PROTECTIVE ENZYMES AGAINST ACTIVE OXYGEN SPECIES DURING NITRATE-INDUCED SENESCENCE OF LUPINUS-ALBUS NODULES

Nitrate application to nodulated legumes inhibits nitrogenase activity and causes the premature senescence of nodules. In this work the activity of antioxidant enzymes and the production of toxic oxygen species during nitrate-induced senescence of Lupinus albus L. cv. Multolupa nodules was investigated. Four-six days of nitrate treatment induced a decrease in the activity of the main enzymes that scavenge H2O2 in the nodule cytosol, catalase and ascorbate peroxidase. Total peroxidase activity increases in response to nitrate treatment, but H2O2 eventually builds up in nodule cytosol. It is suggested that this H2O2 causes the first damage to leghemoglobin apoprotein, which results in a decreased in situ immunolabeling of this hemoprotein. Degradation of Lb continues with the rupture of the heme group after 8 days of nitrate treatment. This may result in the release of catalytic iron, which in turn may promote the formation of hydroxyl radicals from the precursors H2O2 and O-2(-), or from H2O2 through a Fenton-type reaction.