Effect of Osmotic Stress on Photosynthesis Studied with the Isolated Spinach Chloroplast

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Abstract
The effects of reduced osmotic potential on the photosynthetic carbon reduction cycle were investigated by monitoring photosynthetic processes of spinach (Spinacia oleracea L. var. Long Standing Bloomsdale) chloroplasts exposed to increased assay medium sorbitol concentrations. CO2 assimilation was found to be inhibited at 0.67 molar sorbitol by about 60% from control rates at 0.33 molar sorbitol. This level of stress inhibition was greater than that affecting the reductive phase of the cycle; glycerate 3-phosphate reduction was inhibited at 0.67 molar by 27 to 40%. Sorbitol (0.67 molar) inhibited the rate of O2 evolution at saturating and limiting concentrations of NaHCO3, and extended the lag phase of O2 evolution. This indicated that factors which are rate-limiting to the photosynthetic process are adversely affected by reduced osmotic potential. Analysis of photosynthetic products following CO2 fixation in 0.33 molar sorbitol and 0.67 molar sorbitol indicated that reduced osmotic potential facilitated increases in the levels of fructose 1,6-bisphosphate and triose phosphates with reductions in glucose 6-phosphate and fructose 6-phosphate, implicating fructose 1,6-bisphosphatase as a site of osmotic stress. Osmotic inhibition of the reductive portion (glycerate 3-phosphate to triose phosphate) of the photosynthetic carbon reduction cycle was partially attributed to feedback inhibition by the product, triose phosphate, on glycerate 3-phosphate reduction. A saturating concentration of ribose 5-phosphate partially overcame osmotic inhibition of CO2-supported O2 evolution, indicating another but apparently less severe site of stress inhibition in the sequence of ribose 5-phosphate to glycerate 3-phosphate.