The kinetic basis of photosynthetic heat tolerance

Photosynthesis fuels the biosphere and is a key regulator of Earth’s climate. As Earth warms, heat stress threatens to irreversibly impair the molecular machinery of photosynthesis, potentially pushing ecosystem productivity and carbon sequestration beyond a tipping point. Current approaches to quantifying photosynthetic heat tolerance often focus solely on temperature, overlooking exposure time, or rely on temperature-time correlations that do not identify causal mechanisms, limiting inference and prediction. Here we develop a mechanistic theory for heat inactivation of photosynthesis based on principles of chemical kinetics, and test it using data for photosystem II (PSII), the first step in the photosynthetic apparatus. Our framework links the effects of both temperature and exposure time, and enables direct tests of competing hypotheses for how heat impairs photosynthesis. Data from diverse plant species suggest that protein (not lipid membrane) denaturation is the primary mechanism of heat-induced inactivation of PSII. The theory also predicts a general upper temperature limit of 55-60 °C for acclimation of photosynthetic heat tolerance, a prediction supported by global PSII data. This quantitative, mechanistic framework can be incorporated into global change models to improve forecasts of how vegetation and the biosphere will respond to future climate change.