Rubidium was applied to the maize root system as a potassium antagonist. Exogenously applied RbCl dramatically decreased primary root length and caused radial expansion. However, the decrease in fresh weight was not prominent. The endogenous level of potassium decreased accompanying accumulation of rubidium following RbCl treatment. The altered root growth caused by RbCl was restored by KCl, suggesting that potassium is required for directional regulation of root cell growth. Microscopic observations of cells in the elongation zone of control and RbCl-treated roots supported this idea. Gravitropic curvature, which is dependent on ἀne regulation of differential cell elongation, was also impaired in RbCl-treated roots, indicating that potassium is not simply in volved in cell elongation itself, but is necessary for directional regulation of cell growth
