Submitted to: Crop Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/17/2000
Publication Date: N/A
Citation: Interpretive Summary: Legume plants like soybean in symbiosis with soil bacteria are capable of converting atmospheric nitrogen gas to fertilizer through a process called symbiotic nitrogen fixation. This reduces the need for nitrogen fertilizer and contributes to agricultural sustainability. Nitrogen fixation occurs in wart-like structures on roots called nodules. In soybeans nodules begin to senesce (get too old to function) and function inefficiently at the time o seed development. If root nodule senescence could be delayed, symbiotic nitrogen fixation could function throughout seed development and more nitrogen could be available for seed protein formation. Experiments in this report were designed to ascertain what metabolic events occur as root nodules senesce and to determine if genetic variability occurred for root nodule senescence. The results showed that: 1) there is genetic variability for root nodule senescence with root nodules from the genotypes "Hardin" and "Hodgson 78" undergoing senescence later than those of other cultivars and 2) nodule senescence is accompanied by metabolic reactions common to plant pathogenic responses. These findings are important because they show that the active life of nodules can be extended and thus contribute more symbiotically fixed nitrogen to soybean growth. This will be useful to plant breeders and agronomists as they select plants for improved nitrogen fixation.
Technical Abstract: Active N2 fixation in soybeans (Glycine max L.) in Minnesota is limited by cool early-season soil temperatures and by post-flowering nodule senescence. This study examined variation in onset of nodule senescence among Maturity Group I soybean cultivars and sought traits associated with this variation. 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 days after emergence (DAE) and declined rapidly thereafter. In contrast, maximum crown nodule fresh weight in 'Hardin' and 'Hodgson 78' did not occur until 52 DAE, and SNA was still 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 and chalcone synthase gene expression, nodule protease activity, and polyamine accumulation. Morphological changes within the nodule paralleled the biochemical differences, with 'Chippewa' nodules 45 DAE showing more conspicuous deterioration than was evident in 'Hardin.' Because crown- nodule mass and nitrogenase activity in 'Hardin' and 'Hodgson 78' declined later than in other Group I cultivars, with less evidence of host/strain incompatibility, these two lines may have value in breeding programs to extend the period of active nodulation and N2 fixation in soybean.