
DS 2014
WS 2014
WS 2014
WS 2014
WS 2014
No. of O. glaberrima
genotypes evaluated
200
285
74
74
30
30
30
30
Selected O. glaberrima
genotypes
TOG 7400
-
236
236
25
83
270
399
368
TOG 6520
401
-
301
7
-
349
403
455
TOG 6519-A
393
226
5
72
303
286
308
TOG 7442-B
327
-
152
7
69
251
292
443
Checks
O. glaberrima check
CG 14
217
238
51
6
60
104
415
339
O. sativa check
Apo
432
397
472
9
48
242
255
401
FARO 52
472
447
216
9
-
55
-
619
IR 77298-14-1-2-B-10
363
451
298
12
56
263
418
285
Trial mean
246
161
122
6
51
171
241
310
LSD 0.05
142
142
103
5
43
130
119
149
Heritability
0.87
0.75
0.71
0.69
0.70
0.85
0.90
0.84
Adapted from Shaibu et al. (2018).
during drought to retain water, in addition to having small
diameter roots which easily extract water from the soil
(Dingkuhn et al., 1999). The phenological responses of
African rice during times of drought have been found to
be superior to those of traditional and improved O. sativa
cultivars (Dingkuhn et al., 1999). African rice has also
been found to possess the capacity to close stomata
earlier in response to drought as compared to O. sativa
(Bimpong et al., 2011c).
Challenges in breeding for drought tolerance
In most rice breeding programs, grain yield as an
important trait of interest is widely used as an index for
adaptation to drought stress. But several researchers
have reported inconsistency in yield production by rice
genotypes across environments and years (Fukai and
Cooper, 1995; Pantuwan et al., 2002a, b, c). A genotype
performing well in one type of drought environment may
not perform well in other environments (Pantuwan et al.,
2002a, c). It is unclear whether promising materials
selected under drought condition will yield well in full
irrigation/wet-season condition. This explains the large
genotype-by-environment (GxE) interactions and the low
heritability of grain yield of rainfed lowland rice under
drought and the uncertainty in the selection of drought
resistant genotypes (Fukai and Cooper, 1995). To
accommodate the effects of GxE interactions and
improve selection efficiency, a large number of multi-
location trials over years in various drought intensity
conditions could be a solution (Nyquist and Baker, 1991;
Fukai and Cooper, 1995). Unfortunately, such evaluation
processes are costly and time-demanding for making
selections in the breeding program. Therefore, it has
become necessary to identify more efficient breeding
options based on the use of indirect selection
methodology (Falconer, 1989).
Even though there is extensive evidence that selection
under target stresses may accelerate breeding gains for
stress environments (Atlin and Frey, 1990; Ceccarelli et
al., 1992; Ud-Din et al., 1992; Bänziger et al., 1997), the
difficulty of choosing appropriate selection environments,
given a highly variable target environment, may limit the
identification of superior genotypes. While breeding
programs in high-income countries may resort to real-
time GIS information for adequately weighting information
from METs (Podlich et al., 1999), these opportunities
rarely exist in low-income countries as there is a lack of
both real-time GIS information and resources for
conducting a large number of METs. Progress in
improving drought resistance has been slow. This is
partly due to the complexity of the drought environment,
the number of different mechanisms of drought resistance
exploited by rice and the interaction between the two as
well as the genetic complexity of most traits.

Other drought mitigation strategies
In addition to crop improvement and selection of drought
tolerant genotypes, other strategies for mitigating against
long term impacts of drought include development of
irrigation facilities and water harvesting structures such
as dams. Development of water resources is an
important area of protection against drought that is
emphasized in SSA. The large-scale development of
irrigation schemes that was a hallmark of the green
revolution is limited now by high costs and increasing
environmental concerns (Rosegrant et al., 2002).
Moreover, the rationale of establishing new large scale
irrigation schemes may be questioned as many such
schemes have and continue to stall. The collapse of
these schemes, many of which have been established in
partnership with various development partners, brings to
the fore critical issues such as feasibility and
sustainability of such projects. In some cases, the long
term availability of water for these projects is usually not
guaranteed. The technical and financial capacity to
maintain these projects need to be explored before their
establishment. However, there are still substantial
opportunities to provide some protection from drought
through small and minor irrigation schemes and through
land-use approaches that generally enhance soil
moisture and water retention (Shah, 2001; Moench,
2002). Public-sector support for further development,
maintenance and rehabilitation of small and minor
irrigation schemes could make them more effective in
mitigating drought. Public-sector involvement, however,
should be limited to the provision of technical assistance,
while the actual management of these small-scale
schemes is better left to local communities (Kerr et al.,
2002). Hand dug shallow wells are another option for
sourcing water resources particularly for small holder
farmers.
Watershed-based approaches implemented in drought
prone areas of India are providing opportunities to
achieve long-term drought-proofing by improving overall
moisture retention within watersheds (Rao, 2000). As
already stated, one of the causes of drought in Africa is
habitat destruction especially due to population pressure.
Most habitats in many African countries are currently
severely degraded and non-productive. Consequently,
one of the ways to mitigate drought is through the
rehabilitation of these degraded habitats through
ecological restoration. Drought forecasting and timely
provision of such advice to farmers is an important
drought mitigation strategy that can help reduce the
overall economic cost of drought. It also helps improve
preparedness, thereby helping in managing the risk more
effectively. Various indicators such as the Southern
Oscillation Index (SOI) are routinely used to forecast
drought in several countries (Wilhite, 2000; Meinke and
Stone, 2005). Forecasting is especially important in
assisting farmers make more informed decisions
regarding the choice of crops and cropping practices.
Noelle et al. 1267
CONCLUSION
Drought is one of the major climatic hazards even in the
sub humid rice-growing areas of Asia and Africa. It is an
event that reoccurs, affecting agriculture and the
livelihoods of millions of farmers and agriculture laborers.
The socio-economic impact of drought is enormous. It
has huge economic costs, in terms of both actual
economic losses during drought years and losses arising
from foregone opportunities for economic gains. Drought
contributes directly to an increase in the incidence and
severity of poverty.
It is therefore critical that we establish effective
strategies to mitigate the effects of drought in order to
ensure agricultural productivity and environmental
sustainability. Use of adapted genotypes and
improvement in rice production technology are some of
the components of an overall strategy for effective
drought mitigation. Increased moisture availability to
crops through water conservation and harvesting, and
watershed development is an important component.
Improvements in drought forecasting and efficient
provision of such information to farmers can improve their
decisions regarding crop choice and input use.
CONFLICT OF INTERESTS
The authors have not declared any conflict of interests.
ACKNOWLEDGEMENT
The authors thank the Generation Challenge Program
(GCP) for funding drought related research in AfricaRice.
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