INVOLVEMENT OF ACTIVATED OXYGEN IN NITRATE-INDUCED SENESCENCE OF PEA ROOT MODULES

The effect of short-term nitrate application (10 mM, 0-4 d) on nitroge nase (N(2)ase) activity, antioxidant defenses, and related parameters was investigated in pea (Pisum sativum L. cv Frilene) nodules. The res ponse of nodules to nitrate comprised two stages. In the first stage ( 0-2 d), there were major decreases in N(2)ase activity and N(2)ase-lin ked respiration and concomitant increases in carbon cost of N(2)ase an d oxygen diffusion resistance of nodules. There was no apparent oxidat ive damage, and the decline in N(2)ase activity was, to a certain exte nt, reversible. The second stage (>2 d) was typical of a senescent, es sentially irreversible process. It was characterized by moderate incre ases in oxidized proteins and catalytic Fe and by major decreases in a ntioxidant enzymes and metabolites. The restriction in oxygen supply t o bacteroids may explain the initial decline in N(2)ase activity. The decrease in antioxidant protection is not involved in this process and is not specifically caused by nitrate, since it also occurs with drou ght stress. However, comparison of nitrate- and drought-induced senesc ence shows an important difference: there is no lipid degradation or l ipid peroxide accumulation with nitrate, indicating that lipid peroxid ation is not necessarily involved in nodule senescence.