Stromal Acidification Mediates In Vivo Water Stress Inhibition of Nonstomatal-Controlled Photosynthesis

Abstract

Stromal acidification has been reported to mediate reduced osmotic potential (Ypi) effects on photosynthesis in the isolated spinach chloroplast (Berkowitz, G. A.; Gibbs, M., 1983). To det. if stromal acidification mediates osmotic dehydration inhibition of photosynthesis in vivo, the effects of a weak base (NH4Cl), which raises stromal pH, on CO2 fixation of vacuum-infiltrated spinach leaf slices, Chlamydomonas reinhardti cells and Aphanocapsa 6308 cells under isotonic and dehydrating conditions were investigated. Five mM NH4Cl stimulated spinach leaf slice CO2 fixation by 43% under stress (0.67M sorbitol) conditions, and had little effect on the fixation under isotonic (0.33M sorbitol) conditions. Chlamydomonas Cells were more sensitive to reduced Fpi than spinach leaf slices. CO2 fixation in the cells of the green alga C. reinhardti was 99 and 17 micro M/mg chlorophyll/h, resp., at 0.1M and 0.28M mannitol. Five mM NH4Cl stimulated CO2 fixation of Chlamycomonas cells by 147% under stress (0.28M mannitol) conditions. Aphanocapsa 6308 Cells (blue-green alga) were also partially reversed by NH4Cl. These data indicate that in vivo water stress inhibition of photosynthesis is facilitated by stromal acidification, and that this inhibition can be at least partially reversed in situ. [on SciFinder (R)]