Optimal allocation of leaf‐level nitrogen: Implications for covariation of V cmax and J max and photosynthetic downregulation

The maximum rate of carboxylation, V cmax , and the maximum rate of electron transport, J max , describe leaf‐level capacities of the photosynthetic system and are critical in determining the net fluxes of carbon dioxide and water vapor in the terrestrial biosphere. Although both V cmax and J max exhibit high spatial and temporal variability, most descriptions of photosynthesis in terrestrial biosphere models assume constant values for V cmax and J max at a reference temperature ignoring intraseasonal, interannual, and water stress‐induced variations. Although general patterns of variation of V cmax and J max have been correlated across groups of species, climates, and nitrogen concentrations, scant theoretical support has been provided to explain these variations. We present a new approach to determine V cmax and J max based on the assumption that a limited amount of leaf nitrogen is allocated optimally among the various components of the photosynthetic system in such a way that expected carbon assimilation is maximized. The optimal allocation is constrained by available nitrogen and responds dynamically to the near‐term environmental conditions of light and water supply and to their variability. The resulting optimal allocations of a finite supply of nitrogen replicate observed relationships in nature, including the ratio of J max / V cmax , the relationship of leaf nitrogen to V cmax , and the changes in nitrogen allocation under varying water availability and light environments. This optimal allocation approach provides a mechanism to describe the response of leaf‐level photosynthetic capacity to varying environmental and resource supply conditions that can be incorporated into terrestrial biosphere models providing improved estimates of carbon and water fluxes in the soil‐plant‐atmosphere continuum.