A simple theoretical framework to explain leaf acclimation to light availability: Implications for the evolution of plant form and function

Background/Question/Methods . Plants are expected to be under strong selective pressure to maximize their net photosynthesis. In principle, maximum gains can be attained if plants use available resources optimally i.e., if they maximized marginal gains per resource. Given the importance of nitrogen (N) for the photosynthetic apparatus of leaves, different studies have suggested that plants should maximize marginal photosynthetic gains for N (PNUE). Yet, actual PNUE values tend to deviate from theoretical optimal expectations suggesting that N may not be the resource that is been optimized. Alternatively, plants could have evolved to maximize the efficiency with which they use light, photosynthetic photon flux density (PPFD). This hypothesis has recently received support from both theoretical and field studies, which suggest that leaves maximize net photosynthetic PPFD use efficiency (ε_max) along gradients of PPFD in the canopy. Here we hypothesized that ε_max is the principle governing leaf acclimation to PPFD availability and explored some of its consequences for the evolution of plant form and function. We developed a functional-structural model that allowed us to simulate how leaves can attain ε_max through adjustments in both leaf angle and physiology in different environments.

Results/Conclusions . ε_max occurred on a specific point on the ascending portion of a leaf photosynthetic light response curve. To attain ε_max a leaf had to be exposed to a corresponding optimal PPFD (Iε_max), which was typically at a low to medium PPFD value on the curve. A sensitivity analysis showed that plants could acclimate Iε_max through changes in the shape and scale of the photosynthetic light response curve. However, when leaf A_max was limited to a realistic maximum upper value (max-A_max), maximum Iε_max was also constrained. Thus, leaves exposed to high PPFD values had to increase their angle of inclination to be exposed to their optimal Iε_max. Moreover, our simulations showed that plants with a lower max-A_max had steeper leaf angles than plants with higher max-A_max. Given the known relationship between leaf angle and plant leaf area index (LAI), our results suggest that leaf max-A_max and LAI are functionally related through the optimization of Iε_max at the leaf level. Thus, optimization of PPFD use and max-A_max should have far reaching consequences for the evolution of plant form and function.