Availability of Chloride Affects the Balance between Potassium Chloride and Potassium Malate in Guard Cells of Vicia faba L.

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Abstract
Electron probe microanalysis for K and Cl and enzymic determination of malate were performed on epidermal strips of Vicia faba L. which had been incubated with 0.1 equivalent of K+ per liter in the absence or presence of Cl−. In the absence of Cl−, iminodiacetate, a presumed impermeant zwitterion, served as anion. With no Cl− in the medium, 91% of the K+ imported into the guard cells during stomatal opening was neutralized by malate production; import of Cl− (presumably from the rest of the epidermal tissue) contributed 6%. In the presence of Cl−, 50% of the necessary negative charges were provided by malate synthesis, 45% by Cl− import. Stomatal opening was not obviously affected by the chloride concentration in the incubation medium, but malate production declined roughly linearly with the logarithm of [Cl−] between 10−5 and 10−1 equivalent per liter.