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

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Abstract
Effects of removal of external Ca2+ on the cytoplasmic pH (pHc) 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 external concentration of K+. Removal of Ca2+ always decreased pHc when pH0 was below about 6.0; the decrease was about 0.2–0.4 units 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 or no effect on pHc. This situation was not altered by changing the concentration of K+, though in some experiments at pH0 5.0–5.2 there 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 apparently reversed when K+ was higher (5.0 or 10.0 mol m−3). The results suggest that H+ transport continues in the absence of external Ca2+, despite previous suggestions to the contrary, and that the H+ pump does not necessarily run near thermodynamic equilibrium with its chemical driving reaction. They indicate, rather, that the H+ pump is under kinetic control and provide further evidence for the inadequacy of present models for the operation of the H+ pump in charophyte cells, especially in relation to its proposed role in regulating pHc.

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