Light and nitrate induction of nitrate reductase in kinetin- and gibberellic acid-treated mustard cotyledons

Nitrate reductase activity in gibberellic acid and kinetin treated mustard (Brassica juncea 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 ac tivities, assayed by either in vivo or in vitro techniques, whereas, gibber ellic acid was almost ineffective. In the absence of both light and NO3, ho wever, 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 K-m values for NO3 was 0.2 mM. Actinomycin D, cycloheximide and tungstate were equally effective in suppre ssing 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.