Effect of phytohormones and tungsten on light-induced and nitrate-induced nitrate reductase activity of mustard cotyledons

Mustard (Brassica juncea Coss cv. T-59 'Varuna') seedlings pretreated with gibberellic acid (GA) and kinetin (KiN) were grown in light. In vivo nitrat e reductase (NR) activity was estimated and effect of tungsten on light-ind uced and NO3--induced NR activity was investigated. Different concentration s of GA did not show any effect on induction of light-induced NR; addition of nitrate promoted in vivo NR activity but no concentration effect of GA w as evident. Light-induced NR was promoted by KiN and like in GA treatment, addition of nitrate increased NR activity. Addition of Na-tungstate inhibit ed NO3- induced NR while light-induced NR was not much affected in both GA and KiN treated seedlings. The two forms of NRs were further characterized by studying the decay kinetics using Na-tungstate. In light-induced NR, tun gstate did not affect NR activity up to 11 h, while at later periods, a sli ght decay was observed. On the other hand, NO3--induced NR activity increas ed up to 4 h and subsequently a rapid fall was observed. It was therefore a pparent that light-induced NR had a very low turnover rate as compared to N O3--induced NR. These results further support the earlier conclusion that i n mustard seedlings two distinct types of NR enzyme exist and that nitrate requirement for NR induction is not absolute.