Why does photosynthesis exhibit a thermal optimum?

On land, higher plants are successful at performing photosynthesis while experiencing diurnal temperature variation that is typically at least 15°C and in some cases exceeds 30°C. Many decades of research have been devoted to exploring this pattern, and impressive progress has been made in characterizing the temperature-sensitive components of the photosynthetic system. Yet, a fundamental question remains unresolved: which of the myriad temperature-sensitive structures and processes actually limit the photosynthetic rate below, at, and above the thermal optimum? Here, we will propose that this question has persisted because regulatory interactions tend to maintain coordination between electron transport and carbon metabolism, and past experimental approaches have been unable to identify which interactions represent proximate versus ultimate controls of the temperature response. We will describe a new approach to this puzzle based on a mechanistic model of steady-state photosynthesis that resolves the rate-limiting factors in electron transport and carbon metabolism, as well as the regulatory interactions that coordinate these metabolic domains [Johnson, J.E., Field, C.B., and Berry, J.A. 2021. Oecologia 197: 841-866; https://doi.org/10.1007/s00442-021-05062-y]. We will present experiments that use this model to diagnose how the overall temperature responses of C3 and C4 photosynthesis emerge from the temperature-sensitive components of the electron transport system and carbon metabolism. We will emphasize both the importance, and the limitations, of utilizing chlorophyll fluorescence to understand the mechanisms controlling the temperature response. Finally, we will discuss the implications of these findings for interpreting the temperature response of photosynthesis in experiments and observations, and representing the temperature response of photosynthesis in land surface models.