Proton-stimulated opening of stomata in relation to chloride uptake by guard cells

The concentration of potassium chloride required in the incubation medium to open stomata in isolated epidermal tissues of Commelina communis L. and Vicia faba L. could be lowered from 100 mM to 10 mM if the proton concentration of the ambient solution was increased from pH 5.6 to pH 3.5. This acidification effect was formerly attributed to the destruction of epidermal and subsidiary cells resulting in a relief of back pressure upon guard cells. While guard cells remain viable at pH 3.5, as demonstrated by their susceptibility to inhibition by uptake of glucose or to uncoupling by DNP, incipient destruction of the cells surrounding them could first be observed 30 min after the onset of the incubation experiment. By this time, however, the stomata had already opened; the time course of stomatal opening at pH 3.5 did not show any lag phase corresponding to the time required for damaging epidermal cells and showed no difference to that at pH 5.6 Thus, the acid-stimulated opening of stomata appears to be a biphasic phenomenon consisting of a physiologic effect onto which the physical effect of the relief of back pressure is superimposed over longer periods of incubation. To interpret the physiologic role of an increased proton concentration in the ambient solution of isolated epidermal strips, it is suggested that guard cells take up protons and chloride ions in an electroneutral symport. While protons are extruded again to generate the negative membrane potential required for potassium influx, chloride ions are retained to maintain electroneutrality.