FURTHER OBSERVATIONS OF CATION‐STIMULATED STOMATAL OPENING IN ISOLATED EPIDERMIS

S ummary Measurements of apparent osmotic pressures of guard cells in isolated epidermis of Commelina communis indicate a considerable rise (up to 26.3 bars) when K + ions are added to the incubation medium. This rise in apparent osmotic pressure appears to be the major cause of the stomatal opening that is stimulated by K + ions, but reduction in turgor of the epidermal cells, owing to the solutes in the incubation medium, may also contribute to the wide stomatal apertures. Histochemical tests on epidermis taken from leaves with stomata that were closed in darkness, or that were open in light, suggest that K + accumulates in guard cells when stomata open in response to light. No movements of K + or Na + from epidermis to mesophyll could be detected and it is concluded that ion movements associated with changes in guard cell turgor are probably confined to the epidermis in C. communis. Ion uptake (indicated by 86 Rb and 24 Na) into epidermis was stimulated by light but not by a reduction in CO 2 concentration, whereas stomatal apertures were affected by these environmental factors to approximately the same extent. It is tentatively concluded that active transport of monovalent cations into guard cells might explain the CO 2 ‐independent light responses of stomata, but not their light‐independent CO 2 responses.