**4. Effect of drought on wheat nutrition**

According to the most recent assessment report of the Inter-governmental Panel on Climate Change, published in 2014, levels of anthropogenic emissions of greenhouse gases are now at their highest in history [49]. Agricultural production and its effect on land use are major sources of these emissions by sharing methane and nitrous oxide gases. Greenhouse gases causing air temperatures increase, thus more moisture evaporates from land and water bodies. Warmer temperatures also increase evaporation and evapotranspiration in plants, soils, and on other hand, they will also escalate the water stress frequency and intensity with a rise from 1 to 30% in acute drought land area by 2100 [50].

Under dry conditions in the field, 75–100% of the grain yield could be attributed to stored assimilates, compared with 37–39% under high-rainfall conditions. Drought stress severely influenced plant water status by reducing the water potential and the relative water content in wheat [51]. Optimal nutrition levels have also alleviated drought stress damage by sustaining metabolic activities under reduced tissue water potential [52]. Nitrogen supply also has a crucial role in combating drought [53]. Efficiency of nitrogen supply declined with increasing of drought stress [54]. Morgan [55], Arun et al. [56] and Binghua et al. [57] who showed that with an application of nitrogen, plants show positive influence in terms of growth and development under drought stress. Although Li et al. [58] mentioned that different grass species under drought stress did not modify physiological functions under varying N application. Water limitation reduces diffusive conductivity which in turn affects other physiological process such as energy and N metabolism. It is concluded that N uptake and its diffusion depend on environmental condition especially to water supply as also indicated by Abreau et al. [59]. Under water deficiency, roots are unable to get optimal amounts of nitrogen from soil, which has general negative effects on plant metabolisms [60]. The main effect of water restriction is certainly a reduction in N demand due to the marked sensitivity of leaf area expansion [61]. Fewer results have about light reaction affected by genotypic and nitrogen supply variations, mainly under stress conditions. By measuring the yield of chlorophyll fluorescence (Chl-fl), information about changes in the efficiency of photochemistry and heat dissipation can be obtained [62]. Under extreme drought stress when the stomatal resistance just around 0.1 mol H2 O m−2 s−1, poor performance of photosystem II (Fv /Fm) and downregulated activities of CO2 assimilating enzymes such as Rubisco become the dominant limitations to reduced photosynthesis [63]. The optimal photochemical activity (Fv /Fm) values were sensitive for the investigated two environmental factors, and genotype differences were established in tolerance [64]. Chl-fl parameter's sensitivity for detecting nitrogen deficiency is different, but some of them are really applicable for describing nitrogen lack [65]. Previous drought stress studies have reported that photosynthetic rate of the leaf under drought stress is closely related to the leaf chlorophyll contents, N concentrations and stay-green characteristics of the leaf, which in turn increases the grain yield by increasing the photosynthetic process [66]. Palta et al. [67] and Hosenlou et al. [54] reported induction of N remobilisation under drought stress. Application of the high amounts of N under drought resulted to the lowest NUE [68]. Critical, sufficient concentration of nitrogen in leaf is 15–40 mg g−1 DM [69]. Based on Pepó [16] and Zsombik and Seres [70] results, the dry weight production was mainly influenced by environmental factors and modified by fertilisers and genotypes. Water deprivation means higher strain than nitrogen luck with genotype difference based on dry weight value [65]. Plant responses to drought stress vary at different growth stages of the crop [71]. In wheat, tillering capacity of the crop is a major constituent of the final grain yield [72], but has been reported to be highly vulnerable to drought stress [73].
