title: Specific requirement of potassium for light-activated opening of stomata in epidermal strips.
abstract: The effect of various ions on stomatal opening was studied in isolated epidermal strips of Vicia faba L. Stomata in strips floating on 10 mm KCl and in CO(2)-free air opened in light, closed in subsequent darkness, then opened fully again when illuminated. A light-activated highly specific effect of K(-) (and Rb(+)) on opening was found. When strips were floated on high concentrations (50 or 100 meq/liter) of Li(+), Na(+) or Cs(+), stomata opened but light had very little effect on the concentrations required for opening. With K(+), the opening produced in the dark was the same as with the other alkali ions. Light, however, lowered more than 100-fold the concentration of K(+) required for maximal opening. Thus only the effect of K(+) (and Rb(+)) was greatly accentuated by light. NH(4) (+) and Mg(2+) did not produce opening.No specific anion is required in association with K(+). Opening was the same when Cl(-), Br, and NO(3) (-) were used as counter ions, but was less when SO(4) (2-) was used, particularly at higher concentrations and in the dark.The results are discussed in relation to the recent proposal that the basis for stomatal opening is K(+) uptake in amounts sufficient to act as an osmotic agent. This work also demonstrates, for the first time, a physiological process specifically requiring K(+). Assuming that ion uptake is an integral part of stomatal opening, guard cells would appear to have an ion uptake mechanism of a degree of specificity previously unknown in higher plants.
authors.affiliation: Laboratory of Plant-Water Relations, Department of Water Science and Engineering, University of California, Davis, California 95616.
authors.forename: G D
authors.initials: GD
authors.lastname: Humble
authors.forename: T C
authors.initials: TC
authors.lastname: Hsiao