Stomatal and mesophyll conductance are dominant limitations to photosynthesis in response to heat stress during severe drought in a temperate and a tropical tree species

Key message: Stomatal and mesophyll conductance were the dominant limitations to photosynthesis in response to heat stress during severe drought in both a temperate and a tropical tree species. Abstract: Drought and heat stress cause a significant reduction in forest primary production. The current study aims to determine the major limitations to photosynthesis in a temperate and tropical tree species experiencing a combination of drought and heat stress; an increasingly common situation in the face of climate change. Two tree species, one temperate (Fraxinus chinensis) and the other tropical (Radermachera sinica) were subjected in growth chambers to a 2-day heat stress of 40 °C during ca. 2-week period of drought, followed by re-watering. The limitations by three factors that affect photosynthesis (stomatal and mesophyll conductance, and biochemical processes) were partitioned based on measurements of gas exchange, net assimilation rate—intercellular CO₂ concentration (A_net—C_i) curves and chlorophyll fluorescence. Under the drought-only condition, photosynthesis was primarily limited by stomatal conductance in both species. Unexpectedly, heat stress did not further reduce photosynthesis during drought in either tree species. In both species, carbon assimilation during drought and added heat stress was primarily limited by stomatal and mesophyll conductance. Drought decreased mesophyll conductance in both species but the effects were more pronounced in the tropical tree species, R. sinica. Biochemical limitation remained low throughout the stress period. These results suggest the importance of balancing water conservation and transpirational cooling in the diffusive pathway in response to, not only drought, but also the combination of water and heat stress in forest trees.