Abstract: Water use efficiency (WUE) can reflect the coupling status of carbon and water cycles, and is an effective integral trait for assessing the responses of terrestrial vegetations to climate change. In this paper, a moisture control experiment in Jinzhou during 2008-2009 and a field crop trial in Chaoyang during 1990-2010 were conducted, respectively, aimed to examine the instantaneous responses of maize leaf WUE (WUE_l) to the changes of photosynthetically available radiation, leaf surface CO₂ concentration, air temperature and soil humidity as well as the variation characteristics of the WUE at grain yield level (WUE_g) with air temperature and precipitation. The results showed that with the increase of leaf surface CO₂ concentration from 350 to 800 μmol·mol^-1, the leaf photosynthesis increased, while the stomatal conductance and evaportranspiration (ET) decreased, resulting in the increase of WUE_l. The increase of ambient temperature from 26 to 40 ℃ and of soil moisture content from 12% to 26% made the increment of leaf transpiration rate (T_r) be larger than that of net photosynthesis (P_n), causing a significant reduction of WUE_l. The analysis of the effects of ET on the grain yield over the 20 years of monitoring showed that there was a significant quadraticcurve relationship between the WUE_g and ET. When the ET was 400-450 mm, the grain yield and WUE_g were the maximum, and the ET in the growth season was the optimal economical evapotranspiration. No obvious relationship was observed between the mean air temperature from sowing to maturing stage, suggesting that the WUE_g of maize in the study areas was mainly affected by soil moisture condition, but less affected by air temperature.

Key words: plant leaf, environmental factor,  δ¹³C, sub-alpine meadow.