SYMBIOTIC EFFECTIVENESS OF SEVERAL TROPICAL BRADYRHIZOBIUM STRAINS ONCOWPEA UNDER A LONG-TERM EXPOSURE TO NITRATE - RELATIONSHIPS BETWEEN NITROGEN-FIXATION AND NITRATE REDUCTION ACTIVITIES

A greenhouse experiment was conducted on cowpea (Vigna unguiculata L. Walp, cv. IPA 205) to determine the combined effect of nitrate (7 mM N ) and inoculation with eleven Bradyrhizobium strains from several trop ical legumes on nodulation, nitrogenase activities, dry matter and tot al N production in shoots and nitrate reductase activity in leaves, no dules and bacteroids. Only six out of the eleven strains tested nodula ted cowpea. Nitrate did not significantly change most of the numbers a nd weight of nodules produced by the nodulating rhizobial strains. Lik ewise, nitrogenase activities from the effective strains were very tol erant to applied nitrate, especially from those rhizobia not possessin g NRA, namely NFB-436, BR-6156 and 6157; contrarily, those strains tha t displayed high NRA values either in absence or presence of nitrate ( NFB-155 and NFB-700) were very significantly depleted in their acetyle ne reduction values (SARA and TARA) under a long-term exposure to comb ined N. Nitrate significantly augmented dry weights and total N conten ts in shoots for all of the nodulating strains, except NFB-450, which neither fixed N-2 nor possessed NRA. The expression of NRA in bacteroi ds and their importance with respect to nitrogen fixation, as well as nitrate effects on the N economy of these legumes, are discussed.