Yixiang Shan

Summary: As global temperatures and the severity of droughts continue to increase, food crops will more frequently experience high vapour pressure deficit (VPD) during heatwave events. However, the interactive effects of rising atmospheric CO ₂ and high‐VPD heatwaves on crop water fluxes and yields are currently unknown. We investigate stomatal, photosynthetic and productivity changes in wheat during simulated future high‐VPD heatwaves, under ambient (450 ppm) or elevated (720 ppm) CO ₂ concentrations, across four N‐fertiliser treatments. We measured the physiological response of abaxial and adaxial leaf surfaces to elevated CO ₂ concentration and/or high‐VPD heatwave exposure, and quantified drought responses, seasonal water usage and ear weight. Transpiration ( E ) and stomatal conductance ( g _sw ) increased during high‐VPD heatwaves (irrespective of CO ₂ concentration or N‐fertiliser), largely due to increased water fluxes from abaxial leaf surfaces. Higher E and water usage increased wheat vulnerability to drought and led to reduced total ear weight. High‐VPD heatwaves also hindered stomatal responses to light, with g _sw only reducing by 37–38% after 1 h of dark treatment. Our results show that wheat stomata are inhibited from closing under future high CO ₂ , high‐VPD heatwave conditions. This has considerable implications for future wheat water requirements, which in‐turn could significantly impact drought susceptibility and yield potential.

Rising atmospheric CO ₂ concentrations are driving ongoing climatic changes, leading to agricultural crops increasingly experiencing extreme weather events ¹ . Stomata serve as gatekeepers on plant leaves, regulating both CO ₂ capture for photosynthesis and the concomitant release of water. At higher CO ₂ concentrations or higher vapour pressure deficit (VPD), stomatal pores narrow, reducing stomatal conductance to water vapour ( g _sw ) and transpiration ( E ) ^2–6 . Increasing temperatures and/or nitrogen fertilisation promote an opposite stomatal response, enhancing g _sw and E ^7,8 . With atmospheric CO ₂ concentration, temperature and VPD predicted to rise throughout this century ¹ , it is unclear how crops will modify stomatal gaseous exchanges, particularly under differing N-fertilisation regimes. Here, we show in wheat ( Triticum aestivum ), that elevated CO ₂ does not reduce g _sw or E during heatwaves when VPD is high, instead plant water usage increases. High-VPD heatwave events also impact stomatal responsiveness to N-fertiliser application, prompting significantly higher gas exchange contributions from abaxial leaf surfaces, irrespective of CO ₂ growth conditions. Dynamic stomatal responsiveness to light and high CO ₂ are also attenuated during heatwaves in a CO ₂ -independent manner. Taken together, our data suggests that future wheat crops will use significantly more water during heatwaves than might be expected, which has substantial implications for future global food security.