Future heatwave conditions inhibit CO₂-induced stomatal closure in wheat

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 CO₂ 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) CO₂ concentrations, across four N-fertiliser treatments. We measured the physiological response of abaxial and adaxial leaf surfaces to elevated CO₂ 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 CO₂ 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 CO₂, high-VPD heatwave conditions. This has considerable implications for future wheat water requirements, which in-turn could significantly impact drought susceptibility and yield potential.