Abstract

“Isolated” guard cells ofCommelina communis L., in epidermal strips in which all cells other than guard cells have been killed by treatment at low pH, will open to a degree dependent on the K (Rb)/Cl(Br) concentration in the bathing medium. Estimates of the changes with aperture of the ion concentrations in the guard cells were made by measurement of86Rb uptake from RbCl, of82Br uptake from K82Br, and of potassium activity with a potassium-sensitive microelectrode. The osmotic effects of such changes were compared with the previous estimates of the osmotic changes required to change the aperture. The results suggest that a substantial fraction of the osmotic pressure of “isolated” guard cells is contributed by solutes other than KCl (or other potassium salts), and that, even in stomata opened by incubation on KCl solutions, a substantial fraction of the increase in osmotic pressure associated with opening is contributed by solutes other than KCl.

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