Reduction of nitrates in Cucumis sativus L. seedlings II. Influence of tungsten and vanadium on nitrate reductase and adenosine triphosphatase activities

Abstract

ATPases isolated from the roots of cucumber seedlings activated by Mg⁺² ions in experiments in vitro, were fairly distinctly inhibited by Ca^-2 ions, very slightly inhibited by fluorides and molybdenum ions while NO₃^- anions had no effect on the level of ATPase activity studied. Introduction into the nutrient of 10^-4 M Na₂WO₄ or 10^-3 M Na VO₃ (inhibitors of nitrate reductase NR) distinctly inhibited activity of the ATPase under study especially of fractions IIa and III, and inhibited NR activity and lowered uptake of NO₃^-. WO₄^-2 and VO₃ inhibited to the same extent absorption and reduction of NO₃^- in the initial phase of NR induction, whereas at a later stage both inhibitors checked reduction to a greater degree than uptake of NO₃^-. The results indicate the possibility of certain ATPase participation in assimilating nitrates, and suggest that in the initial stage of biosynthesis of the NR enzyme system, activity of the enzyme is distinctly dependent upon NO₃^- transport and the level of NR activity limited by the amount of nitrate taken up. At a later an additional mechanism of NO₃^- transport probably functions, not connected with simultaneous reduction of nitrates. On the basis of results the Butz and Jackson (1977) hypothesis concerning a model for the absorption and reduction of NO₃^- by plant tissues is discussed.