Uncoupling of stomatal conductance and photosynthesis at high temperatures: mechanistic insights from online stable isotope techniques

The strong covariation of temperature and vapour pressure deficit (VPD) in nature limits our understanding of the direct effects of temperature on leaf gas exchange. Stable isotopes in CO₂ and H₂ O vapour provide mechanistic insight into physiological and biochemical processes during leaf gas exchange. We conducted combined leaf gas exchange and online isotope discrimination measurements on four common European tree species across a leaf temperature range of 5-40°C, while maintaining a constant leaf-to-air VPD (0.8 kPa) without soil water limitation. Above the optimum temperature for photosynthesis (30°C) under the controlled environmental conditions, stomatal conductance (g_s ) and net photosynthesis rate (A_n ) decoupled across all tested species, with g_s increasing but A_n decreasing. During this decoupling, mesophyll conductance (cell wall, plasma membrane and chloroplast membrane conductance) consistently and significantly decreased among species; however, this reduction did not lead to reductions in CO₂ concentration at the chloroplast surface and stroma. We question the conventional understanding that diffusional limitations of CO₂ contribute to the reduction in photosynthesis at high temperatures. We suggest that stomata and mesophyll membranes could work strategically to facilitate transpiration cooling and CO₂ supply, thus alleviating heat stress on leaf photosynthetic function, albeit at the cost of reduced water-use efficiency.