Stomatal conductance is the rate at which either water vapor or carbon dioxide passes through the stomata, which are the small pores of a plant. It plays an important role in water exchange between plants and the atmosphere.
The stomatal aperture controls the entry of carbon dioxide into the mesophyll of the plant leaf and the exit of water vapor from the leaf into the atmosphere. Almost all water transpired by plants is lost through the stomata. When carbon dioxide in the atmosphere rises, plants adjust to this change by reducing the stomatal aperture of their leaves and needles. This reduces transpirational water loss and enhances plant water-use efficiency.