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

Abstract

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 ΔpH 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.