ABSTRACT
The pH values in and around roots are frequently measured, and the expected values are well known. Thus soils vary in pH from < 3 to > 9 (Marschner, 1995); the root apoplast (Grignon and Sentenac, 1991; Yu et al., 2000) and the root vacuoles (Guern et al., 1991) are typically mildly acidic; and the pH of the root cytoplasm is usually in the range 7.27.6 (Guern et al., 1991). Our knowledge of these pH values, their regulation, and the physiological significance of the changes observed under normal and adverse conditions depends on the availability of techniques for measuring pH in and around functioning roots. Microelectrodes, fluorescence probes, and nuclear magnetic resonance (NMR) spectroscopy are the principal techniques, and this review of root pH regulation begins with a brief survey of the present status of these methods in root research. All three approaches yield values that ultimately depend on a calibration procedure, and although this inevitably becomes less certain for intracellular measurements, there is reasonable agreement between the pH values obtained with different techniques when direct comparison is possible (Gibbon and Kropf, 1994).
