Canopy Carbon- and Water-Use Efficiencies in Response to Temperature and Water Deficit for Wheat.

The frequency and intensity of extreme climatic events increase the complexity in assessing climate change impacts on (agro)ecosystem functions and crop production. A better understanding of carbon and water fluxes for crop plants under climate change requires research based on direct canopy-scale measurements. By analysing a canopy gas exchange data set synthesised from 8 years' experimentation under semi-field conditions for the post-anthesis period of five wheat genotypes, we examined canopy carbon and water fluxes as well as carbon use efficiency (CUE) and water use efficiency (WUE) under varying environmental conditions. CUE was variable, and was negatively affected by high temperatures. Moreover, CUE responded differently to daily, daytime, and nighttime temperatures, and was most sensitive to nighttime temperatures. The response of WUE to increasing temperatures was dominated by the response of carbon fluxes, while the relative contribution of water fluxes to WUE responses increased under water deficit. WUE based on gross and net photosynthesis responded differently to environmental variables, primarily due to the differences in CUE. The findings increase our understanding of canopy carbon and water fluxes under various environmental conditions and highlight the necessity for future efforts to improve crop CUE and WUE under climate change.