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

Abstract

Mustard (Brassica juncea 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 NO3su−-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 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, 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.