Reduction of nitrates in Cucumis sativus L. seedlings I. Influence of tungsten and vanadium on absorption and reduction of nitrates

Abstract

Investigations were conducted on the influence of tungsten (Na₂WO₄) and meta-vanadate (NaVO₃) on uptake and reduction of nitrates by cucumber seedlings. Tungsten and vanadium almost completely inhibited nitrate reductase activity (NR) after treating the plants for 24 hours with nitrates in the presence of 10^-4 M WO₄^-2 and 10^-3 M VO₃^-. Uptake of NO₃^- in the presence of WO₄^-2 in this same period of time was lowered by about 50 %, while in the presence of VO₃^- by seventy percent. The ratio of NO₃^- absorbed to that reducted in control plants was around 3, whereas in the presence of WO₄^-2 and VO₃^- 9 and 8 respectively. The effect of NR inhibiting activity by WO₄^-2 and VO₃^- was significant but somewhat weaker if both inhibitors were applied to the plants 24 hours after formation of the NR system, whereas NO₃^- uptake was subject to a slight lowering. The above data suggest that WO₄^-2 and VO₃^- after a 24 hour application of these inhibitors to plants, primary inhibit biosynthesis and activity of NR and to a lesser extent effect NO₃^- absorption. On the basis of results reached the Butz and Jackson (1977) hypothesis concerning the model of NO₃^- uptake and reduction by plant tissues is discussed.