Cadmium Regulated Nitrate Reductase Activity in Hydrilla Verticillata (1.f.) Royle

The study was conducted to evaluate the suitability of nitrate reductase activity and the level of some metabolites as an in vivo test system for cadmium toxicity in submerged macrophyte Hydrilla verticillata. Cadmium (Cd) concentrations ranging from 0.01-80 μM affected nitrate reductase activity in a differential way. It had stimulatory effect up to 1.0 μM Cd, while higher concentrations inhibited the enzyme activity significantly. The protein synthesis inhibitor cycloheximide inhibited Cd-stimulated nitrate reductase activity during in vivo and in vitro assays. However, the effect of Cd on NR activity under in vitro assay was more pronounced. Although low Cd exposures had no effect, higher metal exposures augmented nitrate uptake. This Cd-induced NO₃ ^- uptake did not result in recovery of inhibited enzyme activity in vivo. It appears that nitrate reductase activity is more sensitive to Cd toxicity than the eventual products of nitrate assimilation such as total organic nitrogen and soluble proteins. There was a differential response of chlorophyll levels to Cd; lower concentrations enhanced the pigment level while higher ones reduced it. Cadmium exposure always enhanced the levels of carotenoids. Results showed that nitrate reductase activity could serve as an useful bioassay for Cd contamination using H. verticillata.