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Malaysian Journal of Sustainable Agriculture (MJSA) 5(2) (2021) 67-76
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DOI:
10.26480/mjsa.02.2021.67.76
Cite the Article: Bipin Rijal, Prakash Baduwal, Madhukar Chaudhary, Sandesh Chapagain, Sushank Khanal, Saugat Khanal, Padam Bahadur Poudel (2021).
Drought Stress Impacts on Wheat And Its Resistance Mechanisms.
Malaysian Journal of Sustainable Agriculture
, 5(2): 67-76.
ISSN: 2521-2931 (Print)
ISSN: 2521-294X (Online)
CODEN: MJSAEJ
RESEARCH ARTICLE
Malaysian Journal of Sustainable Agriculture
(MJSA)
DOI: http://doi.org/10.26480/mjsa.02.2021.67.76
DROUGHT STRESS IMPACTS ON WHEAT AND ITS RESISTANCE MECHANISMS
Bipin Rijala*, Prakash Baduwala, Madhukar Chaudharya, Sandesh Chapagaina, Sushank Khanala, Saugat Khanalb, Padam Bahadur Poudela
a Institute of Agriculture and Animal Science, Paklihawa, Rupandehi, Nepal.
b Faculty of Agriculture, Agriculture and Forestry University, Rampur, Chitwan, Nepal.
*Corresponding Author email: rijalbpin@gmail.com
This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction
in any medium, provided the original work is properly cited.
ARTICLE DETAILS
ABSTRACT
Article History:
Received 20 November 2020
Accepted 24 December 2020
Available online 06 January 2021
Scarcity of water has been a serious agricultural hindrance to crop productivity since antiquity. Drought-
stressed loss in wheat yield likely exceeds losses from all other causes, since both the severity and duration
of the stress are censorious. Here, we have reviewed the effects of drought stress on the morphological,
physiological, and biochemical attributes along with the growth impacts, water relations, and photosynthesis
impacts in wheat. This review also illustrates the mechanism of drought resistance in wheat. Historical
drought years in Nepal have been identified and the yield losses were assessed. Wheat encounters a variety
of morphological, physiological, biochemical responses at cellular and molecular levels towards prevailing
water stress, thus making it a complex phenomenon. Drought stress affects leaf size, stems elongation and
root proliferation, imbalance plant-water relations and decline water-use efficiency. Different types of
physiological research are ongoing to find out the changes occurs in the wheat plant as a result of drought
stress. Morphological changes can be looked through two ways: changes in root system and changes in shoot
system such as effects on height, leaf senescence, flowering, and so on. Physiological changes involve changes
in cell growth pattern, chlorophyll contents, photosynthetic disturbances, plant-water relations, etc.
Biochemical changes occur in different chemical, biomolecules, and enzymes. Plants portray several
mechanisms to withstand drought stress which can be classified as Drought escape, Drought avoidance, and
Drought tolerance. Selection of wheat genotype that can tolerate water scarcity would be suitable for the
breeding program aiming to development of drought tolerant variety under water limited regions.
KEYWORDS
Agronomic changes, Drought, Nepal, Resistance, Wheat.
1. INTRODUCTION
Wheat, Triticum aestivum, is one of the most widely cultivated cereals,
particularly in the mediterranean region and other semi-arid regions from
temperate to subtropical areas of the world (Ahmed et al., 2019). Most of
the areas of land in which wheat is cultivated lie in arid and semiarid
regions. A key determinant in the favorable outcome of wheat is its
adaptation to a broad range of climatic conditions. Approximately, one-
third of the global population uses wheat as a staple crop and also the first
cereal crop in majority of the developing countries (Bayoumi, 2009). It
serves as an essential food source, as it contains carbohydrates, dietary
proteins, fiber, calcium, zinc, fats, and energy. However, in many countries,
including Nepal the attainable yield hasn’t been achieved through there is
high potential of enhancing the average yield.
Wheat is mostly cultivated under rainfed conditions where fluctuations in
rainfall pattern have caused water insufficiency to act as a determining
factor for declining the crop yield, especially when water deficit stress
occurs during the flowering and grain filling period stages (Bassi et al.,
2017). The likeliness of drought stress in the coming days is high owing to
global climate change and declines in availability of underground water
resources for agriculture. It has been proved through many researches
that wheat production is drastically affected by abiotic stresses. A study
reported that for every 1-degree centigrade increase in temperature, there
is a yield loss of about 4.1% to 6% (Liu et al., 2016). Salinity contributes to
the reduction of wheat yield (Mujeeb-Kazi et al., 2019). On the other hand,
drought is considered as major menace to wheat yield and is gaining much
attention nowadays. By 2025 it is anticipated that nearly 1.8 billion people
will face absolute water shortage and 65% of the world’s population will
face water-stressed environments (Nezhadahmadi et al., 2013b).
The factor limiting the wheat production in many regions is primarily due
to erratic rainfall, reducing average yield up to 50% and often over. Wheat
can be produced in a varied range of agro-climatic environments;
nevertheless, most of these environments have drought stress as one of
the major constraints to their production and yield. The predicted global
warming and climatic fluctuations will increase the frequency of drought,
therefore leads to the losses of the wheat yield. The increase in annual
average temperature accompanied with fluctuations in rainfall patterns
and arising drought risks in many regions have impacted agricultural
Malaysian Journal of Sustainable Agriculture (MJSA) 5(2) (2021) 67-76
Cite the Article: Bipin Rijal, Prakash Baduwal, Madhukar Chaudhary, Sandesh Chapagain, Sushank Khanal, Saugat Khanal, Padam Bahadur Poudel (2021).
Drought Stress Impacts on Wheat And Its Resistance Mechanisms.
Malaysian Journal of Sustainable Agriculture
, 5(2): 67-76.
productions, globally, which has brought limitations on crop yield
potential. Declining underground water availability and rising
temperature is assumed to worsen in coming decades (IPCC, 2013).
Drought stress along with high temperature at reproductive stage
(terminal growth phase of wheat crop) is prime contributing factor
towards low wheat yield in tropics and subtropics.
