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

摘要

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 tropical legumes on nodulation, nitrogenase activities, dry matter and total N production in shoots and nitrate reductase activity in leaves, nodules and bacteroids. Only six out of the eleven strains tested nodulated cowpea. Nitrate did not significantly change most of the numbers and weight of nodules produced by the nodulating rhizobial strains. Likewise, nitrogenase activities from the effective strains were very tolerant to applied nitrate, especially from those rhizobia not possessing NRA, namely NFB-436, BR-6156 and 6157; contrarily, those strains that displayed high NRA values either in absence or presence of nitrate (NFB-155 and NFB-700) were very significantly depleted in their acetylene reduction values (SARA and TARA) under a long-term exposure to combined N. Nitrate significantly augmented dry weights and total N contents in shoots for all of the nodulating strains, except NFB-450, which neither fixed N-2 nor possessed NRA. The expression of NRA in bacteroids and their importance with respect to nitrogen fixation, as well as nitrate effects on the N economy of these legumes, are discussed.