LIGHT-INDUCED CHANGES IN IONIC CONTENT OF CHLOROPLASTS IN PISUM SATIVUM

1. 1. The content of chloroplasts isolated in two minutes from pea plants in the light or dark was ascertained both via osmotic responses in sucrose solutions interpreted by the Boyle-Van 't Hoff relation and from direct determination of the K+, Na+, Mg2+, Ca2+ and Cl- associated with the chloroplasts. 2. 2. From packed-volume and Coulter-counter results, the chloroplasts contained about 3.52 μmoles of osmotically active particles per mg chlorophyll when isolated from plants in the dark and only 2.39 μmoles per mg chlorophyll when isolated from plants in the light. Likewise, the total amount per mg chlorophyll of the five ions determined was about 32% lower for chloroplasts from plants in the light compared with the dark. 3. 3. The concentration of ions in the osmotically responding space was calculated using the chloroplast volume measured at the same osmolality as the cell sap (about 288 mosmolal). Chloroplasts both from plants in the light and the dark contained about 99 mM K+, 92 mM Cl-, 16 mM Mg2+, 15 mM Ca2+ and 10 mM Na+. 4. 4. Based on an extension of the conventional Boyle-Van 't Hoff relation using irreversible thermodynamics, the reflection coefficient of the chloroplast limiting membranes for internal solutes was greater than 0.7. Sucrose was essentially excluded from these chloroplasts. 5. 5. The light-induced decrease of K+ in the chloroplasts and its reversal when the plants were placed in the dark had half-times of about 4 min. This decrease occurring in vivo was 75% inhibited by p-trifluoromethoxycarbonylcyanide phenylhydrazone and was completely abolished by nigericin, indicating that the light-induced K+ efflux from chloroplasts required either ATP or a high-energy intermediate created by photosynthetic electron flow. © 1969.