Proton Transport and pH Control inSinapis albaRoot Hairs: A Study carried out with Double-Barrelled pH Micro-electrodes

Continuous measurements of cytoplasmic pH (pH_c) in Sinapis root hairs have been carried out with double-barrelled pH-micro-electrodes in order to gain information on translocation of protons across the plasmalemma and cytoplasmic pH control. (i) The cytoplasmic pH of Sinapis (7–33 ± 0–12, standard conditions) changes no more than 0.1 pH_c, per pH_o-unit, regardless of whether cyanide is present or not. (ii) Weak acids rapidly acidify pH_c and hyperpolarize, while weak bases alkalize pH_c and depolarize the cells, (iii) 1.0 mol M,³ NaCN acidifies the cytoplasm by 0.4 to 0.7 pH-units, but alkalizes the vacuole. (iv) 20 mmol m⁻³ CCCP has no significant effect on pH_c, if added at pH 9.6 or 7.2, but acidifies pH_c by 1.3 units at pH 4.3. In the presence of CCCP, cyanide acidifies the cytoplasm, (v) Chloride transiently acidifies pH_c, while K⁺, Na⁺, and $NO3-$ have no significant effects, (vi) Cytoplasmic buffer capacity forms a bell-shaped curve versus pH_c with an optimum of about 50 mol m⁻³ H⁺pH_c-unit. The modes of proton re-entry and the effects of active and passive proton 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 to pH_c. The proton export pump, although itself reacting to changes in pH_c, influences pH_c only to a minor extent. It is concluded that buffer capacity and membrane transport play moderate roles in pH_c control in Sinapis, while the interlocked H⁺-producing and -consuming reactions of cellular metabolism are the main regulating factors. This makes pH control in Sinapis quite different from bacterial and animal cells.