Optimal allocation of leaf-level nitrogen
Implications for covariation of Vcmax and Jmax 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.