The abaxial surface of wheat leaves boosts transpirational cooling during current and future heatwave scenarios

Robert S. Caine, Muhammad S. Khan, Holly L. Croft
Plants, Photosynthesis and Soil, School of Biosciences, University of Sheffield, South Yorkshire, S10 2TN, UK

As anthropogenic warming continues to heat the planet, cropping systems are increasingly experiencing heatwaves and droughts. Wheat (Triticum aestivum), like other crops, uses stomatal pores to regulate plant-environment gaseous exchanges, with wheat stomata found on both leaf surfaces and on stems and flowers. Research shows that the abaxial and adaxial surfaces of wheat leaves do not contribute equally to plant gas exchange, with the adaxial surface routinely having a higher overall contribution to photosynthesis and stomatal conductance. Here, we investigate the interactive response of wheat to differential nitrogen fertilisation, elevated CO2 and/or the imposition of heatwave events, to understand how thermoregulation of wheat canopies is achieved under current and future climate scenarios. Using porometry combined with thermal imaging, we show that the abaxial leaf surface of wheat leaves is crucial for boosting plant cooling during heatwaves, particularly when wheat is grown under elevated CO2 conditions and/or during nitrogen limitation. Abaxial surface contributions rise from 8% to 40-45% of total gas exchange during high CO2 heatwaves, resulting in wheat plants that use similar levels of water to those grown under ambient CO2. These results highlight that high CO2 environments will not mitigate against excessive water-loss in wheat during future heatwaves events.