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Stomatal Opening in Isolated Epidermal Strips of Vicia faba. II. Responses to KCl Concentration and the Role of Potassium Absorption ¹

Laboratory of Plant-Water Relations, Department of Water Science and Engineering, University of California, Davis, California 95616

The stimulation by KCl of stomatal opening in isolated epidermal strips of Vicia faba was examined. In dark + normal air the opening response was maximal at 100 mM KCl while in light + CO₂-free air it was maximal at about 10 mM KCl. CO₂-free air was more influential than light in reducing the KCl concentration required for maximal opening. K⁺ was essential while Cl^– seemed to be of secondary importance in these processes.

The use of ⁸⁶Rb⁺ as a tracer for K⁺ showed that the increase in stomatal aperture under various conditions was well correlated with K⁺ uptake. The estimated amount of K⁺ taken up by guard cells, along with a counter ion, was sufficient to account for the changes in solute potential associated with opening. It is suggested that the absorption of extracellular solutes, such as K⁺, may be the primary mechanism of stomatal opening. Both opening and K⁺ absorption are stimulated by light + CO₂-free air.

The increase in stomatal aperture was also well correlated with the decrease in stainable starch in guard cells under all conditions. It is suggested that this is a secondary change, although perhaps closely linked to K⁺ absorption.

¹ Supported in part by grant B-029-CAL from the Office of Water Resources Research, United States Department of the Interior, and by a grant from the Water Resources Center, University of California.

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