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, KCI + 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 ceUls 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, cel 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 ceUls at the cut surface became more negative upon aging the segment in 0.5 mM CaSO4, eventually becoming greater by -25 milivolts 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-tocytoplasm PD of +9 millivolts in 0.5 mM CaSO4 and +35 millivolts in 1 mM KCI + 0.5 mM CaSO4. Most of the cel PD was across the plasmalemma. (e) The reducing supr content of roots in CaSO4 solution was greater than that of roots in the nutrient solution in which ion uptake, parficularly K+ occurred. Many electrophysiologists measure the transmembrane electropotential difference in slices or segments of plant organs such as roots, stems, and leaves. The segments are often aged for periods varying from none to several hours after excision in order to minimize possible side effects due to injury to the tissue during excision. This procedure has led to other problems. For example, in a study using 1-centimeter segments of low-salt barley roots (24), the PD4 of cortex cells along the cut surface became significantly more negative with time after exci- (24). Seeds were surface-sterilized for 20 min in 1% NaOCI, rinsed in deionized H20, and germinated for 24 hr at 20 C in the dark in aerated deionized H20. Germinated seedlings were grown hydroponically at 20 C in the dark for an additional 3 to 6 days. For most studies, the growing solution was 0.5 mm CaSO4 (pH 5.5). Deionized H20 and a nutrient solution referred to as IX (13) were used in a few experiments. The IX solution contained in mM: I KCI, I Ca(NO3)2, 0.25 MgSO4, 0.904 NaH2PO4, and 0.048 Na2HPO, (pH 5.5).The hydroponic growth chamber, illustrated elsewhere (18), was constructed as follows. The base was a 35 x 30 x 14 ...

Single cell electropotentials of barley (Hordeum vulgare L., cv. 'Compana') root cortex were measured at different external concentrations of KCl in the presence of Ca2+. The roots were low in salt from seedlings grown on 0.5 mM aerated CaSO4 solution. Thus, the conditions were equivalent to those used to define the dual nmechanisms found with radioactive tracer-labeled ion uptake. In 0.5 mM CaSO4 alone, there is an increase with time of cell negativity from about -65 millivolts 13 minutes after cutting segments to about -185 millivolts in 6 to 8 hours. Two possible hypotheses, not mutually exclusive, are offered to explain this aging effect: that cutting exposes plasmodesmata which are leaky initially but which seal in time, and that some internal factors, e.g., hormones diffusing from the apex, have a regulatory effect on the cell potential, an influence which becomes dissipated in isolated segments and permits the development of a higher potential difference. In any case changes in selective ion transport must be involved. The cell potentials at KCI concentrations above 2.0 mM are more negative than would be expected for a passive diffuision potential. It is suggested that this discrepancy may be due to an electrogenic pump or to a higher K+ concentration in the cytoplasm than in the remainder of the cell, or perhaps to both. Whether there is a clear relationship between cell potential and mechanisms 1 and 2 of cation transport depends upon whether the cell potentials of freshly cut or of aged tissue represent the values relevant to intact roots. low concentrations (Km = 0.02 mM) and is the major process at concentrations below 0.5 mm. The other component, mechanism 2, is identifiable only at concentrations exceeding 0.5 to 1.0 mM. The two components also differ in selectivity for K+ and Na+: mechanism 1 is highly specific for K+ versus Na+, whereas mechanism 2 shows little discrimination between K+ and Na+. The characteristics of the salt absorption isotherm have been explained by assuming that ion transport takes place via specific sites on a carrier. When the sites are saturated, further increase in concentration, within limits, fails to increase the velocity of uptake unless new sites are brought into action. Consequently, it has been held by some workers that independent ion diffusion does not occur, i.e., ions move only in a combined neutral form (5, 34). This view has not been accepted by others (1,4,21,25). Independent diffusion of some ions may be significant in plants inasmuch as the cell interior is electronegative by a value that can be related to external salt concentration by equations based on passive ion diffusion. In oat coleoptiles, for example, increasing the external KCl or NaCl concentration makes the cell potential less negative by an amount expected if the cell were much more permeable to K+ and Na+ than to 15).With passive transport the potential difference, PD3 (or E, in equations), between the cell sap and the external solution is a diffusion potential which assumes a value s...

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An inhibition of root growth, a decrease in the amount of potassium (as (86)Rb) and phosphate ((32)P) accumulation by the root, and a partial depolarization of transmembrane electropotential were observed to develop with a similar time course and to a similar extent when intact maize (Zea mays L.) roots were treated with 10(-5) M abscisic acid (ABA). Potassium uptake was inhibited by ABA when excised, low-salt roots were bathed in KCl, KH2PO4, or K2SO4. ABA did not affect the ATP content of the tissues, the activity of isolated mitochondria, nor the activity of microsomal K(+)-stimulated ATPases.

Bipolaris maydis (Nisikado) Shoemaker is the fungus which causes southern corn leaf blight. A significant positive correlation exists between cultivar susceptibility to the disease and the susceptibility to toxins produced by race T of the fungus (6, 7, 10, 13,22,23), thus suggesting that host-specific toxins are the primary disease determinant.The objective of the research in our laboratory has been to identify the cellular mechanism(s) which determine host susceptibility to the toxins. The strategy has been to identify the earliest (2,6,9).The purpose of this study was to identify possible nonmitochondrial sites of toxin action. Measurement of ion transport phenomena (kinetics of ion accumulation or membrane electrical potentials) provides in vivo methods to probe for plasmalemma and cytosol sites of toxin action. In this report the effect of race T pathotoxin on K+ uptake is characterized and is shown to be related to host resistance/susceptibility to the disease. The evidence indicates that the plasmalemma is a primary hostspecific site of toxin action.