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Articles

Electrical Properties of the Plasmalemma and Tonoplast in Valonia ventricosa^1,2

Department of Botany, Rutgers University, Newark, New Jersey 01702

Studies were made on the electric potentials of the plasmalemma (E_co) and tonoplast (E_vc) in small cells (1-3 mm diameter) of Valonia ventricosa. To measure E_co, microelectrodes with long tapers were inserted into the vacuole with the path of electrode entry off-center. The microelectrode then was pushed across the vacuole and into the cytoplasm on the opposite side of the cell. A reference electrode was placed in the artificial seawater bathing the cell. A similar method was used to measure E_vc except that the reference electrode was placed in the vacuole.

Both E_co and E_vc were influenced by light. In the light, E_co was –70 millivolts and it changed to –60 millivolts in the dark (cytoplasm-negative to outside). For E_vc, the potentials were +86 millivolts in the light and +69 millivolts in the dark (vacuole-positive to cytoplasm). The vacuole potential (E_vo) was demonstrated to be the algebraic sum of E_co and E_vc. For example, in the light, the sum of the means (±SE) for E_co (= –70 ± 1) and E_vc (= +86 ± 5) is +16 millivolts, which is comparable to the measured E_vo of +17 ± 2 millivolts. In the dark, the sum of E_co (= –60 ± 3) and E_vc (+69 ± 6) is +9 millivolts and the measured value of E_vo is +9 ± 4 millivolts.

The external K⁺ concentration had a controlling effect on both E_co and the direct current resistance of the plasmalemma, which suggests that E_co is largely a K⁺ diffusion potential. The tonoplast electrical properties were affected only slightly by external K⁺.

The data presented are indicative of a K⁺ electrogenic influx pump in the tonoplast. It is also considered possible that H⁺ might be electrogenically pumped from the cytoplasm both into the vacuole and to the cell exterior.

² This paper is presented with affection and respect to the memory of Professor Noe Higinbotham.