Rubidium and Potassium Absorption by Bean‐leaf Slices Compared to Sodium Absorption

At salt concentrations of 0.1 mM as well as of 5.0 mM, the 22Na+ absorption capacity of bean (Phaseolus vulgaris L. cv. ‘Brittle Wax’) leaf tissue increased during the period of leaf expansion and decreased rapidly after leaf maturation. The absorption capacity for 86Rb+ and 42K+ was highest in very young leaves and decreased continuously in expanding and in mature leaves. The 86Rb+ absorption capacity of mature leaves was not increased by detopping the plants; this senescence‐retarding treatment more than doubled 2Na+ absorption. The absorption of 22Na+ by bean‐leaf slices was not enhanced by light, whereas 86Rb+ and 42K+ absorption was much affected. Previously absorbed 86Rb+ and 42K+ were more available for exchange than 22Na+.