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

Abstract

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.