Hot and Hungry: High temperatures induce changes in leaf carbohydrate dynamics and sugar isotope fingerprints

Accurate predictions of vegetation responses to global warming require a precise understanding of physiological temperature responses. We investigated the effects of air temperature (10°C to 40°C) under constant low vapour pressure deficit and sufficient water supply on leaf-level gas exchange, chlorophyll fluorescence, non-structural carbohydrate (NSC) concentrations, and the hydrogen (δ2H) and oxygen (δ18O) isotopic composition of leaf water and leaf sugar in C3 trees, forbs, grasses, and one C4 grass. Rising temperatures significantly altered leaf physiology, NSC composition, and the leaf sugar isotopic composition. We observed a shift from starch to sugar above 30°C, indicating a preference for a more readily available carbohydrate, with a concomitant shift in the hydrogen isotopic composition of leaf sugar. Furthermore, we demonstrate for the first time the close relationship between carbohydrate metabolism and stable isotope fractionation, with 2H enrichment in leaf sugar with increasing temperature. Our results suggest that C3 plants may experience shifts in their carbon metabolism at temperatures above 30°C, which can be detected by δ2H of leaf sugar. Such carbon imbalances may reduce the resilience of C3 plants in an increasingly warming world.