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Studies on H⁺-Translocating ATPases in Plants of Varying Resistance to Salinity ¹

II. K⁺ Strongly Promotes Development of Membrane Potential in Vesicles from Cotton Roots

Department of Botany, The Hebrew University of Jerusalem, Jerusalem, 91904, Israel

Mg²⁺-ATP-dependent H⁺-translocation has been studied in membrane vesicles derived from the roots of Gossypium hirsutum L. var. Acala San Jose 2. Establishment of a positive membrane potential was followed by measuring SCN^– accumulation; establishment of pH across the vesicle membranes by measuring quinacrine fluorescence quenching. High specificity for ATP was shown, and H⁺-translocation was oligomycin stable. The pH profile for H⁺-translocation showed an optimum at 5.5. The relationship between SCN^– accumulation and ATP concentration was approximately Michaelian; the apparent K_m was 0.7 millimolar. K-2-(N-morpholino)ethanesulfonic acid strongly promoted ATP-dependent SCN^– uptake (up to 180% stimulation). The effect was not given by Na-Mes. Carbonyl cyanide p-trifluoromethoxyphenylhydrazone totally inhibited SCN^– accumulation, both in the presence and absence of K-2(N-morpholino)ethanesulfonic acid. Vanadate at 200 micromolar inhibited SCN^– uptake by about 10 to 40% in the absence of K⁺, but more strongly in its presence (about 60%). NO₃^– at 100 millimolar inhibited initial rate of quinacrine quenching by about 25%. The NO₃^– insensitive fraction was activated by K⁺; and inhibited by 200 micromolar vanadate to about 40%, provided K⁺ was present. Saline conditions during the growth of the plants had no appreciable effect on the observed characteristics of H⁺-translocation.