Stomatal opening: role of potassium uptake.

Fischer (I ) proposes that his results "strongly suggest that the basic mechanism of stomatal opening, in response to light plus CO2-free air, is stimulation of the uptake of potassium." I believe that his evidence supports the opposite conclusion (2) that "in order to account for stomatal opening, the guard cells would have to absorb 13 times as rapidly as the most rapidly absorbing cells." Fischer's treatment with KCl did not actually induce opening, but merely doubled the size of the aperture. This increase in size of aperture required 3 to 4 hours, or 6 to 25 times longer than normal opening requires, namely, 10 to 30 minutes. Yet he performed the experiments under conditions that must lead to a far more rapid potassium effect than that in nature, for the following reasons. (i) His KCl solution bathed the whole undersurface of the guard cells, but in nature only about one-fourth of the cell surface is in contact with liquid (from adjacent cells). Bathing the whole undersurface would increase the rate of uptake of both solute and water at least two to three times. (ii) Fischer eliminated the turgor pressure effect of other epidermal cells when he killed them. Stomatal opening would therefore occur at about one-half the cell-sap concentration required in the guard cells under natural conditions. (iii) He eliminated competition for K+ by the mesophyll cells, which are normally also in contact with the epidermal cells, and whose active K+ absorption from them would also be stimulated by light, since, as Fischer points out, this is a general characteristic of cells with chloroplasts. Therefore, the opening described by Fischer as being due to K+ absorption is 10 to 100 times too slow to account for the normal process. Although Fischer's conclusion cannot apply to normal whole leaves, perhaps it is applicable in the case of epidermal strips in general. However, the opening of stomata in epidermal strips floating on distilled water can occur in the complete absence of K+ (2). Fischer proved this when he obtained opening to half the full width in the complete absence of K+. The addition of K+ merely accelerated the opening to full width, which normally occurs in distilled water after a longer time. Fischer's experiments support the classical concept that stomatal opening is due to an increase in turgor pressure of the guard cells, and that this is ini-