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 acti vity of antioxidant enzymes and the production of toxic oxygen species during nitrate-induced senescence of Lupinus albus L. cv. Multolupa n odules 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 a ctivity increases in response to nitrate treatment, but H2O2 eventuall y builds up in nodule cytosol. It is suggested that this H2O2 causes t he first damage to leghemoglobin apoprotein, which results in a decrea sed in situ immunolabeling of this hemoprotein. Degradation of Lb cont inues with the rupture of the heme group after 8 days of nitrate treat ment. This may result in the release of catalytic iron, which in turn may promote the formation of hydroxyl radicals from the precursors H2O 2 and O-2(-), or from H2O2 through a Fenton-type reaction.