Drought is a leading environmental stress declining the global cereals
productivity, with up to half of the agricultural land prone to frequent
drought (Ashraf and Fooled 2007). A rising problem of global warming is
predicted to amplify the frequency and severity of drought in the near
future (Yu et al., 2017). It can bring to the lack of water resources which
influences morphological, biochemical, physiological, and molecular
attributes of the plants. All of these changes retard the plant growth and
the crop yield. Drought stress unfavorably alters the physiological and
morphological parameters of crops. Along with the complexity of the
drought itself, crop response to water shortage is even more complex
because of unpredictable components in the environment and the
interaction among biotic and abiotic factors (Nevo and Chen, 2010). Such
stress leads to significant reduction in photosynthetic efficiency, stomatal
conductance, leaf area and water-use efficiency of wheat (Farooq et al.,
2019; Hussain et al., 2016).
Different researchers have performed study on drought resistance
mechanism in many cereals, but the improvement of wheat for drought
tolerance is limited for many constraints. There is a need to assess the
response mechanism of the plants in response to drought stress. The
objectives of the current study are to assess the possible changes in the
morphological and physiological attributes of the wheat crop due to the
drought stress and plant tolerance mechanism to the stress.
2. DROUGHT STRESS
Faced with inadequacy of water resources, drought is the distinct perilous
hazard to global food security. It was the impetus of the great famines of
the past. As the water supply is limiting worldwide, future food demand
for rapidly increasing population pressures is likely to further aggravate
the impacts of water stress. The severity of drought is uncertain as it relies
on various factors such as quantity and quality of precipitation,
evaporative demands, and soil moisture contents (Wery et al., 1994).
There are various types of stress factors responsible for the reduction of
wheat cultivation among which drought stress plays significant role for
reducing the yield in wheat. About 50% of the cultivated land in the
developing country is under rainfed condition (Paulsen, 2002). The
scarcity of water which induces gradual morphological, biochemical,
physiological and molecular changes is called as drought stress (Sallam et
al., 2019). Due to drought stress, plant transpires less and in order to do
so stomata of the wheat plant close to prevent water loss. And if stomata
close for a long time there occurs an oxidative damage to the plant leaves
tissue which affects the different physiological and biochemical activity of
wheat plant.
Approximately 65 million ha of wheat production was affected by drought
stress in 2013 (Nations, 2020). Different development stages of plant from
germination, vegetative and reproductive stage to grain filling and
maturation of crop are disturbed when the plant suffers from drought
stress. Drought reduces the nutrient uptake efficiency including nitrogen
as a main factor and nutrient utilization by plants. The reduction in
nutrient uptake capacity is due to impaired membrane permeability and
active transport and reduced transpiration rate resulting in decreased
root absorbing power (Ahmad et al., 2018). Different types of research
show that plant height, biomass and yield are more susceptible traits to
drought stress in comparison with number of spikes and 1000 grain
weight (Nouri-Ganbalani et al., 2009). For the development of stress
tolerant plants, we have to know the adaptation methods used by the plant
for surviving during drought stress. Knowing the importance of drought in
wheat yield reduction, a number of researchers have been studying about
the effect of drought and the number of problems caused by it. They are
continuously trying to develop new drought tolerant genotypes which can
perform well under stress conditions.
Drought (water stress) is one of the most crucial environmental stresses
and occurs for various reasons, including erratic rainfall, salinity,
fluctuating temperatures, and high intensity of light. It is a
multidimensional stress and bring alteration in the physiological,
morphological, biochemical, and molecular attributes in plants. Prolonged
drought is a severe curb in landscape restoration in both arid and semiarid
regions. There are numerous kinds of drought; meteorological, caused by
a prolonged lack of rainfall; hydrological, caused by a scarcity in river flow;
pedological, ascribed to a scarcity of water in the soil structure; agronomic,
caused by a insufficiency of water available to plants in order to balance
the physiological needs of evapotranspiration; and sociological, caused by
competing consumptions to meet human and social needs. Landscape
restoration can be drastically affected by all kinds, but notably by
agronomic droughts, which negatively affect seedling establishment and
crop stand establishment. Three major processes decline crop yield by soil
water-deficit; decreased canopy absorption of photosynthetically active
radiation, reduced radiation-use efficiency, and decreased harvest index
(Earl and Davis, 2003).
Wheat has improved its tolerance mechanisms to withstand drought
stress; however, these mechanisms are different and rely on the crop
varieties and the cultivars. It is important to enhance the drought
tolerance of wheat crops under the changing climatic conditions. To date,
there are no efficient feasible technological mechanisms to promote crop
production under drought stress environments. Yet, improvement of crop
plants tolerant to drought stress might be a hopeful approach, which helps
in maintaining the food security. Development of crops for increased
drought resistance needs the sound knowledge of physiological and
genetic mechanisms of the contributing traits at different plant
developmental stages. Relevant research has been carried out on drought
tolerance in wheat crops. Ingram and Bartels, more than a decade ago,
excellently reviewed those appreciable efforts (Bartels, 1996). Similar
reviews have been done by which deal with specific aspects of plant
drought tolerance (Penna, 2003; Reddy et al., 2004; Agarwal et al., 2006).
3. DROUGHT IN NEPAL
In regards to irregular climate change and higher temperature in recent
years than that of global average, Nepal is considered to be among the
most vulnerable countries. From 1975 to 2005, the global mean surface
temperature rises by 0.6 0C while Nepal experienced a significantly higher
temperature rise of 1.5 0C during similar duration of time, from 1982 to
2006 (Biwa et al., 2012). Likewise, rainfall pattern is also becoming more
unpredictable (Wang et al., 2013). Consequently, average rainfall has been
declining by 3.7 mm (−3.2%) monthly, per decade (Ministry of Education,
2010). These ultimately created drought condition particularly for the
rainfed farming, where farmers depend on monsoon rainfall for their
major agricultural activities (Ghimire et al., 2010). Furthermore, the mean
annual temperature is estimated to be rose between 1.3 0C to 3.8 0C by the
2060s, and 1.8 0C to 5.8 0C by the 2090s and annual precipitation
declination could be within the range of 10% to 20%, across the country
(Ministry of Education, 2010).
