Future heatwave conditions inhibit CO ₂ -induced stomatal closure in wheat

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.