The Role of Transmembrane Electrical Potential in Determining the Absorption Isotherm for Chloride in Potato

Measurements have been made of the electrical potential difference between the vacuoles of single potato tuber cells and external Cl- solutions over the range 1–40 mM. With K+ as the counter-ion, the relationship between this transmembrane electrical potential and external Cl- concentration, for fresh cells at 20° C, was found to be one of decreasing negative polarity with increasing Cl- concentration (E at 1 mM Cl- external = – 81 m V; change in E for a 10-fold change in external concentration, ΔE10 = 46 m V). The linearity of this relationship, apparent on a semi-logarithmic plot, was virtually unaltered by low temperature (0.5–2.5° C) or by previous ageing of the cells for periods up to four days (indicating that metabolic ion absorption is not an electrogenic process). When the counter-ion was maintained at a constant high concentration (40 mM K+), the change in potential over the Cl- concentration range was only 4 m V, polarity becoming more negative with increasing Cl- concentration. With Ca++ as the counter-ion, the potential to external Cl- concentration relationship was similar to that found in KCl solutions, except that ΔE10 was only about 20 m V. Curves for the influx of C1- to be expected on the basis of these electrochemical data alone have been shown to run closely parallel to Cl– absorption isotherms previously determined experimentally. This confirms the opinion, already formed on the basis of theoretically derived values for passive Cl- influx, that Cl- uptake by both fresh and one-day-aged potato tissue, from KCl solutions and Cl- solutions with a fixed high K+ concentration, is rate-determined at o° C by passive movement across the plasmalemma. Uptake of Cl- by fresh tissue at 20° C appears to be similarly regulated. No such parallelism was found between observed and expected patterns of Cl- uptake from CaCl2 solutions, or from KCl by two-day-aged tissue, and here factors in addition to the electrochemical ones must determine low temperature Cl-uptake.