Nitrate and hydrogen peroxide metabolism in Medicago sativa nodules and possible effect on leghaemoglobin function

The effect of NO3 (0–20 mM for 7 days) upon NO3 and H2O2 metabolism in lucerne (medicago sativa L. ev. Aragón) nodules initiated by Rhizobium meliloti strain 102F51 has been examines. Ty;pical nitrate reductase, (NR) activities of bacteroids (EC 1.7.99.4) and cytosol (EC 1.6.6.1) of nodules not treated with NO3 were 60 and 45 nmol NO2 formed (mg protein)1h−1 respectively, Inductin of bacteroid NR took place in nodules exposed ot concentrations above 5 mM No3 whereas cytosol NR was induced at 5 mM No3 decreasint at greater NO3 concentrations. In resonse to NO3 additin, NO2 increasingly accumulated in the nodule cytosol at quantities commensurate with those needed to oxidise leghaemoglobin (Lb) in vitro. A comparison of patterns of NO2 accumulation and activities of NRs expressed on a nodule weight basis indicates that plant NR contributes decisively to NO2 production at the earlier phass of nodule senescence (5–10mM No3 while bacteroid NR becomes increasingly important in generating NO2 at nore advanced stages (10–20mM NO3). Specific superoxide dismutase (SOD; EC 1.15.1.1) and catalase (EC 1.11.1.6) activities of bacteroids remained constant during the NO3 induced senescence of nodules whereas SOD activity of cytosol increased 1.5‐fold and catalase activity ws inhibited by 20% at 20 mM NO3 substantial peroxidase (EC 1.11.1.7) activity was found in the plant but none in the bacteroid fraction of nodules. Peroxidase activity increased significantly only at 20 mM NO3 concomitantly with malondialdephyde content. concentrations. Free H2O2 interferes wihjt Lb function in vivo is suggested.