Mechanisms underlying interspecific variation in photosynthetic capacity across wild plant species

Photosynthetic capacity of leaves varies greatly among C(3) species although they have the same photosynthetic metabolisms. Here we discuss mechanisms underlying interspecific variation in photosynthetic capacity. Within-species variation in photosynthetic capacity is generally explained by nitrogen concentration because photosynthetic nitrogen-use efficiency (PNUE, photosynthetic capacity per unit leaf nitrogen) tends to be constant in each species. Among-species variation, on the other hand, involves both variations in nitrogen concentration and PNUE. Species with higher photosynthetic capacity have higher nitrogen concentration per mass and PNUE. Interspecific variation in PNUE is attributable to CO(2) diffusion in the leaves, nitrogen allocation to the photosynthetic apparatus and/or specific activity of photosynthetic enzymes. Previous studies have shown that variations in mesophyll conductance and nitrogen allocation explain the variation in PNUE. As new leaves are constructed by assimilated nitrogen and carbon, increased carbon assimilation rates are expected to dilute nitrogen in the leaves. However, this expectation contradicts the fact that photosynthetic capacity and nitrogen concentration is positively related with each other across species. This paradoxical dilution effect may be compensated by root activity, i.e. species with higher photosynthetic capacity have higher root activity to maintain higher leaf nitrogen concentrations.