Nuclear magnetic resonance (NMR) spectroscopy was
used to measure the intracellular pH of living maize root
tips under nondestructive conditions (Roberts et al. 1992).
In healthy cells, the vacuolar contents are more acidic (pH
5.8) than the cytoplasm (pH 7.4). But under conditions of
extreme O2 deficiency, protons gradually leak from the vacuole
into the cytoplasm, adding to the acidity generated in
the initial burst of lactic acid fermentation. These changes
in pH (cytosolic acidosis) are associated with the onset of cell
death.