The opening of stomata occurs as a result of an increase in turgor of the guard cells which is coupled to an uptake of potassium ions in relatively high concentrations (up to 400 mM: Raschke & Fellows, 1971; Outlaw, 1983). The active K+ transport has to be initiated by a process which requires a supply of energy in the form of ATP. There are two sources of ATP which are known to function in stomatal movement: in the light, photophosphorylation seems to act as a potential ATP source (Ogawa et al., 1979; Assmann et al., 1985; Gotow et al., 1985); in the dark, oxidative phosphorylation has been proposed as the energy source for the swelling of guard cells (Raghavendra, 1981; Shimazaki et al., 1983; Schwartz & Zeiger, 1984).

The role of ATP hydrolysis in ion uptake is still unresolved, although a membrane-bound Mg2+-activated ATPase and a K+-stimulated ATPase have been detected in the epidermis (Fujino (1967), Raghavendra et al. (1976), Kasamo (1979) and Lurie & Hendrix (1979)). The enzyme associated with stomatal opening was active in the light period, and was stimulated by fusicoccin and inhibited by abscisic acid (ABA). The other enzyme, coupled with stomatal closing, showed the opposite behaviour.