Effects of climate change on water use efficiency in rain-fed plants

Water use efficiency (WUE) reflects the coupling of the carbon and water cycles and is an effective integral trait for assessing the responses of vegetated ecosystems to climate change. In this study, field experiments were performed to examine leaf WUE (WUEleaf) in response to changes in CO2 concentration and other environmental variables, including soil moisture and air temperature. We also used yield of maize and soybean, soil water content and precipitation data to calculate water use efficiency at the level of grain yield (WUEgrain) in a manner that enabled us to analyze the effects of climatic factors on WUEgrain. The results showed that the WUEleaf measurements of maize and soybean plants were negatively correlated with soil moisture and air temperature. At a photosynthetically active radiation (PAR) of 1.600 μmol m-2s-1, increasing ambient CO2 concentrations (from 400 to 800 μmol mol-1) improved WUEleaf by 52.0% and 75.8% for maize (a C4 species) and soybean (a C3 species), respectively. Increased annual precipitation stimulated maize WUEgrain up to levels of approximately 500-550 mm, although maize WUEgrain decreased when annual precipitation exceeded 550 mm. It appears that 400-450 mm is an economical evaportranspiration (ET) for spring maize in Chaoyang area of northeast China. For soybean, more water often reduces WUEgrain, and there is a linear relationship between changes in WUEgrain and changes in annual temperature. The different responses of WUEgrain and WUEleaf to climate change suggest that caution should be taken when attempting to up-scale WUE from leaf to grain or biomass levels.