Stomatal control and hydraulic conductance, with special reference to tall trees.

A better understanding of the mechanistic basis of stomatal control is necessary to understand why modes of stomatal response differ among individual trees, and to improve the theoretical foundation for predictive models and manipulative experiments. Current understanding of the mechanistic basis of stomatal control is reviewed here and discussed in relation to the plant hydraulic system. Analysis focused on: (1) the relative role of hydraulic conductance in the vicinity of the stomatal apparatus versus whole-plant hydraulic conductance; (2) the influence of guard cell inflation characteristics and the mechanical interaction between guard cells and epidermal cells; and (3) the system requirements for moderate versus dramatic reductions in stomatal conductance with increasing evaporation potential. Special consideration was given to the potential effect of changes in hydraulic properties as trees grow taller. Stomatal control of leaf gas exchange is coupled to the entire plant hydraulic system and the basis of this coupling is the interdependence of guard cell water potential and transpiration rate. This hydraulic feedback loop is always present, but its dynamic properties may be altered by growth or cavitation-induced changes in hydraulic conductance, and may vary with genetically related differences in hydraulic conductances. Mechanistic models should include this feedback loop. Plants vary in their ability to control transpiration rate sufficiently to maintain constant leaf water potential. Limited control may be achieved through the hydraulic feedback loop alone, but for tighter control, an additional element linking transpiration rate to guard cell osmotic pressure may be needed.