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

Radish (Raphanus sativus L.) seedlings pretreated with different hormones v iz. 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 i n light and dark conditions in the cotyledons. Nitrate reductase activity w as affected differently by hormone application. Nitrate increased and ammon ia decreased nitrate reductase activity; in both light and dark-grown seedl ings KNO3 induced more in vitro nitrate reductase activity. NH4+ when combi ned 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 evi dent with decreased levels after a certain specific ambient N-concentration , despite the presence of high N in the medium. However, the pattern of tra nsition varied with the hormones applied. Further, hormones are found to af fect 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 f actors (light, N-source and phytohormones) which affect nitrate reductase l evels.