Introduction

The pH of an intracellular compartment (pHi) depends on its buffering capacity, i.e. the concentration of weak acids, and on its ionic composition (Stewart, 1983). These properties are determined by metabolism and ion transport and since many biochemical and biophysical processes are sensitive to pH, it is reasonable to assume that any tendency for the pH to drift away from some notionally optimal or normal value will be corrected by altering the balance between the proton-consuming and proton-generating processes within the cell. In fact, Raven (1986) showed that pH regulation is an essential requirement for the growth of all plant cells; and, more generally, the ability to maintain electrochemical potential differences for the proton across membranes (ΔμH+), with the underlying contribution from any difference in pH, is a fundamental requirement of cellular energetics (Nicholls & Ferguson, 1992).

Intracellular pH values in plant cells can be measured by several methods (Guern et al., 1991), and most of the recent work has been done using microelectrodes, fluorescent probes and nuclear magnetic resonance (NMR) spectroscopy. Microelectrodes allow fast, real-time monitoring of single cells, as well as the simultaneous measurement of the functionally related membrane potential (Felle & Bertl, 1986; Felle, 1987, 1993); fluorescent probes allow non-invasive measurements of subcellular pH values (Kosegarten et al., 1997) and, in conjunction with laser scanning confocal microscopy, allow the construction of pH maps within cells and tissues (Gibbon & Kropf, 1994); and NMR provides a range of methods for measuring cytoplasmic and vacuolar pH values, while simultaneously recording other metabolically important information (Ratcliffe, 1994).