Dynamics of stomatal water relations during the humidity response: implications of two hypothetical mechanisms

The feasibility of two hypothetical mechanisms for the stomatal response to humidity was evaluated by identifying theoretical constraints on these mechanisms and by analysing timecourses of stomatal aperture following a step change in humidity. The two hypothetical mechanisms, which allow guard cell turgor pressure to overcome the epidermal mechanical advantage, are: (1) active regulation of guard cell osmotic pressure, requiring no hydraulic disequilibrium between guard and epidermal cells, and (2) a substantial hydraulic resistance between guard and epidermal cells, resulting in hydraulic disequilibrium between them. Numerical simulations of the system are made possible by recently published empirical relationships between guard cell pressure and volume and between stomatal aperture, guard cell turgor pressure, and epidermal cell turgor pressure; these data allow the hypothetical control variables to be inferred from stomatal aperture and evaporative demand, given physical assumptions that characterize either hypothesis. We show that hypothesis (1) predicts that steady-state pi(g) is monotonically related to transpiration rate, whereas hypothesis (2) suggests that the relationship between transpiration rate and the steady-state guard to epidermal cell hydraulic resistance may be either positive or negative, and that this resistance must change substantially during the transient phase of the stomatal response to humidity.