31P NMR measurements of cytoplasmic ph changes in maize root tips

31P nuclear magnetic resonance (NMR) spectroscopy has been used to measure the changes caused by sodium azide and anaerobiosis in the cytoplasmic pH of maize (Zea mays L.) root tips. Measurements of H+influx, lactate accumulation and ATP hydrolysis, together with estimates of the cytoplasmic volume and the cytoplasmic buffering capacity, were used to calculate the expected pH changes under the same conditions. In the case of azide inhibition, where there was a large H+influx, there was good agreement between the calculated and measured pH changes. In contrast during anoxia, where the accumulation of lactate was the most important source of protons, the predicted cytoplasmic pH change was greater than the observed change. It was concluded that in the absence of a compensating H+efflux, the excess H+production was offset by the production of H+-consuming metabolites such as γ-aminobutyrate. © 1985 Oxford University Press.