**2.2. Drought and root signaling**

Effect of drought on the growth and yield of wheat were investigated. Drought during grain filling further decreased yields. Plots with a lower plant density demonstrated a smaller decrease in yield due to drought. There was a significant positive linear relationship between the number of shoots per plant and nodal root axes per plant. There appeared to be a difference between cultivars in root system architecture, and in their response to drought, but these differences were not reflected in grain yield [28]. Drought-induced loss in crop yield probably exceeds losses from all other causes, since both the severity and duration of the stress are critical. Drought stress reduces leaf size, stem extension and root proliferation, disturbs plant water relations and reduces water-use efficiency. Plants display a variety of physiological and biochemical responses at cellular and whole-organism levels towards prevailing drought stress, thus making it a complex phenomenon. Plants display a range of mechanisms to withstand drought stress. The major mechanisms include curtailed water loss by increased diffusive resistance, enhanced water uptake with prolific and deep root systems and its efficient use, and smaller and succulent leaves to reduce the transpirational loss. At molecular levels several drought-responsive genes and transcription factors have been identified, such as the dehydration-responsive element-binding gene, aquaporin, late embryogenesis abundant proteins and dehydrins. Plant growth substances such as salicylic acid, auxins, gibberellins, cytokinin and abscisic acid modulate the plant responses towards drought. Polyamines, citrulline and several enzymes act as antioxidants and reduce the adverse effects of water deficit [29]. The possibility of reducing the proliferation of roots to increase yields at higher seeding rates and conserving the soil water at different growing stages in water-limited environments was studied. In the severe drought towards the end of the growing season, grain yield decreased as the seeding rate increased, but under the more favorable conditions the reverse was true. Averaged over the seeding rates, grain yield was significantly increased; grain yield and yield components were higher and root pruning at spring-growth stage recorded the highest water use efficiency [30]. The leaf net photosynthetic rate and stomatal conductance were significantly decreased under drought. The leaf transpiration rate was decreased by drought. The intercellular CO<sup>2</sup> concentration was increased under drought, while it was decreased most of the time from midday to the afternoon. The leaf stomatal limitation was increased under drought [31].

Root length, root dry weight and seedling dry weight are the major traits to select for studying tolerant genotypes under water stress conditions [32]. It is reported that drought affect the plant water status during ear formation and flowering stage. Water availability mostly affects growth of leaves, roots, photosynthesis and dry mater accumulation [33].

#### **2.3. Phosphorus × drought and root signaling**

Phosphorous availability is correlated with moisture conditions of the soil, because higher water content in soil due to frequent irrigation generally leads to a better mobility and availability of P [34], which also improves the P conversion in the internal of plant [35], by enhancing root-shoot ratio and root elongation releasing of organic acids or protons [36] and phosphatases [37]. The absorbed P by plant to produce more biomass is another adaptive mechanism to P deficiency in soil, thus low-P also limits the yield and quality of wheat [38], because P can effect on photosynthesis, photo-assimilate transportation and stunt growth of plant [39–41]. Further, the coupling effect of water and chemical fertilizers on different crops or varieties have been reported by many studies, they revealed that water and nutrient uptake were two physiological processes that interacted with each other [42–45]. Therefore, soil water content and P fertilizer, and meanwhile, their interaction plays great key role for crop growth [46], and suitable irrigation and fertilization is the main method to increase production. The effects of drought stress on the phosphorus (P), uptake dynamics throughout the growth cycle were studied. Drought stress induced sharp decreases in total P uptake at different developmental stages and, in particular, detrimentally affected the nutrient uptake capability of roots. The results suggested that plants differ in their ability to maintain nutrient uptake under drought stress, and it is highly dependent on the intensity and duration of drought stress and the developmental stage. The decrease in total P uptake caused by both moderate stress and severe stress was accompanied by reduction in biomass production in drought-stressed tissues. The biomass allocation patterns in response to drought stress fluctuated strong mostly because of competitive changes in the shoot and roots at different stages, thus the root: shoot ratio increased at some stages and decreased at other stages. Severe stress induced a dramatic reduction in the harvest index, whereas moderate stress slightly decreased harvest index. Thus, water limitation caused lower P uptake and harvest index [47].

The water content and nutrient in soil are two main determinant factors to crop yield and quality, managements of which in field are of great importance to maintain sustainable high yield. The objective of the study was to measure the uptake, forms, and use efficiency of phosphorus in wheat under irrigation. The results indicated that P fertilizer combined with irrigation not only improved the activity of phosphatase in soil, but also increased P accumulation in wheat, similar results was found in the grain of wheat, the content of total P increased significantly. The interaction between P and irrigation also significantly affected on the P accumulation, grain total P, grain phospholipids P, and P production efficiency [48].
