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

Abstract

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 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 K_m 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.