Cytoplasmic pH of Root Hair Cells ofSinapis albaRecorded by a pH-sensitive Micro-electrode. Does Fusicoccin Stimulate the Proton Pump by Cytoplasmic Acidification?

Abstract

Bertl, A. and Felle, H. 1985. Cytoplasmic pH of root hair cells of Sinapsis alba recorded by a pH-sensitive micro-electrode. Does fusicoccin stimulate the proton pump by cytoplasmic acidification? —J. exp. Bot. 36: 1142–1149. pH-sensitive micro-electrodes, filled with ion-exchanger resin have been fabricated with a turgorinsensitive tip and have been applied to test the intracellular pH and changes thereof in root hair cells of Sinapis alba. (1) The cytoplasmic pH of Sinapis root hairs was determined to be 7.3 ±0.2 (at neutral external pH). (2) 10 mol m⁻³ sodium azide depolarizes the membrane potential by about 100 mV and acidifies the cytoplasm by 0.8 pH-units. (3) The change from 1.0 mol m⁻³ to 10 mol m⁻³ external potassium causes a depolarization of about 45 mV, but no change in internal pH. (4) At an external pH of 5.0, sodium acetate hyperpolarizes the plasmalemma by about 60 mV and acidifies the cytoplasmic pH by 0.2 to 0.3 units. (5) 2.0 mmol m⁻³ fusicoccin (FC) hyperpolarizes the plasmalemma by 20–25 mV, acidifies the cytoplasm by 0.1 to 0.2 pH-units, and acidifies the external medium by about 0–3 pH-units. It is concluded that cytoplasmic acidification stimulates the electrogenic proton pump in Sinapis root hairs, and it is suggested that the FC-induced effects, viz. hyperpolarization and external acidification, can also be interpreted in this way.