Stomatal control of photosynthesis and transpiration

The stomata occupy a central position in the pathways for both the loss of water from plants and the exchange of CO2. It is commonly assumed that they therefore provide the main short-term control of both transpiration and photosynthesis, though the detailed control criteria on which their movements are based are not well understood and are likely to depend on the particular ecological situation. This paper first reviews the main methods available for quantifying the control exerted by stomata over transpiration and photosynthesis in the absence of feedbacks between gas-exchange and stomatal function. The discussion is then extended by using very simple models to investigate the role of stomata in the control of gas exchange in the presence of hydraulic feedbacks and to clarify the nature of causality in such systems. Comparison of a limited number of different mechanistic models of stomatal function is used to investigate likely mechanisms underlying stomatal responses to environment.