Abstract: | Continuous measurements of cytoplasmic pH (pHc) in Sinapis roothairs have been carried out with double-barrelled pH-micro-electrodesin order to gain information on translocation of protons acrossthe plasmalemma and cytoplasmic pH control. (i) The cytoplasmicpH of Sinapis (733 ? 012, standard conditions)changes no more than 0.1 pHc, per pHo-unit, regardless of whethercyanide is present or not. (ii) Weak acids rapidly acidify pHcand hyperpolarize, while weak bases alkalize pHc and depolarizethe cells, (iii) 1.0 mol M,3 NaCN acidifies the cytoplasm by0.4 to 0.7 pH-units, but alkalizes the vacuole. (iv) 20 mmolm3 CCCP has no significant effect on pHc, if added atpH 9.6 or 7.2, but acidifies pHc by 1.3 units at pH 4.3. Inthe presence of CCCP, cyanide acidifies the cytoplasm, (v) Chloridetransiently acidifies pHc, while K+, Na+, and have no significant effects, (vi) Cytoplasmic buffer capacityforms a bell-shaped curve versus pHc with an optimum of about50 mol m3 H+pHc-unit. The modes of proton re-entry and the effects of active and passiveproton transport on cellular pH control are critically discussed.It is suggested that the proton leak, consisting of H+-cotransport(e.g. H+/Cl) rather than H+-uniport, is no threat topHc. The proton export pump, although itself reacting to changesin pHc, influences pHc only to a minor extent. It is concludedthat buffer capacity and membrane transport play moderate rolesin pHc control in Sinapis, while the interlocked H+-producingand -consuming reactions of cellular metabolism are the mainregulating factors. This makes pH control in Sinapis quite differentfrom bacterial and animal cells. Key words: Cytoplasmic pH, double-barrelled pH micro-electrode, pH control, proton transport, Sinapis |