Vacuolar and Cytoplasmic Potassium Concentrations in Pea Roots in Relation to Cell-to-Medium Electrical Potentials.

The cause of the cell-to-medium electrical potentials of plant and animal cells is incompletely understood. In some cases, the potentials appear to be diffusion potentials in which potassium ions play a major role (3,7.) This is indicated A) by potentials which are close to the Nernst potentials for potassium and B) by cell-to-medium potentials which become markedly less negative when the external potassium ion concentration is increased (2, 3, 5, 6, 8, 13). Some evidence indicates that the cell-to-medium potentials are not potassium diffusion potentials. In the squid axon, for example, a large potential remains after the axoplasm has been replaced with a potassium-free solution (14) [There is conflicting evidence here however (1)]. Similar results have been found in the case of vacuolar perfusion of Halicystis and Nitella cells (2,15). The cause of the cell-to-medium electrical potentials of higher plants has not been extensively investigated. Evidence for Avena roots indicates that most of the potential difference occurs between the cytoplasm and the medium, that a negligible potential change occurs between the cytoplasm and the vacuole, and that the cell-to-medium potentials of Avena coleoptiles become less negative when the potassium concentration in the medium is increased (5). Also, flame photometric analysis of extracts from roots and tops of pea and oat seedlings show that the potassium concentrations in the cells are much higher than the concentrations of other inorganic ions (7). The above factors indicate that the cell-to-medium potentials could be diffusion potentials caused principally by the cell-to-medium potassium concentration ratios. In pea roots and stems, however, the cellular potassium concentrations under some conditions did not appear to be high enough to be a major factor in a diffusion potential svstem (4). This finding has to be qualified, however, because only the aver-