Transmembrane electropotential in barley roots as related to cell type, cell location, cutting and aging effects

Transmembrane electropotential difference (PD) was measured in whole roots of barley (Hordeum vulgare L. cvs. Compana and Himalaya). Seedlings were grown 4 to 5 days in aerated 0.5 mM CaSO4 or a nutrient solution. Measurements of PD were made with roots bathed in CaSO4, KCl + CaSO4, or the nutrient solution. The following results were found. (a) There was a radial PD gradient with epidermal cells being 10 to 58 millivolts less negative than cells in the third layer of the cortex (outside to inside). There was no longitudinal PD gradient in the region 0.5 to 4 cm from the root tip, nor was there any difference between the PD of young root hairs and other epidermal cells. (b) Cell PD in excised whole roots was not detectably different from that found in roots attached to the shoot, and was unchanged for 2 hours from excision. (c) In 1-centimeter sections of root, cell PD at the freshly cut surface was depolarized by 90 millivolts from that in the intact root; cells farther than 1 millimeter from the cut surface were not depolarized. The PD of cells at the cut surface became more negative upon aging the segment in 0.5 mM CaSO4, eventually becoming greater by -25 millivolts than that in cells of intact roots. Cells in segments to which the root tips were attached had less negative PDs after aging than those in subapical segments, indicating a possible hormonal effect. PDs in aged, excised segments are not equivalent to those in intact roots. (d) Creeping of cytoplasm over electrode tips inserted into the vacuole gave measurements of vacuole-to-cytoplasm PD of +9 millivolts in 0.5 mM CaSO4 and +35 millivolts in 1 mM KCl + 0.5 mM CaSO4. Most of the cell PD was across the plasmalemma. (e) The reducing sugar content of roots in CaSO4 solution was greater than that of roots in the nutrient solution in which ion uptake, particularly K+ occurred.