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

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Abstract
Stromal acidification has been reported to mediate reduced osmotic potential (ψπ) effects on photosynthesis in the isolated spinach chloroplast (Berkowitz, Gibbs 1983 Plant Physiol 72: 1100-1109). To determine 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 reinhardii cells and Aphanocapsa 6308 cells under isotonic and dehydrating conditions were investigated. Five millimolar NH4Cl stimulated spinach leaf slice CO2 fixation by 43% under stress (0.67 molar sorbitol) conditions, and had little effect on fixation under isotonic (0.33 molar sorbitol) conditions. Chlamydomonas cells were found to be more sensitive to reduced ψπ than spinach leaf slices. CO2 fixation in the cells of the green alga Chlamydomonas reinhardii was 99 and 17 micromoles per milligram chlorophyll per hour, respectively, at 0.1 molar mannitol and 0.28 molar mannitol. Five millimolar NH4Cl stimulated CO2 fixation of Chlamydomonas cells by 147% under stress (0.28 molar mannitol) conditions. Aphanocapsa 6308 cells (blue-green alga) were also found to be sensitive to reduced ψπ, and inhibitions in photosynthesis were 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.