In Nepal, drought usually happens from March through June, which is the
onset of monsoon and winter precipitation has almost declined to zero,
also groundwater has hardly been replenished (Joshi, 2018). Some areas
of the trans-Himalayan regions are intensely dry tall through the year and
droughts occur often in the lowland of Terai and in the western hill of
Nepal. Nepal faced drought in 1972, 1977, 1982, and 1992. The country
has tackled incessant dry spells since 2002, particularly during the years
2002, and from 2004 to 2006—in monsoon (Joshi, 2018). Different
incidences of drought were also noticed during 2012, 2013, and 2015.
Drought in Nepal have created panic in the hill farming system, generally
for crop production and the livelihood support of farmers dependent on it.
However, droughts can generate opportunities to learn different
adaptations strategies that are appropriate in such changing
circumstances (Dulal et al., 2010).
Malaysian Journal of Sustainable Agriculture (MJSA) 5(2) (2021) 67-76
Cite the Article: Bipin Rijal, Prakash Baduwal, Madhukar Chaudhary, Sandesh Chapagain, Sushank Khanal, Saugat Khanal, Padam Bahadur Poudel (2021).
Drought Stress Impacts on Wheat And Its Resistance Mechanisms.
Malaysian Journal of Sustainable Agriculture
, 5(2): 67-76.
Table 1: Major cereal loss in different drought years
S.N
Drought
years
Causes of Drought
Major
cereal
loss (in
Metric
Ton)
Affected
regions
1
1972
Late onset of
monsoon/rainfall
333,380
Eastern and
Central
2
1976
Poor distribution of
rainfall
218,480
Western
3
1977
Late onset of rainfall
322,320
Eastern and
Central
4
1979
Late onset of rainfall
544,820
Western
5
1982
Late onset of rainfall
727,460
Eastern
6
1986
Poor distribution of
rainfall during August
and September
377,410
Western
7
1992
Late onset of rainfall
917,260
Eastern
8
1994
Poor distribution of
rainfall
595,976
All regions
9
1997
Poor distribution of
rainfall
69,790
Eastern
10
2002
Poor distribution of
rainfall
83,965
Eastern and
Central
11
2008
Poor distribution of
rainfall during
November 2008 to
February 2009
56,926
All regions
12
2009
Late onset of rainfall
499,870
Eastern and
Central
13
2012
Summer monsoon late
onset and long dry spell
797,629
Eastern and
Central
14
2013
Inadequate rainfall that
affected wheat
plantation
56,000
Eastern and
Central
Terai
districts
15
2012
Delayed monsoon and
weak at the onset, which
delayed paddy
transplantation
not
available
Eastern
Terai
16
2015 to
2016
Poor monsoon and
drought
30,000
people
highly
insecure
Mid and
Far-
Western
hills and
mountains
Source: (Joshi, 2018)
Table 1 depicts the major drought years in Nepal, main causes of drought,
and damages on chief cereal crops. But this is not the comprehensive data,
and many of the database are still be missing. The major elements
determined for the drought are delayed onsets of monsoon, irregular
rainfall pattern, and decreased intensity of rainfall. For example, late onset
of monsoon likely led to a delay in the sowing of rice, impacting the growth
of wheat, along with reducing the volume of underground water (Joshi,
2018).
4. EFFECTS OF DROUGHT ON WHEAT
The impacts of drought stress may range from morphological to molecular
levels and are detrimental to all physiological performances of plant. An
account of various drought stress effects and their extent is elaborated
below.
4.1 Morphological Changes
As a response of drought there occur various morphological changes in
wheat crop which can be directly observed throughout the different stages
of plant growth. Generally, the morphological response of wheat can be
categorized into two parts i.e. shoot part and root part. The shoot part
contains changes in leaf shape, leaf expansion, leaf area, leaf size, leaf
senescence, leaf pubescence, leaf waxiness, cuticle tolerance and
reduction in shoot length. And the lower root part includes changes in root
dry weight, root density, and root length (Denčić et al., 2000). Several
studies have shown that the correlation between morphological traits
such as grain yield per plant, grain spike per plant, spike fertility and plant
height were considered as a reliable indicator for screening drought
tolerant wheat cultivars. Researchers found the positive correlation
between leaf area, plant height and grain yield. In conclusion we came up
with the various morphological changes like decreased plant size, early
maturity, decreased leaf area, reduced yield, limited leaf extension, small
leaf size, reduced number of tillers, reduced leaf longevity, reduced total
shoot length, decreased plant height, increased in leaf rolling and
reduction in plant biomass in wheat as a response to drought stress.
Among these various morphological responses some of major responses
are discussed below.
4.1.1 Changes in Plant Height
The most common effect of water stress is defective germination and poor
crop establishment (Harris et al., 2002). Drought has been proved to
immensely impaired germination and seedling stand. The quality and
quantity of crop stand rely on these events, which are affected by water
deficit (Figure 2). Cell growth is an important drought-sensitive
physiological process due to the decline in turgor pressure
(TaizandZeiger, 2006). Under serious water shortage, cell elongation of
wheat can be altered by interruption of water flow from the xylem to the
surrounding elongating cells (Nonami, 1998). Drought affects the growth
of wheat plant. Wheat is a plant which is very sensitive to water stress
condition and shows drastic change in growth of plant when exposed to
these types of situations.
However, the duration, time, magnitude of drought and stage of wheat
plant also determines the effect of drought stress. Different experiments
have been carried out at different developmental stages like stem
elongation, booting, grain filling of wheat plant and results shows that the
plant facing drought stress start from stem elongation stage suffered more
as compared to others. Plant height was reduced by 35% and 23% at stem
elongation stage and booting stage respectively while the plant height was
only reduced by 7% at grain filling stage (Caverzan et al., 2016a). Similarly,
reduction in the root and shoot growth of wheat when exposed to drought
conditions is reported by many other researchers also (Azooz and Youssef,
2010; Farooq et al., 2013). Hence drought is one of the major factors
responsible for overall decrease in growth of wheat plant.
