The Transport of Potassium to the Xylem Exudate of Ryegrass

Membrane potentials and potassium activities of the vacuoles of epidermal, cortical, and stelar cells of ryegrass roots were measured with microelectrodes. In 1.0 mM KCl solution a membrane potential of—130 mV was obtained for all the epidermal and cortical cells with an abrupt change to—100 mV at the endodermal-pericycle interface. The vacuolar potassium activity for all cells was approximately 150 mM. This implies uniform potassium transport properties in the cells of all tissues likely to be involved in the radial movement of ions across the root and does not support the existence of the stelar pump proposed by some workers. The pattern of potassium distribution obtained here with microelectrodes differs from that obtained by some workers by electron-probe microanalysis. The possibility of artefacts in both techniques is discussed and it is concluded that the measurements reported here are reliable.