Cytokinin effects on nitrate reductase activity in Leucaena leucocephala Seedlings in the presence of different nitrogenous salts

Excised root and cotyledon segments from Leucaena leucocephala L. seedlings were incubated in ½ strength Hoagland's solution containing KNO₃, NH₄Cl or NH₄NO₃ (10 mM N) as nitrogen source and 1 to 100 μM benzyladenine or kinetin, for 24 h. The effect of cytokinins on in vivo nitrate reductase activity ranged from inhibition to an increase, depending upon the plant organ, the nitrogenous salt and the concentration of the cytokinin. The supply of kinetin reduced metabolic nitrate pool in the cotyledons although it had no effect in the roots at 1 to 50 μM and increased nitrate pool by 67% at 100 μM. The optimum pH for enzyme activity in roots induced by 50 μM kinetin was 7.5, which was similar to that induced by nitrate. It is suggested that kinetin induces nitrate reductase activity in Leucaena, independent of nitrate, although the enzyme is similar to that induced by nitrate, in some of its properties.