Figure 1: Drought effect on growth of wheat
Figure 1 illustrates the mechanisms of growth impact under drought
stress. Under water stress conditions, cell elongation in wheat is retarded
by fall in turgor pressure. Less water uptake leads to a decrease in tissue
water contents. Consequently, turgor is lost. Similarly, drought stress also
lowers down the metabolites required for cell division. As a result,
impaired mitosis, cell elongation and expansion result in reduced growth.
Malaysian Journal of Sustainable Agriculture (MJSA) 5(2) (2021) 67-76
Cite the Article: Bipin Rijal, Prakash Baduwal, Madhukar Chaudhary, Sandesh Chapagain, Sushank Khanal, Saugat Khanal, Padam Bahadur Poudel (2021).
Drought Stress Impacts on Wheat And Its Resistance Mechanisms.
Malaysian Journal of Sustainable Agriculture
, 5(2): 67-76.
4.1.2 Leaf Senescence
A study reported that if drought occurs during reproduction stage, the rate
of senescence increases as a result of drought stress which causes the
remarkable reduction of grain yield (Nawaz et al., 2013). There occurs a
change in leaf color due to breakdown of chlorophyll membrane and water
content when the function of leaf deteriorates. Chlorosis leading to
decrease in photosynthesis is one of the vivid signs of leaf senescence (Ali
et al., 2020). Wheat growing in extreme drought conditions can cause
senescence to the whole plant but it also enhances the mobilization of
stored carbohydrates during parenthesis from the stem and leaves to
developing grains and help in compensating the loss of yield caused by
senescence during drought stress (Farooq et al., 2014). The amount of
total protein content, glutamine synthetase and rubisco (Ribulose Bi-
Phosphate Carboxylase) was used to indicate the beginning and stages of
senescence in wheat. In most of the cases senescence occurs first in older
leaves and then in younger leaves. But in some sensitive varieties, the
sequence of senescence is disturbed due to drought and first occurs in flag
leaves and later on older leaves. It is found that in younger leaves the
amount of glutamine synthetase isoenzyme declined considerably and the
sequence of senescence were disturbed slightly as compared to plants
growing in sufficient water conditions (Nagy et al., 2013). In conclusion,
the leaf senescence of wheat is correlated with drought and by monitoring
the carbon and nitrogen metabolism, we can achieve progress in making
drought sensitive genotype of wheat to make them tolerant.
Figure 2: Impacts of drought on leaf (Source: Agrilife, 2015)
4.1.3 Changes in Root System
Plant root obtains nutrients and water from the ground and plays an
important role during the condition of drought also. When there is scarcity
of water resources plant root goes deep into the soil in order to absorb
water from the soil. Many researchers have reported that the volume,
weight, length and density of root are interrelated with resistance of water
scarcity in crops. In order to survive against drought conditions, the
architecture of the root system is considered very important as the good
architecture of wheat extracts maximum soil water under drought stress
and also improving the yield of the grain (Dodd et al., 2011). The adaptive
mechanism shown by wheat in order to fight against drought stress are
osmotic adjustment of root, increase root penetration to the soil, increased
root density and root increase root to shoot ratio (Ali et al., 2020).
When there is scarcity of water root growth is favored over shoot growth.
If there is reduction in the water potential, osmotic adjustment in the root
helps to maintain level of turgidity up to some level and the water
potential gradient is re-established for water uptake. The formation of
lateral root also increases during drought stress in order to increase
surface area for water absorption. Similarly, there is increase in cross-
section diameter which helps in maintaining water retention in vascular
bundles of wheat. Also, there is increase in sclerenchyma cell diameter and
decrease in aerenchyma cell formation during drought stress (Henry et al.,
2012). So, during breeding programs, genotypes with improved root
systems are used for increasing yield because they can utilize the deep
underground water more properly to survive against drought stress.
4.2 Physiological Changes
Numerous physiological responses have been determined in response to
drought stress. There are many physiological attributes that reduces the
effect of drought stress on wheat crops. There is a direct relationship
between the availability of water and performance of different
physiological processes of plant. When there is reduction in the water
availability, these physiological processes are disturbed and plants are
unable to produce sufficient amount of dry matter. Studies have shown
that during drought condition there is reduction in the plant nutrient
uptake, plant growth rate and height as well as photosynthetic activities
and dry matter production (Todaka et al., 2015; Barbeta et al., 2015;
Ashraf and Harris, 2013). Deficiency of water also leads to decrease in
chlorophyll contents, reduction in the water content and membrane
stability (Sallam et al., 2019).
Due to the drought stress, there is a need to make some physiological
changes in the plant in order to alleviate the effect of drought stress
(Vinocur and Altman, 2005). For surviving the drought situations, plant
have to adapt itself in this situation and for this there is a development of
many tolerant genotypes which helps to maintain the soluble sugars,
proline content, amino acids, chlorophyll content, enzymatic and non-
enzymatic antioxidant activities as well (Abid et al., 2016). The
modifications done during the breeding process of these tolerant variety
help to alter the normal physiological process of wheat and performs its
normal functions on water deficit conditions. For obtaining this, wheat
plant undergoes different adjustments like change in amount of
antioxidant production, proline content, osmotic adjustment, hormone
composition, opening and closing of stomata, cuticle thickness, root depth,
loss of chlorophyll and decrease in transpiration (Rosenberg et al., 1990;
Zhu, 2002).
Different types of research have been done so far in order to understand
the physiological response of wheat to drought stress. A group researcher
observed that transpiration decreased significantly due to drought stress
and then heat can slowly be lost from the leaves and leaf temperature can
be increased (Rosenberg et al., 1990). And due to this, there is increase in
CO2 concentrations and photosynthesis which affects plant growth and
finally water use efficiency can be improved. These different types of
drought tolerance mechanism of plant help in understanding the
physiological response that helps to maintain the growth and productivity
during stress period. Similarly, these traits are also responsible in
breeding programs in order to develop drought tolerant varieties which
can perform well under those regions of the world which have scarcity of
water resources.
