Membrane Potentials in the Xylem in Roots of Intact Plants

The membrane potential differences (PDs) of root cells of intact, illuminated Trifolium repens L. and Lolium perenne L. have been measured. In T. repens the PDs were the same for all cell types except for the xylem vessels, which were more positive, and for some cells immediately adjacent to the xylem vessels which were 10 mV more negative. The mean PD for all cells was emdash 164.6 ± 0.6 mV and the mean for cells adjacent to the xylem vessels with elevated PDs was 178.4 ± 2.4 mV. When the electrode tip was in a xylem vessel a low but stable PD (mean = emdash 89.9 mV) was recorded. The results for L. perenne were similar except that there were no cells with elevated PDs adjacent to the xylem vessels. An inhibitor of ion transport from the root to the shoot, p-fluorophenylalanine (p-FPA), caused a depolarization of 10 mV in the cell PDs but in the xylem vessels the depolarization was 50 mV. The possibility that the elevated PDs of cells adjacent to the xylem vessels are related to the transport of ions into the vessels is discussed.