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 CO2-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. NH4+ and Mg2+ did not produce opening.
No specific anion is required in association with K+. Opening was the same when Cl-, Br, and NO3- were used as counter ions, but was less when SO42- 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.
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Selected References
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