[Hill reaction and photophosphorylation of isolated chloroplasts in relation to water content : I. Removal of water by means of concentrated solutions].

1. Water was removed by means of concentrated solutions from chloroplasts which were isolated from leaves of spinach and beets. During and after the dehydration Hill reaction and cyclic photophosphorylation with PMS as a cofactor were investigated. As osmotic amterial glucose, sucrose, lutrol and NaCl were used. 2. No depression of ferricyanide reduction was obtained in 3 M sugar solution and in 2.5 M lutrol solution. These concentrations correspond to a loss of water amounting to 90% of the total water of leaf cells. In contrast, cyclic photophosphorylation was already decreased in 1-2 M solutions of sugar or lutrol, that means by much less dehydration. In 3 M solutions only 5-25% of the activity of the water saturated controls remained. However, this decrease in cyclic photophosphorylation occurred only when chloroplasts were kept dehydrated during the light reaction. When chloroplasts were permitted to return to optimal water conditions photophosphorylation was no longer inhibited. Therefore, extensive loss of water leads to reversible uncoupling of photophosphorylation from electron transport. 3. Relatively low concentrations of NaCl (as compared with sugar concentrations) damage the ability of chloroplasts to perform Hill reaction and photophosphorylation. Inactivation of the reactions is partly reversible at low concentrations of NaCl and irreversible at high concentrations. 4. The osmotic potential of leaves of sugar beet increased with increasing dehydration. Within a limited range the osmotic behaviour of the cell sap of leaf cells during dehydration was identical with that of NaCl solutions. 5. The possibility of correlating in vitro experiments in which dehydration is simulated by exposure of chloroplasts to various solutions with in vivo experiments using intact leaves which are dehydrated to different degrees is demonstrated.