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 re gulation were investigated using pH-sensitive microelectrodes. To meas ure the pH directly in different cell layers of the apoplast sharp dou ble-barrelled electrodes were applied, whereas blunt pH-electrodes wer e used simultaneously to measure the pH at the root surface. Recording s 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, a s well as perfusion of the test medium. When the medium pH (bulk) diff ers considerably from the apoplastic pH, a small pH gradient is built up between the root surface (unstirred layer) and the outer cortex lay ers. In a standing medium these gradients equilibrate. The apoplastic pH responds to increases in external [K+] and [Ca2+] with an acidifica tion, which is attributed to ion-exchange properties of the cell wall constituents. Stimulation of proton pump activity with fusicoccin acid ifies the apoplast from pH 5.6 to pH 4.8, while deactivation of the pu mp with cyanide/salicylhydroxamic acid increases the pH of the apoplas t from 5.6 to 6.2, and further to pH 6.6 with CCCP. The Ca2+ channel a ntagonists 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.