Stomatal conductance

Stomatal conductance, usually measured in mmol m−2 s−1 by a porometer or similar device, estimates the rate of gas exchange (i.e., carbon dioxide assimilation or transpiration of water vapor) through the leaf stomata per unit difference in the mole fraction of carbon dioxide or water vapor, respectively, between the leaf's internal airspaces and the surrounding ambient atmosphere. Variation in stomatal conductance is driven primarily by variation in the width (aperture) of the stomatal pores and the number of stomata per unit leaf area (stomatal density), and to a lesser extent by variation in the depth of the stomatal pore; temperature also has a small direct effect, independent of these other factors.

The stomatal conductance, or its inverse, stomatal resistance, is under the direct biological control of the leaf through its guard cells, which surround the stomatal pore. The stomatal aperture, and therefore the conductance, is directly related to the turgor pressure of guard cells, which in turn is actively regulated by changes in the osmotic potential of guard cells.

Stomatal conductance is a function of stomatal density, stomatal aperture, and stomatal size. Stomatal conductance is integral to leaf level calculations of transpiration. Multiple studies have shown a direct correlation between the use of herbicides and changes in physiological and biochemical growth processes in plants, particularly non-target plants, resulting in a reduction in stomatal conductance and turgor pressure in leaves.

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