Contrasting leaf‐scale photosynthetic low‐light response and its temperature dependency are key to differences in crop‐scale radiation use efficiency

Summary Radiation use efficiency (RUE) is a key crop adaptation trait that quantifies the potential amount of aboveground biomass produced by the crop per unit of solar energy intercepted. But it is unclear why elite maize and grain sorghum hybrids differ in their RUE at the crop level. Here, we used a non‐traditional top‐down approach via canopy photosynthesis modelling to identify leaf‐level photosynthetic traits that are key to differences in crop‐level RUE. A novel photosynthetic response measurement was developed and coupled with use of a Bayesian model fitting procedure, incorporating a C4 leaf photosynthesis model, to infer cohesive sets of photosynthetic parameters by simultaneously fitting responses to CO2, light, and temperature. Statistically significant differences between leaf photosynthetic parameters of elite maize and grain sorghum hybrids were found across a range of leaf temperatures, in particular for effects on the quantum yield of photosynthesis, but also for the maximum enzymatic activity of Rubisco and PEPc. Simulation of diurnal canopy photosynthesis predicted that the leaf‐level photosynthetic low‐light response and its temperature dependency are key drivers of the performance of crop‐level RUE, generating testable hypotheses for further physiological analysis and bioengineering applications.