Action of Fusicoccin on the Potassium Balance of Guard Cells of Phaseolus vulgaris

Fusicoccin induces stomatal opening in both the light and dark. The stomatal aperture and K content of guard cells was measured to determine whether the action of fusicoccin in inducing stomatal opening is directly related to the uptake of K by the guard cells. Both detached and attached epidermis was treated with fusicoccin and the K content was determined by staining with cobalt sodium nitrite or by electron probe microanalysis. The K content of guard cells in detached epidermal strips floated on 10 μm fusicoccin in 10 mm KCl and aqueous CH3OH (0.02%, v/v) increased in the light and dark as the stomata opened. After exposure to fusicoccin for 6 hr in the light, however, the stomata were closed and no K could be detected in the guard cells. The K content of guard cells of attached epidermis painted with fusicoccin also increased as the stomata opened, but the concentration of K in the subsidiary cells was not significantly altered by fusicoccin-stimulated opening. Moreover, painting with fusicoccin did not significantly change the Ca and P content of the guard or subsidiary cells. Stomata of epidermal strips, opened to their maximum width by fusicoccin, showed only a small and temporary closure when transferred to a solution of 10 μm abscisic acid. The use of metabolic inhibitors suggested that energy for the uptake of the K may be provided by both photophosphorylation and oxidative phosphorylation.