Warming Mitigates Ozone Damage to Wheat Photosynthesis in a FACE Experiment.

Individual effects of elevated ozone (O) and warming on wheat (Triticum aestivum L.) are well documented, their combined effects remain poorly understood. In the present study, we investigated the combined impacts of elevated O (1.5× ambient O) and rising canopy temperature (+2°C) on the photosynthesis of wheat leaves in an open-air field experiment. We found that O-induced oxidative stress reduced the biochemical capacity and inhibited leaf photosynthesis at the end of the grain-filling stage. Night-time warming (NW) increased leaf photosynthesis during the vegetative stage, but whole-day warming (WW) did not. Both WW and NW accelerated wheat development and decreased photosynthesis at the end of the reproductive stage. Neither elevated O nor warming stimulated antioxidant enzymes. Significant interaction between O and WW indicated that WW mitigated the adverse effect of O on leaf photosynthesis. Compared to NW, WW significantly increased daytime canopy temperature and canopy-to-air vapour pressure deficit across O treatments. Decreases in leaf water content and increases in grain oxygen isotope discrimination under warming suggested a link of WW-induced protection against O stress in photosynthesis with declines in stomatal O uptake rather than increases in the antioxidant capacity. Our results indicate the need to consider the warming-induced mitigation of O stress on leaf photosynthesis when predicting the effects of elevated O on crop growth under warmer climate in the future.