An Anomaly in Potassium Accumulation by Barley Roots: II. Effect of Calcium Concentration and Rubidium-86 Labeling.

When excised barley roots were incubated in 0.01 to 0.1 mm KCl solutions containing 0.2 mm CaSO(4), 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 Ca(2+) concentration was decreased and to higher K(+) concentrations when Ca(2+) concentration was increased. Increasing Ca(2+) concentration in the treatment solution was observed to be stimulatory, inhibitory, or neutral depending on the K(+) concentration.When (86)Rb was used as an isotopic tracer for K(+), accumulation of K(+) was grossly overestimated, and the apparent K(+) accumulation curve, as estimated with (86)Rb-labeled K(+), was hyperbolic over the concentration range of 0.01 to 0.1 mm. It was concluded that (86)Rb 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 (86)Rb.