Nitrate and nitrite reduction in the plant fraction of alfalfa root nodules

The plant fraction of alfalfa (Medicago sativa L. cv. Aragon) nodules contained both nitrate reductase (NR) and nitrite reductase (NiR). Specific activity of NADH‐NR from the cytosol of nodules not treated with NO3‐ was about 30 nmol (mg protein)‐1‐h‐1 and was not basically affected by NO3 addition. In contrast, typical specific activity for cytosolic NiR was 1.5 umol (mg protein)‐1h‐1 using methyl viologen as electron donor. This activity strongly increased with NO3 concentration, probably due to substrate induction. Maximal activity was 3.5 μmol (mg protein)‐1h‐1 at 50 to 200 mM NO3. Estimates indicate that the contribution of cytosol to the overall NR and NiR activities of alfalfa nodules is distinctly different: less than 10% and about 70%, respectively. The increasing amounts of NO2 accumulating in the cytosol upon NO3, supply, and the different response to NO3 of bacteroid and cytosolic NRs support the concept that most of this NO2 comes from the bacteroids.