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

Abstract

In the root cortex of Zea mays the apoplastic pH and aspects of its regulation were investigated using pHsensitive microelectrodes. To measure the pH directly in different cell layers of the apoplast sharp doublebarrelled electrodes were applied, whereas blunt pHelectrodes 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.