4.2.1 Changes in Cell Growth Pattern
Wheat is very sensitive to water stress condition and shows drastic change
in growth of plant when exposed to these types of situations. However, the
duration, time, intensity of drought and stage of wheat crop also
determines the effect of drought stress. Different experiments have been
carried out at different developmental stages like stem elongation,
booting, grain filling of wheat plant and results shows that the plant facing
drought stress start from stem elongation stage suffered more as
compared to others. Plant height was reduced by 35% and 23% at stem
elongation stage and booting stage respectively while the plant height was
only reduced by 7% at grain filling stage (Caverzan et al., 2016a). Similarly,
reduction in the root and shoot growth of wheat when exposed to drought
conditions is reported by many other researchers also (Azooz and Youssef,
2010; Farooq et al., 2013). Hence drought is one of the major factors
responsible for overall decrease in growth of wheat plant.
Moreover, the duration, type, and magnitude of drought and the stage of plant
growth also regulate the possible changes. A large body of literature is
available on the growth stage and tolerance level of wheat cultivars under
drought stress. Plant growth is also varied with duration and type of drought.
Shamsi and Kobraee conducted a two-factor experiment with three wheat
cultivars and three different stages of wheat growth (Shamsi and Kobraee,
2011). Drought stress was imposed at stem elongation, booting, and grain
lling stages and continued up to harvest.
Results showed that plants facing water stress from stem elongation stage
suffered more compared to other two stages of plant growth. Plant height was
reduced by 35% and 23% in plants facing drought from stem elongation stage
Malaysian Journal of Sustainable Agriculture (MJSA) 5(2) (2021) 67-76
Cite the Article: Bipin Rijal, Prakash Baduwal, Madhukar Chaudhary, Sandesh Chapagain, Sushank Khanal, Saugat Khanal, Padam Bahadur Poudel (2021).
Drought Stress Impacts on Wheat And Its Resistance Mechanisms.
Malaysian Journal of Sustainable Agriculture
, 5(2): 67-76.
and booting stage, respectively, but only by 7% in plants exposed to drought
at grain lling stage. Almost similar ndings were reported, by who initiated
drought at Moreover, the duration, type, and magnitude of drought and the
stage of plant growth also regulate the possible changes (Akram, 2011). A
large body of literature is available on the growth stage and tolerance level of
wheat cultivars under drought stress. Plant growth is also varied with
duration and type of drought. Some researchers conducted a two-factor
experiment with three wheat cultivars and three different stages of wheat
growth (Shamsi and Kobraee, 2011). Drought stress was imposed at stem
elongation, booting, and grain lling stages and continued up to harvest.
Results showed that plants facing water stress from stem elongation stage
suffered more compared to other two stages of plant growth.
Plant height was reduced by 35% and 23% in plants facing drought from stem
elongation stage and booting stage, respectively, but only by 7% in plants
exposed to drought at grain lling stage. Almost similar findings were
reported by who initiated drought at Moreover, the duration, type, and
magnitude of drought and the stage of plant growth also regulate the possible
changes (Akram, 2011). A large body of literature is available on the growth
stage and tolerance level of wheat cultivars under drought stress. Plant
growth is also varied with duration and type of drought. Some researchers
conducted a two-factor experiment with three wheat cultivars and three
different stages of wheat growth (Shamsi and Kobraee, 2011). Drought stress
was imposed at stem elongation, booting, and grain lling stages and continued
up to harvest. Results showed that plants facing water stress from stem
elongation stage suffered more compared to other two stages of plant growth.
Plant height was reduced by 35% and 23% in plants facing drought from stem
elongation stage and booting stage, respectively, but only by 7% in plants
exposed to drought at grain lling stage. Almost similar ndings were
reported, who initiated drought aposed to drought (Akram, 2011). Moreover,
the duration, type, and magnitude of drought and the stag.
4.2.2 Change in Chlorophyll Content and Photosynthetic Rate
With reduction in the volume of available water, plant closes their stomata
(plausibly via ABA signaling), which reduces the CO2 influx. Reduction in
CO2 not merely decreases the carboxylation directly but also directs more
electrons to form reactive O2 species. Serious drought stress restrict
photosynthesis due to the reduction in the activities of ribulose-1, 5-
bisphosphate carboxylase/oxygenase (Rubisco), phosphoenolpyruvate
carboxylase (PEPCase), NADP-malic enzyme (NADP-ME), fructose-1, 6-
bisphosphatase (FBPase) and pyruvate orthophosphate dikinase (PPDK).
Lowered tissue water contents also enhance the activity of Rubisco
binding inhibitors. Chlorophyll is a green pigment and is responsible for
the photosynthetic process. Mainly there are two types of chlorophyll
found in wheat i.e. chlorophyll a and chlorophyll b. The ratio between
chlorophylls a and b is generally 3:1 depending upon cultivars, plant
growth, and various environmental factors (Ahmad et al., 2018). Many
researchers and scientists have reported that whenever wheat plant goes
through drought stress, there is significant reduction in the leaf
chlorophyll content (Fotovat et al., 2007).
The effect of this stress is more in the chlorophyll b and the number of
chlorophyll b has decreased to more extend as compared to chlorophyll a.
This is explained by the fact that the part of the decrease in chlorophyll a
could be because of conversion to chlorophyll b (Fang et al., 1998).
Scientists have found that when wheat plant is exposed to light there
occurs enzyme activation reaction of chlorophyll synthesis which
increases the chlorophyll content in young leaves but the chlorophyll
content decrease by 13-15% in older leaves due to activation of
chlorophyllase and inactivation of enzyme under drought condition
(Nikolaeva et al., 2010). As the drought stress damages the chlorophyll
components there occurs change in the photosynthetic machinery which
resists the photosynthesis. Different studies have shown that there is
decrease in the photosynthesis of cereal crops because of the drought
stress. Electron transport chain is also affected by drought stress which
ultimately results for the production of ROS that are harmful for plant cells
and organelles like mitochondria, chloroplast and perioxisomes (Farooqi
et al., 2020).
