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

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 SO42- 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-� KSO42- if the p.d. were solely a diffusion potential, then substituting SO42- for Cl- would be expected to reduce it. This prediction is based on the Goldman equation with a term for SO42- 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.