Accumulation of the phytoalexin, glyceollin, in root nodules of soybean formed by effective and ineffective strains ofBradyrhizobium japonicum.

Nitrogen fixation in root nodules formed by strain 2143 ofBradyrhizobium japonicum andGlycine max (L.) Merr. cv Williams 82 reaches a maximum at 21 to 28 days postinoculation and then begins to decline. The phytoalexin, glyceollin, accumulates in nodules coincident with the decline in nitrogen fixation. Nodules formed by strain 3122, which are unable to fix nitrogen, accumulate even higher levels of glyceollin and do so beginning 21 days postinoculation even though these nodules contain no recoverable bacteria. The typical phytoalexin response occurs within days of infection. The mechanism by which this response in theBradyrhizobium japonicum-soybean combination is delayed 2 to 3 weeks after infection is presently unknown but phytoalexin accumulation could contribute to the inability of the soybean-Bradyrhizobium japonicum combination to maintain high levels of nitrogen fixation throughout the growing season.