Electrical Potential Differences in Cells of Barley Roots and Their Relation to Ion Uptake

Abstract

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...

Potassium Fluxes in Chlamydomonas reinhardtii (I.Kinetics and Electrical Potentials)

“…8B). In a similar study of barley root AI) values, Pitman et al (1971a) observed depolarizations of 43, 28, and 53 mV per decade of [K+], between 1 and 2.5, 2.5 to 10, and 10 to 25 mM [K+],, respectively. Furthermore, our measurements of AI) in Chlamydomonas by the use of TPP+ revealed that AI) was hyperpolarized as externa1 pH increased (Fig.…”

Induction and Development of Increased Ion Absorption in Corn Root Tissue

“…There was no change in respiration rate during washing (just as for traygrown roots, Table I). and there was no correlation between respiration rates and phosphate absorption rates (Table VIII) (22). The fact that the cell potential starts rising immediately suggests that induction may be linked to this phenomenon.…”

Increase in Electrogenic Membrane Potential with Washing of Corn Root Tissue

“…2). Pitman et al (10) suggested that PD might rise with the removal of a source of hormone, sealing of plasmodesmata, or an increase in the salt concentration in the cytoplasm. We have no data for evaluating these possibilities.…”

“…There is a growing belief that proton pumps are the source of the potential for active transport (3,12 (Fig. 4) (10), but in both the transformation is complete in 4 to 6 hr. We find the entire increase lies in the electrogenic PD, which levels out at the same time as the increase in phosphate absorption.…”

“…Macklon and Higinbotham (8) and Pitman et al (10) have reported that aging, or washing, of pea epicotyl and barley root segments is accompanied by an increase in cell potential. In other work, Higinbotham and associates have shown that a portion of the measured potential difference across the cytoplasmic layer of higher plant cells is electrogenic (4).…”