Extracellular Potassium Activity in Attached Leaves and its Relation to Stomatal Function

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Abstract
Apoplastic potassium activities (ak) in leaves of Commelina communis L., Vicia faba L. and Pisum sativum L. var. argenteum were recorded with neutral-carrier-based, potassium-sensitive micro-electrodes. Measurements were carried out in 0.3–1.4 nl volumes contiguous with the extracellular space of attached leaves and were held for periods of 7–68 min. Mean steady-state aK values recorded from all three species were below 50 μM. Similar potassium activities were attained, regardless of the initial values obtained after washing with distilled water or potassium additions, and the activities recorded showed only minimal dependence on the relative vapour pressure difference. Tissue capacity for K+ absorption was increased 15–30-fold in the presence of added Ca2+. By contrast, cyanide reduced both the initial rate of potassium absorption by the tissues and their apparent capacity for the cation. These observations indicate that the free potassium pool in the leaf apoplast is significantly smaller than has previously been assumed. The results contradict the notion that high concentrations of potassium accumulate locally as a result of transpiration, and may indicate the presence in the leaf tissues of potassium transport activity sensitive to Ca2+ and dependent on metabolism.