CHLORIDE TRANSPORT INENTEROMORPHA INTESTINALIS(L.) LINK

Abstract

SummaryRates of36Cl−labelling, efflux and uptake were measured onEnteromorphaplants grown in sea‐water (547 mM Cl−), and also in brackish water medium [Artificial Cape Banks Spring Water (ACBSW), 25.5 mM Cl−, 20.4 mM Na+ and 0.5 mM K+]. Efflux experiments showed that, inEnteromorphaplants grown in seawater, light did not affect Cl−fluxes at the plasmalemma and tonoplast. Typical experiments exhibited two exchanged phases but a significant number (8/32) exhibited a single exchange phase; this was more likely to occur in darkness. Influx experiments also showed no effect of light on the tonoplast flux. Transfer of plants grown and labelled in seawater to low salinity medium caused a rapid loss of36C1−label; however, this was related to the change in osmotic potential of the medium rather than to changes in [Cl−o] or [K+0]. Exchange of36Cl−did not depend on [K+o] in seawater. Cyanide decreased36Cl−uptake in the dark but not in the light. In low salinity medium (ACBSW),36Cl−labelling and the plasmalemma flux inEnteromorphaplants were independent of light; however, the intracellular compartmentation of Cl−differed between light and dark. The tonoplast flux was also greater in the light. Intracellular Cl−was about 300 mmol kg−1in seawater plants and about 159 mmolkg−1in ACBSW plants. The cytoplasmic Cl−concentration ([CI−0]) based on compartmental analysis was about 200 mM inEnteromorphaplants in seawater and ACBSW medium. Use of this [Cl−0] value and the Nernst equation suggests active Cl−uptake in plants in both seawater and ACBSW. However it is unlikely that the cytoplasmic [Cl−] is above about 70 to 100 mM since many cytoplasmic enzymes are inhibited by high [Cl−0]. Taking this lower estimate of [C1−0], the Nernst criterion suggests passive accumulation of Cl−across the plasmalemma in seawater but active transport would be likely in plants in ACBSW medium. Where separate plasmalemma and tonoplast fluxes were detectable, plasmalemma fluxes were higher in low salinity medium than in seawater (about 500vs300 nmol m−2s−1); the tonoplast flux was about 30 nmol m−2s−1in seawater plants and about 20 and 10 nmol m−2s−1in light and dark, respectively, in ACBSW‐grown plants. In experiments where only a single exchange phase could be detected the apparent plasmalemma flux was only about 30 nmol−2s−1. Apparently the plasmalemma can exist in two permeability states to Cl−(0.2 to 1.0vs0.03 nm s−1).