Phosphorus-31 NMR Studies of Wild-Type and NaCl-TolerantCitrusCultured Cells

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Abstract
Ben–Hayyim, G. and Navon, G. 1985. Phosphorus–31 NMR studies of wild–type and NaCl–tolerant Citrus cultured cells.—J. exp. Bot. 36: 1877–1888. The internal pH of the cytoplasm and vacuole and the relative distribution of internal Pi concentrations between those two cell compartments have been determined by 31P nuclear magnetic resonance spectroscopy in wild–type and NaCl–tolerant cell lines of Shamouti orange (Citrus sinensis L. Osbeck). Wild–type cells accumulate higher amounts of Pi than the NaCl–tolerant cells when exposed to equal external Pi concentrations. This additional Pi is located mainly in the vacuole. When both types of cells are exposed to increasing external Pi concentrations, the internal Pi concentrations increase. The cytoplasmic Pi concentration reaches saturation at a rather low external Pi concentration while the vacuolar Pi can be increased by a large factor. Transfer of cells from aerobic to anaerobic conditions causes an immediate increase of Pi in the cytoplasm and a slow acidification. Exposure of cells to NaCl during the period of their growth results in an increase in total Pi concentration with a large increase in the ratio of vacuolar to cytoplasmic Pi levels. When these cells are exposed to NaCl for a short time, total internal Pi concentration docs not change significantly but its proportions change in favour of the vacuole. pH values of the cytoplasm and the vacuole under all these conditions are rather constant, the value being 5.8–6.0 for the vacuole and 7.4–7.6 for the cytoplasm. Moreover, subjecting these cells to a wide range of external pH values does not change their intracellular pH. These results indicate that a strong regulation of internal pH is operating in both types of cells. The presence of a phosphorylated metabolite with an unusual pH titration curve, located in the vacuole, is also reported.