Nitrate reductase activity of radish cotyledons as affected by phytohormones and different nitrogen sources

Radish (Raphanus sativus L.) seedlings pretreated with different hormones viz. kinetin, gibberellic acid and abscisic acid were subjected to different N-forms. The seedlings were treated with different concentrations of KNO3, NH4Cl and NH4NO3 and the changes in nitrate reductase activity were seen in light and dark conditions in the cotyledons. Nitrate reductase activity was affected differently by hormone application. Nitrate increased and ammonia decreased nitrate reductase activity; in both light and dark-grown seedlings KNO3 induced more in vitro nitrate reductase activity. NH4+ when combined with NO3-, however, could level up to some extent, with KNO3 in light, except in kinetin. A transient response of induction of NR activity was evident with decreased levels after a certain specific ambient N-concentration, despite the presence of high N in the medium. However, the pattern of transition varied with the hormones applied. Further, hormones are found to affect induction of different isoforms of nitrate reductase by NH4+ and NO3-. NH4+ induced isoform was prominently promoted by kinetin treatment in dark. The data documents a particular kind of interaction between controlling factors (light, N-source and phytohormones) which affect nitrate reductase levels.