Photosynthesis acclimates to warming through predictable changes in photosynthetic capacities, stomatal sensitivity and enzyme kinetics

Abstract

Acclimation of photosynthesis allows plants to adjust to seasonal environmental changes and gradual long-term changes of similar magnitude. However, current global photosynthesis models diverge in their representation of temperature acclimation. Here, we applied fundamental principles of acclimation to identify key processes for predicting the response of photosynthesis to global warming.   We investigated how the instantaneous temperature response of photosynthesis changes due to acclimation of the photosynthetic capacities (changes in base rates of carboxylation, electron transport and respiration), the stomatal response (sensitivity to changes in vapour pressure), and the enzymatic response (changes in the enzyme kinetics of carboxylation and electron transport). Using a dataset of gas exchange measurements from globally distributed sites, we compared predicted and observed relationships between prevalent growth temperature (Tgrowth) and the optimal temperature of photosynthesis (Topt), the photosynthesis rate at Topt (Aopt), and the temperature sensitivity (width of the temperature response curve, Tspan).   The observational data showed a significant linear increase of  Topt with Tgrowth (0.74 °C/°C) and no correlations between Tgrowth and Aopt, respectively Tspan. To accurately predict Topt, all acclimation processes were required (R2 = 0.74). Acclimation of the enzymatic response was a key driver but caused an underestimation of Topt in tropical climates. This underestimation was resolved through acclimation of the base rate of respiration and the stomatal sensitivity to vapour pressure changes. Both decreased with increasing Tgrowth, resulting in an upwards shift of Topt and accurate predictions in tropical climates. Additionally, acclimation of the photosynthetic capacities was necessary to avoid an otherwise falsely predicted increase of Aopt with Tgrowth. The model predicted a linear decrease of Tspan with Tgrowth, indicating an incomplete formulation for the acclimation of the enzymatic response.   Our results demonstrate that the thermal acclimation of Topt and Aopt is predictable from the environment across species and that global photosynthesis models should adopt acclimation of the photosynthetic capacities, stomatal sensitivity and enzymatic response to predict the response of photosynthesis to global warming accurately.