Host variation in traits associated with crown nodule senescence in soybean

Active N-2 fixation in soybean [Glycine max (L.) Merr.] in Minnesota is lim ited by cool early-season soil temperatures and by postflowering nodule sen escence. This study examined variation in onset of nodule senescence among Maturity Group I soybean cultivars and sought traits associated with this v ariation. Host genotype markedly affected onset of crown nodule senescence. For most cultivars, crown nodule fresh weight and specific nodule activity (SNA) peaked 31 to 38 d after emergence (DAE) and declined rapidly thereaf ter. In contrast, maximum crown nodule fresh weight in 'Hardin' and 'Hodgso n 78' did not occur until 52 DAE, and SNA was stilt high 45 to 52 DAE. Two cultivars, Chippewa and Alpha, that exhibited early change in crown nodule mass and SNA, accumulated glyceollin I 10 to 45 DAE at rates significantly greater than for Hardin and Hodgson 78. The four cultivars also differed in phenylalanine ammonia lyase (PAL) and chalcone synthase (CHS) gene express ion, nodule protease activity, and polyamine accumulation. Morphological ch anges within the nodule paralleled the biochemical differences, with Chippe wa nodules 45 DAE showing more conspicuous deterioration than was evident i n Hardin, Because crown-nodule mass and nitrogenase activity in Hardin and Hodgson 78 declined later than in other Maturity Group I cultivars, with le ss evidence of host-strain incompatibility, these two lines may have value in breeding programs to extend the period of active nodulation and N-2 fixa tion in soybean.