Keywords

1 Introduction

Drought is an environmental worldwide problem, especially in arid regions, affecting crop productivity earnestly, and recent climate change has made this status more critical (Anjum et al. 2011). Across the world, the projections point out an increase in water request used in agriculture leading to more drought stress (Turral et al. 2011). Also, Qadir et al. (2007) stated that 60% of the people in the world may suffer from water scarcity by 2025. High temperature combined with absence each of rainfall or water applied, for a period long, cause soil moisture depletion, which in turn causes disturbance of physiological processes in plants (Wang et al. 2011; Ahmed et al. 2017). Wheat (Triticum spp.) is a sensitive crop to drought stress specifically during germination, heading stage, and grain filling period. Exposing wheat plants to drought adversely affects phenological development, physiological and biochemical processes, and yield (Waraich and Ahmad 2010; Akram et al. 2014; Hammad and Ali 2014). Water often limits crop growth and development. Plant roots grow into moist soil and absorb water until a critical water potential (Ψw). Soil moisture availability is affected by some soil properties (texture, electrical conductivity, pH, etc.). A clay soil concludes more available water ranged 18–20% of its weight, whereas the sandy soil holds about 6–7%.

Stomatal closure during water stress period reduces CO2 uptake and consequently carbohydrates accumulation. Wheat responses to water deficit are relative to their genotypes performance, growth stage, metabolic activity, the timing of stress, and degree of stress (Blum 2011; Nakhforoosh 2014).

This chapter focuses on wheat responses to drought through understanding the drought mechanisms and the changes in phenological development and physiological process in plants grown under water deficit levels.

2 Drought Mechanisms

Adaptation to water stress is mainly identified through three mechanisms, i.e., drought tolerance, drought escape, and drought avoidance. Enhancement of wheat growth and yield by agricultural practices and/or breeding for drought adaptation strongly depends on drought duration and occurrence time (Blum 2011). It could affirm that the three ways of drought adaptation are not separated because the same plant can possess more than one mechanism of water deficit adaptation. This occurs through physiological management and genetic modifications. Wheat faces water stress by drought escape via reducing transpiration rate and vegetative growth, early flowering, and maturity, which enable the plant to evade water deficit stress (Shavrukov et al. 2017). Drought avoidance by water saving via increasing root distribution and reducing stomata numbers and leaf area/canopy ratio. Improving topsoil root length density, root volume, and deep rooting caused an increase in soil water absorption (Nakhforoosh 2014). Water-use efficiency is referring to yield production in relation to water used during growing season to produce this yield. High water-use efficiency (WUE) reduces water loss and allows the plants to ready up for changing environmental condition and be adaptive with drought (Shavrukov et al. 2017) especially during the grain filling period. A modification of genetics by plant breeding programs through conventional and molecular methods can be an effective way for wheat breeding to drought tolerance via improving osmotic adjustment ability and increasing cell wall elasticity (Araus et al. 2008; Xu et al. 2010). New wheat varieties that had superior WUE and photosynthesis rate via high leaf chlorophyll content produce more yield (Nakhforoosh 2014).

3 Wheat Responses to Drought

Wheat plants response to water deficit condition by complex mechanisms via molecular genetic expression, biochemical metabolism, and physiological processes (Fig. 1). Wheat plants are a response to water deficit relative to its growth stage, metabolic activity, and yield potential, as well as the timing of water stress and its degree (Blum 2011).

Fig. 1
figure 1

Wheat plant responses to drought. (Adapted from http://www.plantimpact.com)

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3.1 Germination and Seedling Vigor

Germination is considered as a critical stage for water stress. Germination is an important stage affecting seedling vigor and plant population in the unit area. Increasing RNA and protein in wheat grain are the first indicators that indicate germination beginning (Zhang et al. 2009). After grain absorbs water directly, these processes are established. Increasing water stress degree causes a decrease in shoot elongation of seedlings (Esfandiari et al. 2008).

