Photosynthesis of isolated chloroplasts and protoplasts under osmotic stress : Reversible swelling of chloroplasts by hypotonic treatment and its effect on photosynthesis.

1. Isolated intact spinach chloroplasts respond to changes of the sorbitol concentration of the suspending medium as near-perfect osmometers within a large range of osmotic potentials. Under isotonic conditions (π=9-10 bar), their average osmotic volume is 24 μm(3) and the total volume 36 μm(3). The osmotic volume can be increased to 63 μm(3) by lowering the sorbitol concentration until a critical osmotic potential of π=4 bar is reached. Below that value chloroplasts rupture. Between 10 bar and 4 bar, volume changes are reversible. 2. Increasing the chloroplast volume above 24 μm(3) causes inhibition of photosynthesis, with 50% inhibition occurring at an osmotic potential of π=5-6 bar. This corresponds to an osmotic volume of 45-55 μm(3). Depending on the duration of hypotonic treatment, inhibition of photosynthesis is more or less reversible. 3. Between 4 and 10 bar, the chloroplast envelope exhibits a very low permeability for ferricyanide, many metabolites, and soluble stroma proteins. 4. Electron transport is not inhibited by swelling of chloroplasts. Also, the ATP/ADP-ratio remains unchanged. 5. The solute concentration in the chloroplasts appears to be optimal for photosynthesis at 10 bar. Increasing the chloroplast volume causes inhibition of photosynthesis by dilution effects.