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Transmembrane Electropotential in Barley Roots as Related to Cell Type, Cell Location, and Cutting and Aging Effects ^1,2

^a Department of Botany, Washington State University, Pullman, Washington 99163

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.

¹ This research was supported by National Defense Education Act Title IV Fellowship 66-8369 to S.M.M. and by National Science Foundation Grants GB-5117X and GB-19201 to N.H., and is a portion of a thesis in partial fulfillment of the requirements for the Ph.D. degree by S.M.M.

² This paper is dedicated to Dr. Leon Bernstein who showed an early interest in plant electrophysiology.

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