Epidermal transpiration and stomatal responses to humidity: Some hypotheses explored

Abstract Stomatal responses to humidity as affected by both evaporation from the epidermis and the hydraulic conductance of the transpiration stream to evaporation sites on the epidermis are discussed. Recent estimates of evaporation from the inner walls of the epidermis are too high because the cell wall surfaces were assumed completely wet, and leaves have usually been considered isothermal. It is suggested that a fall in humidity increases evaporation from the epidermis, and that stomata respond to the consequent fall in water potential. Cuticular transiration is inversely related to stomatal conductance. Thus, evaporation from the epidermis is dependent on the stomatal, boundary layer, and cuticular conductances, and on evaporation from the inner walls of the epidermis. Stomatal responses to humidity will change as the boundary layer conductance changes. The conductance of the transpiration stream is a determinant of the water potential of the epidermis. Water potentials of adjacent cells will be more similar if flow is symplastic than if it is apoplastic. It is concluded that flow in living tissues is primarily symplastic over long distances, but over shorter distances it is increasingly apoplastic, and that stomatal responses to humidity are mediated by the water potential of the whole epidermis.