Abstract: Using a ratio of photosynthetic rate (Pn) and transpiration rate (Tr) to express water use efficiency (WUE), the WUE of two varieties of spring wheat and its physioecological affecting factors under different water treatments are analyzed . It is found that the WUE of both varieties is higher in the morning, with a peak at 8～10 am than in the afternoon.The change of WUE is determined by non symmetry and contrary characteristics of Pn and Tr, i.e.,Pn is higher in the morning than in the afternoon, but Tr is on the contrary.The Pn and Tr values are all lower under water stress condition, but the WUE of different varieties is not the same. The results indicate that some physiological factors, such as leaf water potential(ψ_w) and stomatal resistance (Rs), and some ecological factors, such as relative air humidity (RH), solar radiation (Q) and canopy temperature(Tc), are significantly correlated with WUE , although both physiological and ecological factors have different effects on it. 40% of RH is an important threshold value to WUE change.

Key words: Spring wheat, Water use efficiency, Diurnal change, Physioecological factors, Correlative analysis, ana-lytic hierarchy process (AHP), entropy method (EM), object-based classification, land use/ land cover, habitat suitability index(HSI)