Photosynthetic characteristics of chloroplasts isolated fromMesembryanthemum crystallinum L., a halophilic plant capable of Crassulacean acid metabolism.

Photosynthetically highly active chloroplasts were routinely obtained by rupture of leaf protoplasts from the halophyteMesembryanthemum crystallinum which exhibited the photosynthetic characteristics of either a C3 plant when grown with 20 mmol l(-1) NaCl in the rooting medium, or a Crassulacean-acid-metabolism (CAM) plant when grown with 400 mmol l(-1) NaCl. Photosynthesis rates of C3 and CAM chloroplasts were 150-250 and 90-150 μmol mg(-1) chlorophyll h(-1), respectively. Because of osmotic adjustment, CAM chloroplasts required higher sorbitol concentrations (0.7-0.8 mol l(-1)) in the assay medium than C3 chloroplasts (0.3-0.4 mol l(-1)) for optimum activity. Substitution of sorbitol by NaCl as the osmoticum strongly reduced photosynthesis of CAM chloroplasts. Rates of electron transport (ferricyanide reduction, uncoupled) remained unaffected over a range of sorbitol concentrations (0 to 1 mol l(-1)). Sensitivity of electron transport to increasing levels of NaCl was less pronounced than the NaCl-sensitivity of CO2 fixation by intact chloroplasts. The CAM chloroplasts showed a broad pH optimum of photosynthesis between pH 7.0 and 8.2; photosynthesis of C3 chloroplasts dropped markedly below pH 7.6. The CAM chloroplasts maintained a higher transenvelope proton gradient than C3 chloroplasts both in the light and dark. External pyruvate (5 mmol l(-1)) inhibited photosynthesis of CAM chloroplasts, but not of C3 chloroplasts. Inhibition was reduced by increased external concentrations of orthophosphate.