Physiological Basis of Stomatal Response

Abstract

Stomata fulfill three major functions in the physiology of the plant. (1) They permit the entrance of CO₂ into the green leaf at a rate sufficient to support an adequate rate of photosynthesis for the normal growth and development of the plant. They have become adapted to this function by opening in response to low CO₂ concentrations inside the leaf. (2) They permit the entrance of O₂ into the leaf at a rapid enough rate to support an aerobic respiration sufficient to provide the metabolic needs of the leaf. They have become adapted to this function by opening in response to low O₂ concentrations inside the leaf. (3) If these two were the only functions of the stomata they would fulfill them by remaining constantly open. Accompanying the uptake of CO₂ and O₂, however, there must also be an increased loss of water vapor, as long as the water potential of the external atmosphere is below that of the leaf. When such a loss of water approaches the point of water stress injury, the third function of the stomata is to close, reducing the rate of water loss to that due to cuticular transpiration—a small fraction of stomatal transpiration. They have become adapted to this function by means of a guard-cell structure which leads to closure when loss of water from these cells lowers their turgor pressure to a sufficient degree. Recent evidence has revealed another adaptation which permits them to close even before the evaporative loss of water is sufficient to lower the guard cell turgor (Lange et al., 1971).