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.