The coupled response of stomatal conductance to photosynthesis and transpiration

Increases in the net rate of photosynthesis and stomatal conductance of leaves are perceived to occur simultaneously. Published data are re-analysed here to show that when increases in the net rate of photosynthesis are a result of transient increases in the ambient CO2 concentration then the reverse is the case, i.e. increases in net photosynthesis are closely correlated to reductions in stomatal conductance. Rather than explain this observation in terms of a direct effect of CO2 on stomata this article develops the idea that stomata respond to a signal in proportion to the degree to which the photosynthetic capacity is realized. This interpretation of stomatal function facilitates the formulation of a simple static model of stomatal responses to the rates of both carbon gain and water loss.