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Alkaline Band Formation in Chara corallina

Due to OH^– Efflux or H⁺ Influx?

¹ Department of Botany, University of California, Davis, California 95616

The nature of the transport system responsible for the establishment of alkaline bands on cells of Chara corallina was investigated. The transport process was found to be insensitive to external pH, provided the value was above a certain threshold. At this threshold (pH 5.1 to 4.8) the transport process was inactivated. Transport function could be recovered by raising the pH value of the external solution. The fastest rate of recovery was always obtained in the presence of exogenous HCO₃^–.

Experiments in which plasmalemma integrity was modified using 10 millimolar K⁺ treatment were also performed. Alkaline band transport was significantly reduced in the presence of 10 millimolar K⁺, but the system did not recover, following return to 0.2 millimolar K⁺ solutions, until the transport site was reexposed to exogenous HCO₃^–.

The influence of presence and absence of various cations on both alkaline band transport and total H¹⁴CO₃^– assimilation was examined. No specific cation requirement (mono- or divalent) was found for either process, except the previously established role of Ca²⁺ at the HCO₃^– transport site. The alkaline band transport process exhibited a general requirement for cations. This transport system could be partially or completely stalled in low cation solutions, or glass-distilled water, respectively. The results indicate that no cationic flux occurs across the plasmalemma in direct association with either the alkaline band or HCO₃^– transport systems.

It is felt that the present results offer support for the hypothesis that an OH^– efflux transport system (rather than a H⁺ influx system) is responsible for alkaline band development in C. corallina. The results support the hypothesis that OH^– efflux is an electrogenic process. This OH^– transport system also appears to contain two allosteric effector sites, involving an acidic group and a HCO₃^– ion.