EFFECT OF FORM AND LEVEL OF APPLIED NITROGEN ON NITROGENASE AND NITRATE REDUCTASE ACTIVITIES IN FABA BEANS

The effects of nitrogen applied at increasing levels of 0, 4, 8, 16 an d 32 mM N (KNO3 or NH4Cl) were studied in faba bean (Vicia faba) nodul ated by Rhizobium leguminosarum bv. viceae RCR lool. Nitrogenase activ ity was higher at 4 and 8 mM N than the zero N treatment (control), bu t 16 and 32 mM N significantly reduced the efficiency of nodule functi ons. Nitrate reductase activities (NRA) of leaves, stems, roots, nodul es and nodule fractions (bacteroid and cytosol) were increased with ri sing the NO3- or NH4+ levels. NRA decreased in the order of nodules > leaves > stems > roots. Cytosolic NR was markedly higher than that rec orded in the bacteroid fractions. Nitrate levels were linearly correla ted to NRA of nodules. Accumulation of NO2- within nodules suggests th at NO2- inhibits nodule's activity after feeding plants with NO3- or N H4+.