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Soybean Cyst Nematode and Rhizobium Strain Influences on Soybean Nodulation and N₂-fixation¹

The spread of the soybean cyst nematode (SCN) (Heterodera glycines Ichinhoe) has required the breeding of resistant soybean [Glycine max (L.) Merr.] cultivars. The SCN causes a reduction in nodulation and N₂-fixation which may explain part of the yield reductions associated with SCN infestations. This study was designed to determine the interaction effects of SCN resistant and susceptible cultivars, Rhizobium japonicum (Kirch.) Buchanan strains, and SCN races on nodulation and N₂-fixation.

The susceptible cultivar ‘Williams’ and resistant cultivars ‘Bedford’, ‘Forrest’, and ‘Franklin’ were grown on sterile sand culture in the greenhouse and inoculated with all possible combinations of SCN races 3 and 4 and R. japonicum strains 3, 110, and 123. Bedford and Forrest inoculated with strain 110 had the highest specific N₂(C₂H₂)-fixation activity (SA), but Bedford, Forrest, and Franklin inoculated with strain 123 had the largest nodule mass (NM). SCN race 3 did not reduce NM, but reduced both total N₂(C₂H₂)-fixation activity (TA) and SA. Race 4 did not influence TA and was inconsistent in its influence on SA and NM.

Total N₂(C₂H₂)-fixation activity and SA was estimated at the R2 (determinate) and R4 (indeterminate) stages on several field-grown soybean cultivars. The number of SCN cysts per 250 cc of soil (cyst index) also was determined. Resistant cultivars had low cyst indices and high N₂-fixation potentials. Susceptible cultivars had higher cyst indices and generally lower N₂-fixation rates but ‘Williams’, ‘Wayne’, and ‘York’ exhibited high fiiation potentials despite high cyst indices. Nodule serology indicated a significant cultivar x strain interaction which may account for the TA and SA differences.

These results indicate that nodulation and N₂-fixation are influenced by cultivar x Rhizobium strain and SCN race interactions and that the SCN is not equally deleterious to all susceptible cultivars. Williams and Wayne may be especially valuable in breeding resistant cultivars since they exhibited good nodulation and N₂-fixation despite high cyst indices.

Key Words: Acetylene reduction assay • Glycine max (L.) Merr. • Nodule efficiency • Heterodera glycines • Serogroups • Serology

² Former graduate research assistant (presently research assistant, Dep. of Soils, Univ. of Minnesota, St. Paul), associate professor, and professor, respectively.

Received for publication January 29, 1979.