3.2 Phenological Development

Differences among drought stress regimes found to be significant for growth traits, i.e., plant height, numbers of tillers and leaves per individual plant, root and shoots dry weights plant−1, flag leaf area, and leaf area plant−1. All characters were negatively affected by lower water supplies during the growth stage. Root elongation and its dry weight are not affected as much as leaf area, stem elongation, and shoot dry weight. Roots were expanding into an area where available water is not depleted. High root densities occur in upper soil zone where water is extracted rapidly, but as water becomes limiting in this zone, roots expand into lower soil profile where water is more abundant (Anjum et al. 2011). The inhibition in stem elongation, leaf area, and shoots dry weight of wheat plants grown under moderate drought were amounted to 17.35, 14.50, and 18.54% while reached 29.53, 32.34, and 43.90% under severe water stress, respectively, compared with well-watered plants (Hammad and Ali 2014). The most dramatic effect of the early vegetative water deficit was a reduction in leaf area and shoots dry weight. Restricted internode elongation and leaf expansion can occur under water deficit through inhibiting cell expansion (Waraich and Ahmad 2010). Drought causes losses in tissue water contents that reduce turgor pressure in the cell, thereby inhibiting enlargement and division of cell causing reduction of plant growth and dry mass production.

3.3 Stomatal Conductance and Photosynthesis

The stomatal response can be different in wheat genotypes and stage of development. The stomatal closure had to be the factor reducing the photosynthesis. Stomatal closure during water stress period reduces CO2 uptake. A continuation of water deficit causes a more reduction in photosynthesis rate. Stomatal conductance and photosynthetic rates were reduced when plants exposed to water deficit, which include some signals like ABA accumulation. The photosynthetic system may be damaged under severe drought condition (Le et al. 2017). The stronger association could be detected between chlorophyll content and photosynthetic rate. Carotenoids are as signaling molecules and have been engaged to have a role in the interactions of plants with their environment (Esteban et al. 2015). Statistical analysis shows significant decreases in chlorophylls and carotenoids content by increasing water deficit as compared with normal water applied (Hammad and Ali 2014). The decrease in photochemical activities of chloroplast occurred by water deficit can be correlated with the decrease in the accumulation of chlorophyll. A decrease in net photosynthetic rate under water stress is also related to disturbances in biochemical processes of a non-stomatal nature, caused by oxidation of chloroplast lipids and changes in the structure of pigments and proteins (Marcinska et al. 2013). A significant decrease in CO2 assimilation rate with increasing water deficit levels (Waraich and Ahmad 2010). Many investigators have proposed the response of wheat photosynthetic rate to water deficit (Ali et al. 2007; Maria et al. 2008).

3.4 Plant Water Relations

Relative water content , osmotic pressure (OP), and membrane integrity (MI) are important indicators that affected plant-water relations. Wheat plants grown under severe drought condition followed by moderate drought condition produced the highest significant values of OP and MI compared to well-watered plants. However, there was a gradual reduction in RWC by increasing drought stress degree, which reached about 15.15% under severe drought (Hammad and Ali 2014) compared to normal water applied (irrigation at depletion 50% of available water). Relative water content has been an important indicator of water stress in leaves, which is directly related to soil water content (Rampino et al. 2006; Waraich and Ahmad 2010). This indicates greater resistance to water flow at the soil-root interface or decreased hydraulic conductivity of soil at low soil moisture. In response to water deficit, wheat plants can minimize the deleterious effects by increasing osmotic adjustment through the accumulation of solutes within the plant. Under stress condition, cell membranes are subject to changes often associated with the increase in the cell permeability (Iqbal 2009). Wheat drought-tolerant genotypes maximize WUE by lowering the water loss. Improved crop management and plant breeding have led to substantial acquire in WUE. The authors usually conclude that plants need strong and deep root systems during water deficit condition, and increases in rooting depth increase the total quantity of water available for extraction during the growing season (Evans and Sadler 2008).

3.5 Biochemical Responses

Under water deficit, the expression of many genes is enhanced, influencing the metabolism of several biochemicals, e.g., enzymes, hormones, amino acids, and carbohydrates (Yang et al. 2010). Water stress significantly affected TSS, TC, TAA, and TP. The stress plants recorded lowest values of TC and TAA, while well-irrigated plants obtained the highest values of TSS and TP (Iqbal 2009; Hammad and Ali 2014). The reduction in severe plants could be ascribed to that water-induced loss of solutes from guard cells, leading to stomatal closure. Soluble carbohydrate in well-watered wheat plants exhibited higher content than those obtained from stressed plants (Zhang et al. 2009).

A gradual increase in proline content when exposing wheat plants to drought stress compared to its content in well-watered plants. Proline appears to assist plants in drought tolerance (Ahmed et al. 2017). It does not only act as an osmolyte , but it also contributes in stabilizing subcellular structures (e.g., membranes and proteins) under water deficit (Iqbal 2009; Alaei et al. 2012). Proline is believed to stabilize membrane phospholipids which helps the plants to overcome periods of drought stress (Rampino et al. 2006).