ROS is also responsible for reduction of chlorophyll from the leaves. This
changes the inner structure of chloroplast, mitochondria, chlorophyll
content and minerals. As a result of imbalance between the light capture
and its utilization there is metabolic distortions of photosynthetic
activities, decrease in Rubisco activity, reduction of chloroplast
membranes, degradation of chloroplast structure and photosynthetic
apparatus, chlorophyll photo-oxidation, loss of chlorophyll substrate,
inability of chlorophyll biosynthesis, and the increase of chlorophyllase
activity (Kabiri et al., 2014; Kingston-Smith and Foyer, 2000). Some of the
major components limiting photosynthetic rate is CO2 diffusional
limitation due to early stomatal closure as a response to the drought
induced loss of turgor, reduced activity of different photosynthetic
enzymes, decrease of biochemical components which help in the
formation triose-phosphate and most of all there is reduction in the
photochemical efficiency of photosystem II (Pandey and Shukla, 2015).
The decrease in photosynthetic amount under drought condition is a
result of inhibition of RuBisCO (ribulose-1, 5-bisphosphate
carboxylase/oxygenase) enzyme activity and development of ATP (Dulai
et al., 2005).
Figure 3: Effect of Drought on Photosynthetic activity.
4.2.3 Membrane stability
Biological membranes are the first and foremost target of different abiotic
stresses. It is assumed that the maintenance membranes stability under
water stress is a major element of drought resistance in crops. The
membrane integrity is changed by drought stress. A plausible explanation
of this is the rise of the cell permeability accompanied by electrolyte
leakage from the cell. Drought has a huge effect on the plant cell, damaging
the selective permeability of the plasma membrane. As a result of drought
stress cell membrane stability (CMS) depicts the ability of plant tissue to
prevent electrolytes leakage by keeping the cell membrane in safe mood
(Larson et al., 1971). Measurement of solute leakage from the plant tissue
was used to estimate the damage to the cell membrane caused by drought
and heat under field conditions. The MSI (Membrane Stability Index) is
highest i.e. 82.1% in drought susceptible varieties compared to
moderately tolerant i.e. 79.4% and tolerant one i.e. 80.4% at vegetative
stage, however no differences were recorded between tolerant and
moderately tolerant varieties but at anthesis stage moderately tolerant
varieties showed lowest MSI values i.e. 75.7% and the highest value were
recorded in drought tolerant varieties i.e. 78.8%, in general MSI decreased
as plant advanced in age (Almeselmani et al., 2012). Due to the drought
stress, there is loss of water from plant tissues which affects the both
membrane structure and function. A group researcher found that there is
a correlation between electrolyte leakage and drought and the leakage was
caused due to damage of cell membranes which becomes more permeable
(Martin et al., 2006). Drought stress affects the plant more having lower
CMS value than those genotypes which have higher CMS values (Mehraban
and Miri, 2017). The genotypes having less than 50% values are highly
susceptible to drought while the genotypes with 71-80% values are
considered to grow with full potential under drought condition (Mehraban
and Miri, 2017). A group researcher reported that under drought
condition cell membrane stability (CMS) have positive relationship with
tiller per plant, grain yield but negative relationship with 100 kernel
weights (TGW) (Farshadfar et al., 2011).
Malaysian Journal of Sustainable Agriculture (MJSA) 5(2) (2021) 67-76
Cite the Article: Bipin Rijal, Prakash Baduwal, Madhukar Chaudhary, Sandesh Chapagain, Sushank Khanal, Saugat Khanal, Padam Bahadur Poudel (2021).
Drought Stress Impacts on Wheat And Its Resistance Mechanisms.
Malaysian Journal of Sustainable Agriculture
, 5(2): 67-76.
4.2.4 Relative Water Content (RWC)
Relative water content, leaf water potential, stomatal tolerance,
transpiration rate, and leaf temperature are important attributes that
influence plant-water relation. Relative water content in leaves of wheat
was more initially during leaf development and reduced as the leaf
matured (Siddique et al., 2001). Undoubtedly, drought stressed wheat
crops had lesser relative water content than non-stressed ones. Exposure
of these crops to drought stress significantly reduced the leaf water
potential, relative water content and transpiration rate, with a substantial
rise in leaf temperature (Siddique et al., 2001). Among the different types
of water potential leaf RWC is considered as more important parameter
under water deficit conditions. Due to the drought stress, there is
significant reduction in the RWC of wheat during its development stages.
The effect of drought is more in later stage (after 6 weeks of emergence)
and effects on water relations, nutrient uptake, growth, and yield than in
early stage (after 3 week of seedling emergence) in wheat (Nawaz et al.,
2014). As a result of drought stress there is reduction in water status
during crop growth, soil water potential and plant osmotic potential for
water and nutrient uptake which ultimately reduce leaf turgor pressure
which results in upset of plant metabolic activities (Mehraban and Miri,
2017). Excised leaf water retention (ELWR) is enhanced by drought stress
which reflects the water retention mechanism in the leaf under stress that
may cause leaf rolling or decrease in exposed leaf surface area. Many
researchers have found that there is continuous variation in the relative
water content during drought stress because it is controlled by multiple
genes with additive effect.
High turgor potential and relative water content is maintained by drought
tolerant genotypes to signify water had a little effect on their protoplasmic
structures as compared to sensitive genotypes which represent a highly
positive correlation between water content and photosynthetic rate
(Moayedi et al., 2010). The final impact of lower relative water content is
reducing in water status and osmotic potential in plants. Under water
deficit conditions, maintenance of leaf turgor pressure is a crucial adaptive
mechanism that plays a remarkable role in stomatal regulation and
photosynthetic activities. For the preservation of turgor pressure osmo-
regulation plays an important part which help in the absorption of soil
water and helps in plant metabolic activities for its survival (Mehraban
and Miri, 2017). Total grain yield per plant, biological yield per plant and
harvest index of wheat has a positive correlation with relative water
content (Abdul et al., 2010). Hence relative water content is a useful
parameter for selecting drought tolerant wheat genotypes (Hasheminasab
et al., 2012).
