Effect of external pH on the cytoplasmic and vacuolar phs in mung bean root-tip cells: A ³¹P nuclear magnetic resonance study

The effect of the external pH on the intracellular pH in mung bean (Vigna mungo (L.) Hepper) root-tip cells was investigated with the ³¹P nuclear magnetic resonance (NMR) method. The ³¹P NMR spectra showed three peaks caused by cytoplasmic G-6-P, cytoplasmic P_i and vacuolar P_i. The cytoplasmic and vacuolar pHs could be determined by comparing the P_i chemical shifts with the titration curve. When the external pH was changed over a range from pH 3 to 10, the cytoplasmic pH showed smaller changes than the vacuolar pH, suggesting that the former is regulated more strictly than the latter. The H⁺-ATPase inhibitor, DCCD, caused the breakdown of the mechanism that regulates the intracellular pH. H⁺-ATPase appears to have an important part in the regulation of the intracellular pH.