The apoplastic pH of the Zea mays root cortex as measured with pH-sensitive microelectrodes:: aspects of regulation

In the root cortex of Zea mays the apoplastic pH and aspects of its regulation were investigated using pH-sensitive microelectrodes. To measure the pH directly in different cell layers of the apoplast sharp double-barrelled electrodes were applied, whereas blunt pH-electrodes were used simultaneously to measure the pH at the root surface. Recordings carried out 8-10 mm behind the root tip show that the apoplastic pH is maintained between 5.1 and 5.6, depending on the given experimental conditions, i.e. varying external [K+], [Ca2+], pH, weak buffering, as well as perfusion of the test medium. When the medium pH (bulk) differs considerably from the apoplastic pH, a small pH gradient is built up between the root surface (unstirred layer) and the outer cortex layers. In a standing medium these gradients equilibrate. The apoplastic pH responds to increases in external [K+] and [Ca2+] with an acidification, which is attributed to ion-exchange properties of the cell wall constituents. Stimulation of proton pump activity with fusicoccin acidifies the apoplast from pH 5.6 to pH 4.8, while deactivation of the pump with cyanide/salicylhydroxamic acid increases the pH of the apoplast from 5.6 to 6.2, and further to pH 6.6 with CCCP. The Ca2+ channel antagonists nifedipine and La3+ also increase the apoplastic pH. It is suggested that not only the proton pump, but also the cation channels may contribute to the regulation of the apoplastic pH.