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 respira tion were studied in mesophyll protoplasts of pea (Pisum sativum). Osm otic stress was induced by increasing the sorbitol concentration in th e 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 c arbon assimilation (but not PS II mediated activity) soon after exposu re 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 mu ch 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 photoinhib itory light. The photosynthetic or respiratory activities of protoplas ts recovered remarkably when they were transferred from hyperosmotic ( 1.0 kmol m(-3) sorbitol) back to iso-osmotic medium (0.4 kmol m(-3) so rbitol), demonstrating the reversibility of osmotic-stress-induced cha nges in protoplasts. Respiration was more resistant to osmotic stress and was quicker to recover than photosynthesis. We suggest that the ex perimental system of protoplasts can be useful in studying the effects of osmotic stress on plant tissues.