CHLORIDE IN CELLS OF OHARA AUSTRALIS By H. G. L. COSTER* [Manuscript received December 21, 1965] Summary A potentiometric method of measuring the vacuolar and cytoplasmic chloride activity in cells of Ohara australis is described. It was found that the activity of chloride ions in the vacuole was approximately 100 mN (average for 13 cells), and in the cytoplasm chloride activity was found to be approximately 10 mN (average for 13 cells). The electrochemical equilibrium potentials, Eel, for chloride across the plasmalemma and tonoplast are far removed from the resting potentials across these membranes. During an action potential the membrane potential shifts towards ECl, as would be expected if the action potential were produced by a transient increase in the permeability of the membrane to chloride ions. 1. INTRODUCTION In cells of Ohara australis a fast and large action potential occurs across the plasmalemma, and a small and slow action potential occurs across the tonoplast (Findlay and Hope 1964a). Since it appears that chloride plays a major role in the production of the action potential, it is important to know the activity of chloride ions in the cytoplasm. From this the electrochemical potential for chloride ions across both membranes can be calculated. If in fact chloride ions were in electro- chemical equilibrium then one would not expect a transient change in permeability to chloride ions to cause the action potential. The concentration of K+, Na+, Cl-, and Ca 2 + ions in the vacuole of cells of O. australis can be fairly easily estimated by analysis of extracted cell sap. The K+, Na+, and Ca 2 + are readily determined with a flame-photometer while Cl- can be determined by potentiometric titration. This method does not necessarily give the activity of chloride ions in the vacuole. Until now no direct measurements of chloride ion concentration or activity in vivo either in the vacuole or in the cytoplasm have been made. Extraction of the cytoplasm is difficult because of its small volume and because of its distribution as a thin layer between the cell wall and the vacuole; this makes direct chemical analysis very difficult. However, MacRobbie (1962, 1964) obtained values of 130 mN for K + and 40 mN for N a + in the cytoplasm of N itella translucens. Similar results were obtained by Spanswick and Williams (1964), except for a much lower value for Na+ (14 mN). The chloride concentration in the flowing cytoplasm, obtained by these authors, was 65 mN. * Plant Physiology Unit, Division of Food Preservation, CSIRO, and School of Biological Sciences, University of Sydney; present address: School of Physics, University of New South Wales. Aust . .1. BioI. Sci., 1966, 19, 545-54