Tonoplast action potential inNitella in relation to vacuolar chloride concentration

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Abstract
The action potential ofNitella internode was studied in relation to K+ and Cl− concentrations in the vacuole. When the vacuole ofNitella pulchella was filled with an artificial solution with extremely low Cl− concentration, a diphasic action potential (DAP) was observed. The first phase consists of a rapid depolarization followed by a relatively rapid repolarization, and the second one consists of a strong hyperpolarization followed by a gradual return to the resting potential. When the cell was stimulated immediately after the generation ofDAP, a monophasic action potential which resembles an action potential of the natural cell was observed, indicating that theDAP consists of two components with different refractory periods. The refractory period of the component responsible for the depolarizing phase is shorter than that of a component responsible for the hyperpolarizing phase. Measuring the plasmalemma potential and vacuolar potential separately, it was demonstrated that the hyperpolarizing component ofDAP originates from the tonoplast. The action potential of the tonoplast, in contrast with that of the plasmalemma, could be generated independently of concentration of K+ in the vacuole. Since the maximum amplitude of hyperpolarization decreased significantly by increasing Cl− concentration of the vacuole, it is concluded that the tonoplast is very sensitive to Cl− during excitation.