© 2004 Heron Publishing—Victoria, Canada
Stomatal control and hydraulic conductance, with special reference to tall trees
Peter J. Franks (1, 2)
1. School of Tropical Biology, James Cook University, P.O. Box 6811, Cairns, Queensland, 4870, Australia (peter.franks@jcu.edu.au) / 2. Department of Organismic and Evolutionary Biology, Harvard University, Cambridge, MA 02138, USA / Received September 16, 2003; accepted February 15, 2004; published online June 1, 2004
Summary
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.
Keywords:
hydraulic conductivity, stomatal conductance, stomatal models, transpiration control, transpiration rate, tree water use.
