Unsaturation of Leaf Air Spaces Sheds New Light on the Role of Aquaporins

Summary statement Aquaporins have long been known to facilitate water transport across membranes inside leaves. However, their role in regulating the water potential ( ψ ) difference between the cytosol and cell wall has been questioned, as the ψ of the cytosol and cell wall would be in equilibrium under the assumption of water‐vapour‐saturated leaf intercellular air spaces. Recent advances suggest that intercellular air spaces are unsaturated at high vapour pressure deficit (VPD) and that aquaporins that up‐regulate water transport might simultaneously down‐regulate CO 2 transport in a competitive manner. Therefore, the currently assumed mechanisms of CO 2 and water transport in the mesophyll under varying VPD must be re‐examined. We incorporated the competitive aquaporin hypothesis into the leaf gas exchange pathways with unsaturated intercellular air spaces. We show that the putative competitive control of CO 2 ‐ and water‐facilitating aquaporins is fully effective only when there is a large ψ gradient between the cytosol and cell wall at high VPD. In this context, the down‐regulation of water‐facilitating aquaporins and the up‐regulation of CO 2 ‐facilitating aquaporins could protect the cytosol from drying and maintain the CO 2 supply for photosynthesis, respectively. While it remains unclear whether unsaturation drives aquaporin activity or vice versa, we identify research challenges that need to be addressed.