**4. Adverse effects of drought on wheat production**

Drought is one of the most limiting stress factors for crop growth and development, dry matter production and potential yield [15, 54]. The major processes required for plant growth and development are hampered by the drought condition. Water deficit conditions lessen the rate of photosynthesis by inhibiting chlorophyll synthesis, impede cellular elongation and metabolism, decline the CO2 assimilation rates due to reduction in stomatal conductance and gaseous exchange, reduce dry matter biomass production and alter root morphology [54, 55]. As a result leaf size, stem elongation, root production and finally the rate of growth and yield are affected by drought [54].

Drought is not a static stress, it can occur at any crop growing period, its severity and frequency can vary and also it can recurrently happens in combination with other abiotic stresses, such as salinity and heat [56]. Drought stress can fluctuate diurnally (high during peak photosynthetic period and low overnight) and different organs of plants respond differently to drought stress [57]. Yield contributing traits vary according to growth stage of plant, so the level of seriousness of drought stress eventually relies on the particular growth stages that are impacted by drought. The nature of plants' response also differ depending on whether the plant is experiencing stress for the first time or after several exposures and whether they are recovering from stress after a rainfall or irrigation event [58].

Water is needed for the entire growth period of wheat but some specific stages are more sensitive to water limitations. Various morphological, physiological and biochemical alterations are occurred in plants body under drought environment (see **Table 3**). In case of wheat, the extent of drought stress may vary according to different growth stages. Specific critical growth stages of wheat plants such as germination and seedling stages [60]; tillering and stem elongation stages [61, 62]; heading, anthesis and grain filling stages [16, 60] may be more vulnerable


**Table 3.**

*Effect of water-deficit stress on morphological, physiological and biochemical traits of wheat [59].*

to drought stress. Long term droughts (starting from stem elongation through to maturity) cause more drastic yield reduction compared to those initiating at later stages through to maturity [63]. Although the influence of drought stress on heading and grain filling stages are more severe in terms of yield, drought can also negatively affect the multiple growth stages of wheat comprising germination, tillering, booting, heading, anthesis, and maturity [64].

## **5. Crop traits and mechanisms adaptive to drought stress**

In drought condition, sometimes very swift growth responses are generated even due to a little water pulse that can vigorously activate plant growth and safeguard survival [65]. Constantly fluctuating nature of drought events makes it indispensible to understand the plants' aptitude to adapt and recover from the stress [66]. To overcome the harmful effects of drought stress, naturally plants are well furnished with various adaptive mechanisms. These adaptive mechanisms support plants for an optimal maintenance of growth for metabolic regulation and survival [67]. The more the extent of these mechanisms, the more will be the plants' capability to overcome stress condition. But the adaptive mechanism is not as simple as it sounds; it comprises diverse morphological, physiological and anatomical modifications in plant under stress condition. Morphological and metabolic adaptation processes of plants vary according to cultivars in response to water deficit condition. As, plants' may have unique adaptation capabilities irrespective to cultivars [68]. Different physiological processes in plant such as photosynthesis, heat dissipation and chlorophyll fluorescence are occurred in rivalry with each other in response to drought events i.e. any upsurge in the efficacy of one will bring diminution to others [69]. The normal fluctuation values of these physiological processes can denote plant fitness with the magnitude of environmental stress [55].

Drought adaptation is complicated that experiences diverse anatomical and morpho-physiological and biochemical amendments in plants such as alterations in leaf traits or canopy cover, leaf water relations with modification of growth rates, reduction of stomatal opening and associated components [70, 71]. The plants' response to water deficit condition has been extensively studied to recognize tolerance mechanisms [72]. So, detailed knowledge about underlying behavior of plants under drought stress is required to develop drought tolerant plants. Although being complex, mainly three kinds of drought-resistance mechanisms are exhibited by plants to evade the resulting devastating effects of droughts: (i) drought escape (ii) drought avoidance and (iii) drought tolerance [73]. Drought escape happens when plants grow quickly and reproduce before severe drought conditions. In this mechanism, plants evades drought season by modifying flowering time thus they try to complete their life cycle before drought condition. In drought avoidance mechanism, plants avoid water-deficit situation by enhancing their water-use efficiency (WUE) through closure of stomata, reduction of transpiration, limitation of vegetative growth, or by increment in root growth. In case of drought tolerance, drought stress is fought by plants at cellular level through osmotic adjustment by developing antioxidants and production of molecules that stabilize proteins [73].