4.3 Biochemical changes
Wheat crops are provided with internal defense mechanism equipped with
antioxidant enzymes such as superoxide dismutase (SOD), catalase (CAT)
and peroxidase (POX) for ROS scavenging under stressed conditions (Chen
et al., 2012). Therefore, water stress contributed to the drastic changes in
the biochemical attributes of the wheat plants as described below:
4.3.1 Proline Content
Proline can be defined as one of the major amino acids which is used in the
biosynthesis of proteins. The response of wheat to water stress by
accumulating proline is a useful tool to understand the mechanisms of
drought tolerance. The accumulation of some organic compatible solutes
in wheat which adjust the intercellular osmotic potential is highly affected
by drought stress. As there is accumulation of organic compatible solutes
it increases the solute potential of plant which prevents loss of water
(Naeem et al., 2015). Due to the lack of water, wheat plant accumulates
proline content in larger extend than any other osmoregulators (Maralian
et al., 2010). It is reported that there is increase in the proline content of
wheat plant after it had been subjected to drought stress (Vendruscolo et
al., 2007). It is found that the maximum amount of proline increases in
heading stage of wheat when it is under water stress condition (Maralian
et al., 2010). The genotypes of wheat which have more accumulation of
proline under drought have the ability to bear drought stress and it is
different for different wheat genotypes because different genotypes have
variable water stress threshold. Hence the estimation of proline content of
wheat can be a useful trait for selecting drought tolerant wheat genotype.
4.3.2 Antioxidant Properties
Due to the drought stress, there is accumulation of reactive oxygen species
(ROS) in the cells, which can cause severe oxidative damage to the plants
which inhibits the growth and grain yield of wheat plant. The equilibrium
between the production and scavenging of ROS is knows as redox
homeostasis (Caverzan et al., 2016a). However, if the production of ROS
exceeds the cellular scavenging capacity, it creates the unbalancing of the
redox homeostasis which results in rapid and transient excess of ROS,
known as oxidative stress (Sharma et al., 2012). Therefore, plants have
antioxidant mechanisms for scavenging the excess ROS and prevent
damage to cells. Enzymatic and non-enzymatic antioxidants are
responsible for maintaining the equilibrium between the production and
detoxification of ROS (Mittler, 2002). In wheat, several studies have
showed that there is change in the activity of the antioxidant defense
system in plant to control the oxidative stress induced by many
environmental factors like drought.
There is activation of both enzymatic and non-enzymatic system which is
used to detoxify the toxic levels of ROS which is harmful to plant produced
as a result of drought stress (Caverzan et al., 2016b). Different studies have
showed that the responses of these enzymatic and non-enzymatic systems
vary with different genotypes, Different genotypes shows different
responses under same condition. Generally tolerant genotypes show
higher antioxidant capacity resulting in lower oxidative damage to the
plant. This response also depends upon several others factors like tissue
type, length, and intensity of the stress as well as on developmental stage
proving the complexity of the mechanism of production and detoxification
of ROS and the effect of ROS on antioxidant system (Caverzan et al., 2016a).
Hence having the information about the antioxidant response of wheat
during drought stress, it helps us to develop different improved genotypes
having more antioxidant properties.
5. RESISTANCE MECHANISM OF DROUGHT STRESS
Through the induction of different morphological, biochemical, and
physiological responses, wheat crops respond and adapt to and survive
under severe drought stress conditions. Drought stress disturbs the water
circulations at different levels, causing unwanted reactions and finally
adaptation reactions (Beck et al., 2007). To survive under such conditions,
susceptible plants have defense mechanisms against drought stress, which
need to be studied comprehensively. As a result of drought stress, there
occurs a rapid loss in the yield and yield performance of wheat. The
different process which occurs in plant in order to suppress the stress in
the given condition and producing the higher yield as compared to normal
water availability conditions is known as drought resistance. According to
the agriculture point of view drought resistance can be defined as the
process in order to minimize the loss of economic yield under limited
water availability conditions (Bohnert et al., 1995). Generally, there are 3
different forms of drought resistance i.e. drought escape, drought
avoidance and drought tolerance (Bohnert et al., 1995).
Figure 4: Resistance mechanism of Drought Stress
Drought
stress
Drough Escape
Early flowering
Early maturity
Developmental
Plasticity
Assimilates
Remobolization
Drought
Avoidance
Less water
loss
Closing of stomata
Leaf Rolling
Leaf Firing
Glaucousness
More water
absorption
Higher Root Depth
High root density
High root-shoot ratio
Drought
Tolerance
Osmoregulation
Osmotic
adjustment
Antioxidant
Enzymes
Genetic
modification
Malaysian Journal of Sustainable Agriculture (MJSA) 5(2) (2021) 67-76
Cite the Article: Bipin Rijal, Prakash Baduwal, Madhukar Chaudhary, Sandesh Chapagain, Sushank Khanal, Saugat Khanal, Padam Bahadur Poudel (2021).
Drought Stress Impacts on Wheat And Its Resistance Mechanisms.
Malaysian Journal of Sustainable Agriculture
, 5(2): 67-76.
5.1 Drought Escape
The process of shortening the life cycle or growing season of plant in order
to avoid the dry environmental conditions is known as drought escape.
The condition required for drought escape is, when the phenological
development is successfully matched with periods of soil moisture
availability and it occurs when the growing season is reduced and terminal
drought stress prevails (Araus et al., 2002). In order to achieve drought,
escape we can use different process like early maturity, developmental
plasticity, assimilate remobilization (Vani et al., 2017). Among these
various factors, early maturity of wheat is considered as one of the major
aspects for escaping the drought stress. For obtaining early maturity we
have to reduce the developmental time at different growing stages of
wheat. Among different stages, shortening of time at flowering stage is
considered as one of the most effective time for escaping drought
(Shavrukov et al., 2017). Through selection of different genotypes of wheat
having early flowering behavior, their vegetative growth can limit and
enables the reproductive growth to occur before the terminal stress
(Bodner et al., 2015). Early flowering and maturity can be considered as
an effective mechanism for drought escaping however it has some
drawbacks and can limit the grain yield potential due to reduction in time
for photosynthesis and seed nutrient accumulation required for higher
grain yield (Bidinger and Witcombe, 1989; Radhika and Thind, 2014).
