N level is related to plant growth and development by affecting the water physiology in plants (Wu and Zhao, 2010).
N level is proportional to that of ABA in plants, thus involving in the regulation of plant stomata activity.
Optimal N level in plants could depress ABA level, sustain the stomata aperture, and increase the photosynthetic rate and yield (Garcia and Lamattia, 2002).
Besides, nitrogen is important in improving WUE (water use efficiency) and soil water use.
Possible mechanisms to enhance WUE by improving N nutrition in crop plants are presented in Fig. 1 (Waraich et al., 2011).