Properties of the Isolated Intact Chloroplast at Cytoplasmic K + Concentrations

Abstract

Photosynthesis, stroma-pH, and internal K+ and Cl− concentrations of isolated intact chloroplasts from Spinacia oleracea, as well as ion (K+, H+, Cl−) movements across the envelope, were measured over a wide range of external KCl concentrations (1-100 millimolar). Isolated intact chloroplasts are a Donnan system which accumulates cations (K+ or added Tetraphenylphosphonium+) and excludes anions (Cl−) at low ionic strength of the medium. The internally negative dark potential becomes still more negative in the light as estimated by Tetraphenylphosphonium+ distribution. At 100 millimolar external KCl, potentials both in the light and in the dark and also the light-induced uptake of K+ or Na+ and the release of protons all become very small. Light-induced K+ uptake is not abolished by valinomycin suggesting that the K+ uptake is not primarily active. Intact chloroplasts contain higher K+ concentrations (112-157 millimolar) than chloroplasts isolated in standard media. Photosynthetic activity of intact chloroplasts is higher at 100 millimolar external KCl than at 5 to 25 millimolar. The pH optimum of CO2 fixation at high K+ concentrations is broadened towards low pH values. This can be correlated with the observation that high external KCl concentrations at a constant pH of the suspending medium produce an increase of stroma-pH both in the light and in the dark. These results demonstrate a requirement of high external concentrations of monovalent cations for CO2 fixation in intact chloroplasts.