The effects of nitrogen applied at increasing levels of 0, 4, 8, 16 and 32 mM N
(KNO 3 or NHaCI ) were studied in faba bean (Vicia faba) nodulated by Rhizobium
leguminosarum bv. viceae RCR 1ool. Nitrogenase activity was higher at 4 and 8 mM
N than the zero N treatment (control), but 16 and 32 mM N significantly reduced the
efficiency of nodule functions. Nitrate reductase activities (NRA) of leaves, stems,
roots, nodules and nodule fractions (bacteroid and cytosol) were increased with
rising the NO 3" or NH4 + levels. NRA decreased in the order of nodules > leaves >
stems > roots. Cytosolic NR was markedly higher than that recorded in the bacteroid
fractions. Nitrate levels were linearly correlated to NRA of nodules. Accumulation of
NO 2" within nodules suggests that NO 2 inhibits nodule's activity after feeding plants
with NO 3" or NH4 §