Nitrogen absorption by plants is related to the water conductivity of root-shoot system.
Under drought condition, N uptake was regulated by lowering the root-shoot water conductivity to sustain high water use efficiency (Radin and Eidenbock, 1982).
The respiration rate of root system is enhanced by N supply.
Roots of plants with sufficient N supply have higher respiration rates compared with those having N deficiency under water stress (Xu, 2010).
Unlike water use efficiency of wheat is improved by N addition, the improvement in NUE can only be obtained with low N supply (Li et al., 2003).