Why is physiological water-use efficiency lower under low nitrogen supply?

Nitrogen efficiency is an important factor for sustainable agriculture, especially for crops such as winter-oilseed rape that has an inherently low N-harvest index but a high N demand. Low N-supply will not only result in lower yield but will also reduce water-use efficiency (WUE). In order to test the relationship between N-supply and WUE, winter-oilseed rape was pot-grown during early vegetative growth. Carbon isotope discrimination (D) was significantly affected by N-supply and clearly related to WUE. A larger discrimination (more positive D) under low N-supply indicates higher internal concentration of carbon dioxide and was reflected in lower WUE. The consideration of the root fraction did not improve the correlation between measured and calculated WUE.