Photosynthetic responses to stress in Mediterranean evergreens: mechanisms and models

Plants in Mediterranean ecosystems face multiple environmental stresses during the growing season. Such stresses greatly affect ecosystem function, but are poorly represented by current ecosystem models. Here, we review recent advances in understanding the responses of ecosystem photosynthesis to drought and photoinhibition. Reductions in canopy net assimilation rates by recurrent water limitations during the day and during drought periods result from multiple limitations at the biochemical, cellular and structural levels, including reductions in stomatal, and mesophyll diffusion conductances, foliage biochemical potentials, light harvesting efficiency and modifications in canopy structure. Overall, a hierarchy of environmental effects, from instantaneous, short- and long-term influences, shape photosynthetic responses to stress in the field. The processes involved can be incorporated into photosynthesis models to different degrees, given our current understanding of each. However, fully mechanistic consideration of certain phenomena such as non-reversible, hysteretic and lagged heat and drought effects, or photoinhibition, would require the use of models including dynamic damage and repair mechanisms. We outline further model and experimental work needed to describe the parameter space for quantitative simulation of such phenomena, especially considering the need to simulate plant production under global change.