Water availability is a major determinate of agricultural productivity in arid and semi-arid regions of the world. Therefore, higher water use efficience should be center of the agricultural production in these regions. One solution to the problem is to improve water use efficiency (WUE) of plants themselves. In this study, we examined the differences of carbon isotope composition (δ~(13)C), long-term water use efficiency (WUE_L), instantaneous water use efficiency (WUE_i), total water consumption and droug...