Relationships Between Chloride Transport and Electrical Potential Differences in Carrot Root Cells

Abstract

The Cl^- influx across the tonoplast increases at about 3 days after excision, and is inhibited by carbonyl cyanide m-chlorophenylhydrazone in well-washed tissue, while the influx of Cl^- across the plasmalemma and the cellular electrical potential difference both remain constant under these conditions. The transport of Cl^- within the cell therefore appears not to be electrogenic.

When SO₄^2- is substituted for Cl^-, keeping the external K⁺ concentration constant, the cell potential difference (p.d.) increases from - 54 mV in 10 mol m^-3 KCl to -65 mV in 5 mol m-³ KSO₄^2- if the p.d. were solely a diffusion potential, then substituting SO₄^2- for Cl^- would be expected to reduce it. This prediction is based on the Goldman equation with a term for SO₄^2- introduced, and various estimates of the parameters involved. It is therefore suggested that a small part of the p.d. in carrot may be due to the activity of a Cl^- pump inwards across the plasmalemma, which is linked either to a larger cation influx or to a larger anion (e.g. OH^-) efflux.

During accumulation of KCl, the cell p.d. increases slightly, from - 54 mV in the non-loaded cell in 10 mol m^-3 KCl to -59 mV in the KCl-loaded cell in 10 mol m^-3 KCl. This small change is not inconsistent with the response of the p.d. to changes in external salt concentration.

From these results, the electrochemical potential of Cl^- in the vacuole is calculated to be greater than in an external solution of 1 mol m^-3 KCl by 17 kJ mol^-1 in non-loaded tissue and by 23 kJ mol^-1 in KCl-loaded tissue. This increase in the gradient opposing Cl^- entry is probably not sufficient to account for the fall in the active influx of Cl^- during accumulation of KCl.