Effects of Cl⁻on HCO3− and Malate Fluxes and CO₂Fixation in Carrot and Barley Root Cells

Cl⁻ and $HCO3−$ ions interact apparently competitively during influx across the plasmalemma of carrot root cells. Cl⁻, however, reduces $HCO3−$ influx much less than predicted from the effect of $HCO3−$ on Cl⁻ influx. Cl⁻ and $HCO3−$ plasmalemma influxes both increase with time after excision of carrot tissue. Cl⁻ and $HCO3−$ may therefore be transported by a common mechanism. The effect of pH changes on the influx of malate across the plasmalemma in barley roots shows that malate crosses the plasmalemma as the singly charged anion. Stimulations of influx by both K₂SO₄ and KCl suggest that the malate anion crosses in association with. K⁺. If malate entry is passive, P_mal- is about 2×10⁻⁸ cm s⁻¹, but it is thought that malate entry is partly an active process. A slight, apparently competitive inhibition by Cl⁻ of malate flux into the vacuole of barley root cells suggests that the two anions may be transported by a common process at the tonoplast, but this is not thought to be physiologically significant. The accumulation of ¹⁴C from 1 mM $HCO3−$ is drastically reduced by 10 mM Cl⁻. A quantitative analysis of the kinetics of ¹⁴C exchange shows that Cl⁻ directly inhibits the formation of malate from $HCO3−$. The decreased influx of endogenously produced malate to the vacuole in the presence of Cl⁻ is probably a secondary consequence of the fall in the cytoplasmic concentration. The nature of the Cl⁻ inhibition of malate formation is discussed. In KCl-loaded tissue the influx of external malate and the accumulation of ¹⁴C from external $HCO3−$ are reduced. The location of these effects is not certain, but the effects suggest that regulation of malate synthesis and accumulation may be related to the negative-feedback regulation of Cl⁻ and $NO3−$ transport.