Thermal acclimation as an emergent property of economically optimal photosynthesis

Summary Leaf‐level photosynthesis acclimates to growth conditions over days to weeks, with the instantaneous optimal temperature for photosynthesis ( T opt ) increasing with growth temperature ( T growth ). We investigated how this behaviour emerges in the P model, a primary production model based on eco‐evolutionary optimality hypotheses. We evaluated the P model estimates against experimental data for three herbaceous species grown at different temperatures and examined mechanisms underlying the responses of T opt and the photosynthetic rate at T opt ( A opt ). The model yields a realistic increase in T opt with T growth and a peaked relationship between A opt and T growth . These responses emerge largely from adjustment of the stomatal sensitivity parameter ξ, which determines the internal‐to‐ambient CO 2 ratio ( c i : c a ) and its sensitivity to vapour pressure deficit. Rubisco kinetics ( K ) does not acclimate in the model, but its temperature dependence enters acclimation through optimality, mainly via ξ, shifting T opt . The decline in A opt at high growth temperatures, often attributed to heat stress, reflects the narrowing gap between T opt and T growth because the T opt shift has a slope below one. The P model provides a mechanistic explanation for observed trends in T opt and A opt and a scalable approach for representing biochemical and stomatal acclimation in land‐surface models.