EFFECT OF NITRATE ON NODULATION AND N-2-FIXATION OF LUPINUS-MUTABILISMUTANTS AT DIFFERENT GROWTH-STAGES

The inhibitory effect of nitrate on biological dinitrogen fixation by Rhizobium-legume symbiosis is well known, but the mechanisms involved are still not clear. One of the main techniques used to study this eff ect has been the use of mutants of Rhizobium strains or host plants wh ich do not express the usual pattern of nodulation. The present work w as conducted using parental genotypes and mutants of Lupinus mutabilis with hypernodulation or low nodulation phenotypes that were submitted to two levels of nitrate (0 and 5 mM) and harvested at two different times (7 and IO weeks after emergence). The five millimolar concentrat ion appeared to be very important for inhibition of the initial nodula tion of Lupinus, although mutants L-114 and L-105 were more efficient in nodulation, N-yield and nitrogenase activity than the cv. 'SCG-25' and may allow increased nitrogen fixation in the presence of nitrate. There were no differences in the GS activity of nodules among L-114, L -105 and cv. 'SCG-25' when the plants were grown without nitrate, yet there was a tendency for mutant L-114 to show higher GS activity in th e presence of nitrate. The results indicate that mutant L-114 is more capable of assimilating fixed nitrogen, as also suggested by its PEP-c arboxilase activity. Thus, the partial tolerance to nitrate presented by L-114 and L-105 is apparently associated with the hypernodulated ph enotype. This is in fact illustrated in the results of total and speci fic acetylene reduction activity, which were higher for mutant L-114 t han for cv. 'SCG-25' both in the absence and presence of nitrate.