INHIBITION OF PHOTOSYNTHESIS BY OSMOTIC-STRESS IN PEA (PISUM-SATIVUM) MESOPHYLL PROTOPLASTS IS INTENSIFIED BY CHILLING OR PHOTOINHIBITORY LIGHT - INTRIGUING RESPONSES OF RESPIRATION

The effects of reduced osmotic potential on photosynthesis and respiration were studied in mesophyll protoplasts of pea (Pisum sativum). Osmotic stress was induced by increasing the sorbitol concentration in the medium from 0.4 kmol m(-3) (-1.3 MPa) to 1.0 kmol m(-3) (-3.1 MPa). Protoplasts lost up to 35% of the maximum capacity of photosynthetic carbon assimilation (but not PS II mediated activity) soon after exposure to 1.0 kmol m(-3) sorbitol. The response of protoplast respiration to osmotic stress was intriguing. Respiration was stimulated if stress was induced at 25 degrees C, but was inhibited when protoplasts were subjected to osmotic stress at 0 degrees C. Photosynthesis was also much more sensitive to osmotic stress at 0 degrees C than at 25 degrees C. The inhibitory effects of osmotic stress on photosynthesis as well as respiration were amplified by not only chilling but also photoinhibitory light. The photosynthetic or respiratory activities of protoplasts recovered remarkably when they were transferred from hyperosmotic (1.0 kmol m(-3) sorbitol) back to iso-osmotic medium (0.4 kmol m(-3) sorbitol), demonstrating the reversibility of osmotic-stress-induced changes in protoplasts. Respiration was more resistant to osmotic stress and was quicker to recover than photosynthesis. We suggest that the experimental system of protoplasts can be useful in studying the effects of osmotic stress on plant tissues.