Water stress negatively affected N, P, and K percentages and their uptakes in leaves’ tissues compared to plants grown under normal condition. The maximum reductions were recorded under severe stress, which reached about 22.11, 29.73, and 15.98% for N, P, and K, respectively (Hammad and Ali 2014). Uptake of NPK also took the same trend with increasing water stress degree in comparison with well-watered plants (Baque et al. 2006 and Maria et al. 2008).

POD, POX, CAT, and NR enzymes in wheat leaves were negatively affected under drought conditions. Enzyme activities were decreased in response to water stress, and maximum decreases were recorded in severe stress (Caravaca et al. 2005; Iqbal 2009; Hammad and Ali 2014) with increasing duration of water stress at both booting and grain filling stages.

Plant responses to water deficit are known to be generally determined by endogenous phytohormones (Pandey et al. 2015). Water deficit causes change in the balance of endogenous phytohormones by increases in the growth inhibitory hormones with a decline in growth-promoting hormones. Water stress caused a significant increase in ABA while decreases in IAA and GA contents of wheat leaves (Barnawal et al. 2017). Stressed plants growing in a soil water level near permanent wilting point showed a significant increase in endogenous ABA content in leaf and root compared with well-watered plants. Under water deficit, the inhibition of plant elongation is as likely to be due to elasticity loss as to ABA accumulation. It is well known that the changes in endogenous hormones levels after heading period might indirectly influence starch and protein accumulation in wheat grains by affecting the regulatory enzymes (Xie et al. 2003).

3.6 Yield

The timing of water applied and the quantity of irrigation water applied were important factors controlling biomass yield. Grain yield is the final product of many of the developmental processes revolving throughout growth, influenced by environmental conditions (Mwadzingeni et al. 2016). Wheat grain yield is mainly based on three components, i.e., number of spikes per unit area, number of grains per spike, and 1000 grain weight. These components are determined according to the plant responses to resource availability at different development stages (Akram et al. 2014). Delays in first irrigation, crown root initiation stage, have reduced yield by 27%. In addition, the flowering stage and grain filling periods are considered as critical stages, which need water supplements (Hunsigi and Krishna 1998). At anthesis , soluble sugars can represent 5–7% of the total dry matter. It seems likely that this stem reserve of carbohydrate buffers grain filling and grain yield against reductions in post anthesis photosynthesis caused by water deficit. After 3 weeks after pollination , water deficit no longer affected grain number per spike but did decrease grain weight, indicating that the moisture stress reduced photosynthetic and translocation to spikes. The translocation rate to formed spikes under water deficit was not enough to develop their grains normally. Grain yield of modern wheat varieties has increased considerably over the last periods. This yield has been achieved without much increase in seasonal evapotranspiration (ET) because the WUE has increased along with increases in the yields. It is supposed that new wheat varieties that have higher WUE and strong grain sink under water deficit condition had more drought tolerance and produced more yielding than other varieties (Khakwani et al. 2012; Nakhforoosh 2014). Numerous studies showed that wheat varieties differed extensively in their physiological processes which determine yield (Ali et al. 2015; Ahmed et al. 2017). Water stress-affected wheat plants might lie not only in the variations in physiological processes like an accumulation of osmolytes , antioxidant capacity, and stomatal conductance but also in changes in the phytohormonal balance (Iqbal 2009). During water-limiting conditions, photosynthesis reduction happens, resulting from reducing the efficiency of biochemical processes, which led to suppressing vegetative growth and dry matter production. Decreasing grain, straw, and biomass yields could have occurred due to inhibition of physiological and biochemical processes. Increasing drought stress decreases the grain yield by about 14.63% and 41.37% under moderate and severe drought, respectively, compared with normal irrigation condition (Hammad and Ali 2014).

3.7 Nutritive Value of Grains

Significant depression in total protein and carbohydrates content, while a gradual increase in total fibers of wheat grains were occurred when exposing plants to severe drought. Irrigation until soil water reached 50 up to 65% depletion had the effect of increasing grain protein and carbohydrates content (Kilic and Yağbasanlar 2010; Hammad and Ali 2014). This could have occurred due to more NPK uptake from the soil compared to severe drought condition (Dromantiene et al. 2009; Bakry et al. 2012).

4 Conclusion

Exposing wheat plants to drought is adversely affecting phonological development, physiological and biochemical processes, and yield. New wheat varieties that have higher WUE and strong grain sink under water deficit condition had more drought tolerance and produced more yields than other varieties. Therefore, there will have to be concerted efforts to integrate genetic response with good agronomic practices of wheat crop in order to assure that there is maximum grain supply for all the world’s populations.