5.2 Drought Avoidance
There are numerous ongoing physiological and metabolic activities on the
plant which are not exposed to drought stress and are continuously
performing their normal functions even in the water deficit conditions.
The ability of plant to maintain relatively higher content of water in tissue
of plant despite the fact of having lower water content in the soil is known
as drought avoidance (Levitt and others, 1980). It helps to control the
water loss by controlling stomatal transpiration and also maintain water
uptake through an extensive and deep root in the soil. During the drought
condition wheat maintains its water status by closing their stomata. In
contrast, there are some negative effects on photosynthesis and
respiration as a result of stomata closing. Moreover, there occurs a leaf
rolling in response to drought in order to save water content in plant
which later unrolls when the leaf-water relations of plant improve (Sirault
et al., 2015). It has been reported that the epi-cuticular wax layer of wheat
also called as glaucousnessis also responsible for maintaining the leaf-
water relations during the drought stress and is considered as important
trait for drought avoidance (Richards et al., 1986). Drought avoidance is
also influenced by several root characters like root length, root density and
root biomass (Kavar et al., 2008). Greater thickness of root, higher root
depth and higher root density are responsible for excessive water uptake
during drought stress (Aina and Fapohunda, 1986). Hence there are
various characters of plant like stomatal transpiration, glaucousnessis, leaf
rolling and different structural and functional aspects of root are
responsible for avoidance of drought under stress conditions.
5.3 Drought Tolerance
The ability of plant to maintain their growth and development during
water deficit conditions is known as drought tolerance. Drought tolerance
is a complex mechanism and plants modify its different physiological and
biochemical factors to fight against the water deficit conditions in order to
maintain its normal growth and yield capacity. Different studies have
shown that osmoregulation, osmotic adjustment and activity of
antioxidant enzymes plays a vital for dealing with the drought stress
situation in plants (Nemeskéri and Helyes, 2019). In order to fulfill the
increasing demand of food for growing population it is must to develop the
more advanced wheat tolerant genotypes. Thus, the main aim of the
drought related research program is to identify such genes which can be
used in the breeding program in order to develop new more drought
tolerant genotypes of wheat. Drought tolerant variety of the wheat have
some modifications over the normal physiological and biochemical
processes in order to survive in the water deficit conditions. Drought
tolerance mechanism involves the activation of different physiological and
biochemical processes at cell, tissue, organ and whole plant level. Some of
the major fields for genetic modifications of wheat in order to obtain
drought tolerant variety are in table 2.
Table 2: Modification field for improving wheat resistance
S.N
Field for genetic modifications
1
Drought-Induced Gene Expression/Single Action Gene
2
Osmo-protectants, Metabolites and Protective Genes
3
Transporter Genes
4
Carbon Metabolism
5
Transcription Factors
6
Post-Translational Modification
7
Protein Kinase
8
Nuclear Factor
Source: (Khan et al., 2019)
Until now, different studies have been done in order to exploit the genes
that are responsible for drought stress and have been categorized through
RNA sequencing and the Affymetrix GeneChip technology (Dugas et al.,
2011). Researchers found that different types of kinase likeCDPKs
(calcium-dependent protein kinases), CIPK (CBL interacting protein
kinase), and SnRK2 (sucrose non-fermenting protein-related kinase 2)
and MAPKs (mitogen-activated protein kinases) also shows some
response to drought stress (Malone and Oliver, 2011). There is a
correlation between the drought and antioxidant system and shows
positive response towards it. Reports suggested that reactive oxygen
species (ROS) like OH (Hydroxide), H2O2 (Hydrogen Peroxide), SOD (Super
Dioxide) and oxygen which is singlet are created in drought conditions
(Nezhadahmadi et al., 2013a). Some of the studies shows that wheat
genotype having lower malondialdehyde (MDA) content and greater
osmotic regulator has helpful for obtaining tolerance against drought
(Nezhadahmadi et al., 2013a). All these parameters have important role in
drought tolerance and it can be useful for selecting drought tolerant
varieties and lines particularly at reproductive stage (M. Almeselmani,
2012).
6. CONCLUSION
Different years of drought in Nepal have been identified and the impacts
of those stresses on crop production have been assessed. Historical
evidences have shown that there was a huge loss in crop yield in the past.
Drought stress retards crop growth and development, leading to the
changes in morphological, physiological, and biochemical attributes of the
crop. Since majority of the global wheat cultivation area lies in arid and
semi-arid regions, drought is one of the major problems for obtaining the
potential yield. It reduces the proper growth and development of plants
hampering fruiting and grain filling which eventually leads to reduced size
and number of wheat grains. Injury biochemical reactions under drought
stress are among the major deterrents to growth. Due to this reason, there
is great economic loss in the production of wheat all around the world. For
improving yield under drought condition, it is essential to understand the
physiological response of wheat under these situations. Various resistance
mechanisms have been developed in the plants to cope with drought
stress. CO2 assimilation by leaves is decreased primarily by stomatal
closure, membrane damage and disturbed activity of various enzymes,
especially those of CO2 fixation. By understanding the physiological,
morphological, and biochemical responses of wheat under this situation,
it helps us to identify drought tolerance mechanism and develop drought
tolerant varieties of wheat.
ACKNOWLEDGEMENT
We would like to express our sincere gratitude to Asst. Prof. Dr. Mukti Ram
Poudel, Department of Agronomy, Plant breeding and Agri-statistics,
Paklihawa Campus, Institute of Agriculture and Animal science for the
continuous support during the manuscript preparation. Also, special
thanks to the author’s parents whose guidance and motivation are always
with us.
Malaysian Journal of Sustainable Agriculture (MJSA) 5(2) (2021) 67-76
Cite the Article: Bipin Rijal, Prakash Baduwal, Madhukar Chaudhary, Sandesh Chapagain, Sushank Khanal, Saugat Khanal, Padam Bahadur Poudel (2021).
Drought Stress Impacts on Wheat And Its Resistance Mechanisms.
Malaysian Journal of Sustainable Agriculture
, 5(2): 67-76.
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