Potassium content and aperture in “intact” stomatal and epidermal cells ofCommelina communis L

SummaryMeasurements of potassium activity with a potassium-sensitive microelectrode have been made in the cells of the stomatal complex, and in epidermal cells, ofCommelina communis L., as a function of stomatal aperture. The estimated osmotic effects of the changing accumulation of potassium salts in the guard cell have been compared with the previous estimates of the osmotic changes required to open/close the pore. The results suggest that a significant fraction of the osmotic pressure of the guard cells, particularly when closed, is contributed by solutes other than potassium salts. The degree of potassium accumulation may determine the aperture of wide-open stomata, but the potassium changes in the early stages of opening are much too small to account for the osmotic changes required. The difference in potassium contents of “intact” and “isolated” guard cells is close to that required to overcome the previously estimated effect of subsidiary cell turgor on the water relations of the guard cell. In some tissue (but not in all) much more K is lost from epidermal cells than appears in other cells of the complex as the stomata open, and extracellular storage would be required.