Photosynthesis under osmotic stress : Inhibition of photosynthesis of intact chloroplasts, protoplasts, and leaf slices at high osmotic potentials.

1. Photosynthesis of leaf slices, mesophyll protoplasts, and intact chloroplasts of spinach was inhibited in hypertonic sorbitol solutions. Sorbitol could be replaced by other nonpenetrating osmotica such as sucrose or glycinebetaine. As a penetrating solute, ethyleneglycol was also inhibitory, but osmolarities required for inhibition of photosynthesis were considerably higher than in the case of non-penetrating osmotica.-2. With leaf slices and protoplasts, 50% inhibition by sorbitol was usually observed at osmotic potentials between 25 and 40 bar. With isolated intact chloroplasts, the osmotic potentials producing 50% inhibition varied considerably. Depending on the growth conditions of the plant material, 50% inhibition occurred between 14 and 40 bar. The integrity of the chloroplast envelope as measured by the accessibility of the thylakoid system for ferricyanide was not affected by osmotic stress.-3. Quantum requirements for CO2 assimilation and reduction of 3-phosphoglycerate or nitrite by intact chloroplasts increased under osmotic stress. The increase was larger for CO2 reduction than for reduction of 3-phosphoglycerate or nitrite.-4. In intact chloroplasts, electron transport to methylviologen was not much affected by osmotic stress. Basal electron transport was not stimulated, suggesting absence of uncoupling.-5. The increase in ATP/ADP ratios on illumination of intact chloroplasts was slower at an osmotic potential of 36 bar than at 11 bar.-6. The results indicate that inhibition of photosynthesis is not caused by the sensitivity of a single photosynthetic reaction to increased osmotic potentials. Rather, several reactions are sensitive to water stress. Osmotic stress acts on the photosynthetic apparatus mainly at the level of dark reactions and ATP synthesis, and much less on primary photoreactions or electron transport, between water and the primary oxidant of photosystem I.-7. The different sensitivity of chloroplasts to penetrating and non-penetrating solutes and the observed variability of chloroplast sensitivity to stress suggests that the reduction in water potential is not directly responsible for damage to the photosynthetic apparatus during osmotic stress. Rather, the composition of the chloroplasts appears to be a decisive factor which determines sensitivity or resistance to osmotic stress.