Leaf photosynthetic light response: a mechanistic model for scaling photosynthesis to leaves and canopies

1. The response of photosynthesis to radiation is an often-studied but poor ly understood process, represented empirically in most photosynthesis model s. However, in scaling photosynthesis from leaf to canopy, predictions of c anopy photosynthesis are very sensitive to parameters describing the respon se of leaves to Photosynthetic Photon Flux Density (PPFD). 2. In this study, a mechanistic, yet still simple, approach is presented th at models the degree of light saturation in leaves explicitly, assuming a h eterogeneous environment of PPFD and chlorophyll. 3. Possible mechanisms determining the ratio of chlorophyll to nitrogen are considered, including a direct dependence on PPFD, a mechanism involving t he red/far-red ratio of light in the canopy, and an approach based upon max imizing photosynthesis. 4. Comparison of model predictions with two data sets of light, nitrogen an d chlorophyll from canopies of Populus and Corylus suggests that the red/fa r-red mechanism is the most realistic. The data also show that the trees st udied do not always optimize their nitrogen partitioning to maximize photos ynthetic yield. 5. We then apply the model to the data sets, to predict the shape of light response curves of leaves within canopies and assess the applicability of s imple scaling schemes, in which full acclimation of photosynthesis to PPFD justifies the use of big-leaf models. We conclude that, at least for the da ta used, basic assumptions of such schemes do not hold.