NITRATE REDUCTION IN BRADYRHIZOBIUM SP (LUPINUS) STRAINS AND ITS EFFECTS ON THEIR SYMBIOSIS WITH LUPINUS-LUTEUS

Three Bradyrhizobium sp. (Lupinus) strains (750, IM 43B and UPM 861), expressing hydrogenase uptake activity (EC 1.18.99.1), were inoculated on Lupinus luteus seedlings planted in Leonard jars. Six different nitrate levels (0,1, 2, 4, 8 and 12 mM N) were kept for 6 weeks, when combined N was removed and a fresh N-free solution was supplied to all seedlings for 3 more weeks. Hydrogen uptake activities decreased as a result of growth in nitrate; only one strain (IM 43B) held similar values after N withdrawal. Each strain displayed a different type of nitrate reductase activity in its bacteroids; whereas 750 showed a constitutive NR (EC 1.6.6.2), NR was induced (EC 1.6.6.1) in IM 43B by the presence of nitrates, and there was no detectable activity in UPM 861. The increasing N levels produced quite dissimilar nodulation patterns, being much reduced with strain 861, while nodulation formed by 750 resisted high rates of nitrate. Likewise, the latter strain gave rise to the largest nitrogenase activities (EC 1.18.6.1) over 8 mM N. Nitrogen fixation recovered completely after removal of all nitrates. Rates of hydrogen evolved from the nodules were more depleted than the C2H2 reduced by increasing N contents in the medium; therefore, the Relative Efficiencies improved. The nodules possessing a NR in their bacteroids correlated with the best N and NO3- contents in the shoots. H2 release by the nodules and concentrations of nitrate in the plants were the variables most significantly affected by the two factors studied (strain and nitrate). The presence and type of NR and Hup activities seemed to influence the modulation, nitrogen fixation and nutrition of lupine plants. © 1990, Gustav Fischer Verlag, Stuttgart. All rights reserved.