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Plant and Cell Physiology, 1984, Vol. 25, No. 6 1009-1016
© 1984

Article

Membrane Potential and Resistance in Relation to Cytoplasmic pH in Nitellopsis

Tetsuro Mimura and Masashi Tazawa

Department of Botany, Faculty of Science, University of Tokyo Hongo, Tokyo 113, Japan

The responses of membrane potential and membrane resistance of tonoplast-free Nitellopsis cells to step changes of internal pH (pHi) from 7 were studied during continuous perfusion with media containing either 1 mu ATP or no ATP. Whether ATP was present or not, the time course of Em responses was composed of an initial rapid change (initial phase) and a subsequent slow change (second phase). At the end of the second phase, Em attained nearly stable values. When Em values of ATP-containing cells obtained at the peak of the initial phase were plotted against pHi, Em was found to hyperpolarize most at pHi 6.5. This was also found for steady Em values measured at the end of the second phase. The Em values of ATP-lacking cells were almost insensitive to pHi changes between 4 and 8, but more positive than those of ATP-containing cells at pHi 4–7.5. Above pHi 8, no difference in Em was observed between the two types of cells. In this range of pHi, the Em change in the initial phase amounted to about 60 mV per unit of pHi change.

The light-induced hyperpolarization still occurred at pHi 6.5 where the electrogenic potential was maximal, and over the wide pHi range, from 6.0 to 7.5 even when pHi was strongly buffered. Thus, we concluded that the pHi change may not be the cause of light-induced hyperpolarization.

(Received December 5, 1983; Accepted June 18, 1984)
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