Electrochemical data on compartmentation into cell wall, cytoplasm, and vacuole of leaf cells in the CAM genusKalanchoë

Summary

Transcellular electrical profiles ofKalanchoë leaf cells were obtained by pushing a glass micro-saltbridge through cells with the tip consecutively in the cell wall, cytoplasm, and vacuole. The electrical resistance of the cell wall was too small to be detectable, that of the plasmalemma and tonoplast was about 0.18–0.21 and 0.16–0.18 Ωm², respectively. The electrical potential difference between the cytoplasm and the external medium,ψ _co , was ≈−180 mV, the potential difference between the vacuole and the medium,ψ _vo , was ≈−155mV, and thus the mean potential difference at the tonoplast,ψ _vc , was about +25 mV. Potential difference,ψ _vo , was independent of proton concentration in the external medium between pH 9 and 5.5, and behaved like an H⁺-electrode between pH 5 and 3. Depolarizations and hyperpolarizations ofψ _vo obtained by increasing and decreasing, respectively, the Na⁺-concentrations in the medium were smaller than with changing K⁺-concentrations, suggesting that permeabilities areP _Na ⁺/P _K ⁺≈-0.23. Assessment of K⁺-compartmentation by flux analysis gave K⁺-concentrations in the cytoplasm including chloroplasts (c _c) and vacuole (c _v) asc _c between 200 and 400 mmol kg⁻¹ FrWt andc _v ≈-15 mmol kg⁻¹ FrWt. The Nernst criterion suggests that metabolically regulated K⁺ transport out of the vacuoles concentrates K⁺ in the cytoplasm. Fusicoccin (10⁻⁵ m) hyperpolarizedψ _co by about 100 mV and depolarized the positiveψ _vc by about 10 mV, the latter presumably being an insignificant effect. The evidence for the existence of proton pumps exchanging H⁺ and K⁺ at the plasmalemma and at the tonoplast is discussed.