Abstract: The effect of cytokinin on the formation of NR activity were studied with tobacco callus tissues and wheat seedlings. Cytokinin could not induce the NR activity alone but could enhance the NR inducibility (Table 1). The enhancement of NR formation was detected in the tissues pretreated with cytokinin for over 12 hours. It showed that there was a precondition in the tissues for the induction of NR (Fig. 3). The precondition could not be improved by cytokinin when cycloheximide (inhibitor of protein synthesis) was added into the medium during cytokinin pretreatment (Table 2). Thus, it was thought that cytokinin might enhance synthesis of a protein which participated in the NR activity induction. In immunological test (Fig. 5) the existence of a nonactive apoenzyme of NR in higher plant tissues was demonstrated. It is, therefore, suggested that there are two major steps in the NR activity formation: (l) the synthesis of a nonactive NR apoenzyme, (2) the activation of this nonactive apoenzyme. The former step might be stimulated by cytokinin and the latter was mediated by nitrate.

Key words: Cytokinin, Nitrate reductase, Tobacco callus, Wheat seedlings