An overview of models of stomatal conductance at the leaf level

Stomata play a key role in plant adaptation to changing environmental conditions as they control both water losses and CO(2) uptake. Particularly, in the context of global change, simulations of the consequences of drought on crop plants are needed to design more efficient and water-saving cropping systems. However, most of the models of stomatal conductance (g(s)) developed at the leaf level link g(s) to environmental factors or net photosynthesis (A(net)), but do not include satisfactorily the effects of drought, impairing our capacity to simulate plant functioning in conditions of limited water supply. The objective of this review was to draw an up-to-date picture of the g(s) models, from the empirical to the process-based ones, along with their mechanistic or deterministic bases. It focuses on models capable to account for multiple environmental influences with emphasis on drought conditions. We examine how models that have been proposed for well-watered conditions can be combined with those specifically designed to deal with drought conditions. Ideas for future improvements of g(s) models are discussed: the issue of co-regulation of g(s) and A(net); the roles of CO(2), absissic acid and H(2)O(2); and finally, how to better address the new challenges arising from the issue of global change.