Photosynthetic Responses to Rising CO2 and Temperature: Mechanisms, Models, and Field Evidence.

Combined increases in atmospheric CO2 and warming temperatures impact photosynthesis in complex yet mechanistically predictable ways. Seminal work in this area showed that temperatures above a thermal optimum reduce photosynthesis primarily by increasing photorespiration, but that elevated CO2 and higher temperature can act synergistically to raise the thermal optimum of photosynthesis and increase absolute photosynthetic rates. The modeling work that led to this prediction outlined both the synergistic effects of CO2 and temperature and hypothesized scenarios that could deviate from theory. Here, the assumptions underlying models of CO2 × temperature interactions in photosynthesis are reviewed, with emphasis on advances in in vivo enzyme kinetics, diffusive limitations, photosynthetic acclimation, and atmospheric feedbacks. Advances in Rubisco biochemistry, mesophyll conductance, field experiments, and canopy-scale modeling show why photosynthetic responses under future climates often deviate from the theoretical CO2 × temperature response. Scaling from leaf-level physiology to canopy carbon uptake is addressed to identify priorities for next-generation experiments and models. Together, this review highlights the work of the late Prof. Stephen P. Long, beginning with his pioneering manuscript that first addressed the synergistic effects of CO2 and temperature on photosynthesis.