Wheat plants exhibit a tight network of morpho-physiological and photoprotective mechanisms to alleviate the drought stress [66]. To escape reproductive failure from severe drought stress, plants displayed phenological alterations of earlier anthesis and maturity [66]. Previous literature revealed constitutive traits that confer dehydration avoidance mechanisms in plants include leaf waxy layer, leaf rolling and osmotic adjustment [74], high root length density [75] and high fine roots with small diameters [76], a deep root system and the number of seminal roots [77–79],

*Drought Affected Wheat Production in Bangladesh and Breeding Strategies for Drought Tolerance DOI: http://dx.doi.org/10.5772/intechopen.95283*

#### **Figure 2.**

*Morpho-physiological and biochemical derivatives of drought tolerance in wheat.*

high total root length and total root surface area [80, 81], root-to-shoot dry matter ratio [82] and root partitioning of assimilates to shallow or depth roots in response to drought [83]. There are range of morphological, physiological and biochemical derivatives of drought tolerance in wheat [59] (shown in **Figure 2**). Water use, water use efficiency, biomass yield and flag leaf relative water contents are the important drought tolerance traits in wheat [84, 85]. Selection of wheat plant with high transpiration efficiency, high percentage of relative water contents and cell membrane thermo-stability and greater osmotic adjustment capacity leads to produce drought tolerant plants [59, 86]. When drought stress is imposed on seedling stage of wheat, cell membrane thermo-stability, fresh and dry weight of seedlings are considered major traits to govern drought responses under stress conditions [87]. Therefore, greater morphological adaptation with limited down-regulated physiological activities followed by high recovery in wheat cultivars designate its capability to effectively endure drought events [66].

#### **6. Prospects of breeding for drought tolerance in wheat**

Plant breeders around the world have to deal with great challenges to work with drought stress. Polygenic nature of drought makes the breeding efforts more complicated than other abiotic stresses [88]. Global climate change will result in frequent drought events as per predicted in country like Bangladesh [89]. So, for improving wheat production in Bangladesh, research priority should be focused on breeding new high yielding drought tolerant wheat varieties. Majority of the studies under drought stress focus on the response of natural drought in field conditions where drought events are ambiguous and irregular using conventional techniques. Generally the conventional breeding techniques such as introduction, selection, hybridization and mutation are being used by the breeders of Bangladesh. Whereas throughout the globe, wheat breeders are now using different novel breeding methods including *in situ* and *in vitro* techniques. Under drought stress, several morpho-physiological and biochemical mechanisms are activated in plant body to withstand the stress. But poor conceptual knowledge about the developmental and physiological basis of yield related traits under water-deficit environments make the drought stress more complex [90]. Therefore, better understanding about the detailed physiological and genetic adaptive strategies of wheat cultivars during water-deficit stress would offer the appraised benchmarks of breeding methods

for pursuing drought tolerance in wheat [59]. Hence, selection procedures based on physiological traits have potentiality to improve the final productivity of wheat under drought stress [66].

In recent times, as part of empirical breeding based programs, breeders have been embracing replicated, multi-locational and multi-year variety testing for finding out the best adaptive varieties to stress environments. Expanding grain yield under drought stress can be performed to a limited extent through selection process [91, 92]. For being recurrent and season indefinite stress event, trait evaluation under drought condition may cause losing of potential genetic resources which perform better in normal wheat-growing environments [89]. This may ultimately hamper the variety development process. Therefore, evaluation including diverse testing environments including both normal and stressed conditions will be more suitable and competent for the development of high yielding, stable varieties amended to water-deficit conditions [89].
