Mechanisms of light-induced structural change in chloroplasts II. The role of ion movements in volume changes

Abstract

1. The effect of certain anions in enhancing light-induced light-scattering increments in isolated spinach chloroplasts has been studied, and the relation of these effects to volume changes and ion movements has been investigated. 2. Changes in transmission or absorbance have been shown to provide a better indication of volume changes of chloroplasts in suspension than do changes in 90° light scattering. Absorbance measurements were therefore employed to study osmotic behavior of chloroplasts suspended in a variety of sodium and ammonium salts. Chloroplasts swell rapidly in the dark in the ammonium salts of weak acids. Since the rate of swelling is greatest with anions whose p K a is highest, it is concluded that chloroplasts are permeable to undissociated acids and ammonia. 3. A mechanism has been proposed to explain the effects of anions in enhancing light-scattering changes in illuminated chloroplasts. It is suggested that on acidification of the chloroplast interior by light-induced hydrogen ion uptake, weakly acidic anions are lost as a result of the displacement of the equilibrium of undissociated acid across the chloroplast membrane, and that the volume changes observed are by osmotic equilibration. The kinetics and pH dependence of the light-induced light-scattering and hydrogen ion changes are consistent with this mechanism. 4. The light-induced movement of cations in suspensions of chloroplasts in acetate and chloride media has also been measured. On illumination in Tris-acetate, the small amount of cation retained by chloroplasts is irreversibly lost but a reversible uptake of cation on illumination of chloroplasts suspended in choline or Tris-chloride is observed. The extent of the light-induced cation movements is too small to account for chloroplast volume changes.