Regulation of the Cytoplasmic pH of Chara corallina in the Absence of External Ca2+: Its Significance in Relation to the Activity and Control of the H+ Pump

Effects of removal of external Ca²⁺ on the cytoplasmic pH (pH_c)of Chara corallina have been measured with the weak acid 5,5-dimethyl-oxazolidine-2,4-dione(DMO) as a function of external pH (pH₀) and of the externalconcentration of K⁺. Removal of Ca²⁺ always decreased pH_c whenpH₀ was below about 6.0; the decrease was about 0.2–0.4units at pH₀ 5.0, increasing to about 0.5 units at pH₀ 4.3.When pH₀ was 6.0 or higher the removal of Ca²⁺ had little orno effect on pH_c. This situation was not altered by changingthe concentration of K⁺, though in some experiments at pH₀ 5.0–5.2there was a slight decrease in pH₀ (about 0.2 units) when K⁺was increased from 0.2 to 2.0 mol m^–3, an effect apparentlyreversed when K⁺ was higher (5.0 or 10.0 mol m^–3). Theresults suggest that H⁺ transport continues in the absence ofexternal Ca²⁺, despite previous suggestions to the contrary,and that the H⁺ pump does not necessarily run near thermodynamicequilibrium with its chemical driving reaction. They indicate,rather, that the H⁺ pump is under kinetic control and providefurther evidence for the inadequacy of present models for theoperation of the H⁺ pump in charophyte cells, especially inrelation to its proposed role in regulating pH_c. Key words: Chara corallina, Cytoplasmic pH, Calcium