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Nitrate reductase activity in gibberellic acid and kinetin treated mustard (Brassicajuncea Coss. cv. T-59 'Varuna') seedlings, grown in the presence or absence of light and/or NO3 was investigated. While both light and NO3, alone could induce NR activity, their combination showed additive effects. Kinetin treatment significantly promoted both light-and NO3-induced NR activities, assayed by either in vivo or in vitro techniques, whereas, gibberellic acid was almost ineffective. In the absence of both light and NO3, however, phytohormones alone could not induce NR activity. Both light-induced and NO3 induced NR fractions had a pH optima of 7.5, preferred NADH as an electron donor (NADH : NADPH ratio 2.5) and Km values for NO3 was 0.2 mM. Actinomycin D, cycloheximide and tungstate were equally effective in suppressing the development of NR activity after exposure to light or NO3. These results indicate that two independent NR fractions operate, with apparently identical properties but separate control mechanisms.List of abbreviations: GA = gibberellic acid; KN = kinetin; NR = nitrate reductase

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Mustard (Brassica flmcea Coss cv. T-59 'Varuna') seedlings pretreated with gibberellic acid (GA) and kinetin (KIN) were grown in light. In vivo nitrate reductase (NR) activity was estimated and effect of tungsten on light-induced and NO3-induced NR activity was investigated. Different concentrations 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 was evident. Light-induced NR was promoted by KiN and like in GA treatment, addition of nitrate increased NR activity. Addition of Na-tungstate inhibited NO3-induced NR while tight-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, tungstate did not affect NR activity up to 11 h, while at later periods, a slight decay was observed. On the other hand, NO3--induced NR activity increased up to 4 h and subsequently a rapid fall was observed. It was therefore apparent that light-induced NR had a very low turnover rate as compared to NO3--induced NR. These results further support the earlier conclusion that in mustard seedlings two distinct types of NR enzyme exist and that nitrate requirement for NR induction is not absolute.List of abbreviations: GA = gibberellic acid; KiN = kinetin; NR = nitrate reductase