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.