Rubisco deactivation and chloroplast electron transport rates co-limit photosynthesis above optimal leaf temperature in terrestrial plants.

Net photosynthetic CO 2 assimilation rate (A n ) decreases at leaf temperatures above a relatively mild optimum (T opt ) in most higher plants. This decline is often attributed to reduced CO 2 conductance, increased CO 2 loss from photorespiration and respiration, reduced chloroplast electron transport rate (J), or deactivation of Ribulose-1,5-bisphosphate Carboxylase Oxygenase (Rubisco). However, it is unclear which of these factors can best predict species independent declines in A n at high temperature. We show that independent of species, and on a global scale, the observed decline in A n with rising temperatures can be effectively accounted for by Rubisco deactivation and declines in J. Our finding that A n declines with Rubisco deactivation and J supports a coordinated down-regulation of Rubisco and chloroplast electron transport rates to heat stress. We provide a model that, in the absence of CO 2 supply limitations, can predict the response of photosynthesis to short-term increases in leaf temperature.