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An Anomaly in Potassium Accumulation by Barley Roots

II. Effect of Calcium Concentration and Rubidium-86 Labeling ¹

^a Department of Agronomy, University of Kentucky, Lexington, Kentucky 40506

When excised barley roots were incubated in 0.01 to 0.1 mM KCl solutions containing 0.2 mM CaSO₄, there was a peak in the K⁺ accumulation against concentration curve at 0.02 to 0.04 mM KCl. The peak in the K⁺ accumulation curve was shifted to lower K⁺ concentrations when Ca²⁺ concentration was decreased and to higher K⁺ concentrations when Ca²⁺ concentration was increased. Increasing Ca²⁺ concentration in the treatment solution was observed to be stimulatory, inhibitory, or neutral depending on the K⁺ concentration.

When ⁸⁶Rb was used as an isotopic tracer for K⁺, accumulation of K⁺ was grossly overestimated, and the apparent K⁺ accumulation curve, as estimated with ⁸⁶Rb-labeled K⁺, was hyperbolic over the concentration range of 0.01 to 0.1 mM. It was concluded that ⁸⁶Rb is a poor tracer for K⁺ over the concentration range of 0.01 to 0.1 mM. Accumulation of Rb⁺ in the presence of K⁺ was accurately measured by ⁸⁶Rb.