ENVIRONMENTAL AND BIOCHEMICAL CONTROL OF STOMATAL MOVEMENT IN LEAVES

Summary The chloroplasts of guard cells are largely responsible for the response of stomata to light. The opening movement is probably associated with the synthesis of ATP by non‐cyclic photophosphorylation, and the obligatory fixation of carbon dioxide to produce glycolic acid. The oxidative metabolism of glycolic acid is related to ATP synthesis as well as to the possible formation of carbohydrate. In the absence of oxygen in the gas phase, stomata open poorly, and the synthesis of glycolic acid is also inhibited. α‐Hydroxysulphonates, which are inhibitors of glycolate oxidase, prevent stomatal opening in a competitive manner. High concentrations of carbon dioxide close stomata and inhibit the formation of glycolic acid, and the effect of high carbon dioxide on closing can be reversed by providing the tissue with glycolic acid. When leaf hydration is not limiting, raising the temperature in the environment causes an increase in the steady state of the stomatal aperture. There is little effect of temperature on the rate of closing in darkness, hence opening is the active process. A large number of chemical inhibitors were found to prevent opening, either by interfering with metabolic reactions in the guard cells concerned with the ‘pump’ responsible for the increased turgidity, or by acting on the membranes that serve as a ‘check valve’. Sodium azide is a substance which acts by both mechanisms, but the maximal effect occurs at different concentrations, and experiments with this inhibitor have shown that the opening and closing reactions operate independently.