Warming Mitigates Ozone Damage to Wheat Photosynthesis in a FACE Experiment

Individual effects of elevated ozone (O₃) 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 O₃ (1.5× ambient O₃) and rising canopy temperature (+2°C) on the photosynthesis of wheat leaves in an open-air field experiment. We found that O₃-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 O₃ nor warming stimulated antioxidant enzymes. Significant interaction between O₃ and WW indicated that WW mitigated the adverse effect of O₃ on leaf photosynthesis. Compared to NW, WW significantly increased daytime canopy temperature and canopy-to-air vapour pressure deficit across O₃ treatments. Decreases in leaf water content and increases in grain oxygen isotope discrimination under warming suggested a link of WW-induced protection against O₃ stress in photosynthesis with declines in stomatal O₃ uptake rather than increases in the antioxidant capacity. Our results indicate the need to consider the warming-induced mitigation of O₃ stress on leaf photosynthesis when predicting the effects of elevated O₃ on crop growth under warmer climate in the future.