The kinetic basis of photosynthetic heat tolerance.

Photosynthetic heat tolerance constrains primary productivity and defines the upper temperature limit of the biosphere. However, current approaches to heat tolerance either overlook the duration of heat exposure or describe temperature-time relationships without characterizing causal mechanisms, constraining hypothesis testing and prediction under novel conditions. Heat inactivation of photosynthesis has been posited to result from protein denaturation or lipid membrane breakdown, but their relative contributions are not well resolved. Here we develop and test a quantitative theory of photosynthetic heat inactivation based on chemical kinetics and use the theory to evaluate these competing mechanisms. By analysing photosystem II temperature responses in 177 plant species from 157 families, we show that protein denaturation, not lipid membrane breakdown, governs heat tolerance. The theory also predicts an upper acclimatization limit of ~60 °C, set by the thermal stability of heat shock proteins and supported by global data for 826 species. Our findings provide a predictive, mechanistic basis for photosynthetic heat tolerance and the thermal limits of the biosphere in a changing climate.