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Improving the ability for nodulation and nitrogen fixation of common beans and soybean

Background

All legumes so far studied show genetic difference in the ability to fix N₂ in symbiosis. The trait is probably multigenic, but breeding for enhanced nodulation and nitrogen fixation has usually beeen successful. Phaseolus beans are generally considered relatively weak in the ability to fix N₂; even in soybeans the potential for high N₂ fixation can begin to decline where the crop is grown for a number of years under conditions of high soil N. Our goal is to enhance the potential for N₂ fixation in both bean and soybean as a normal component of a breeding program.

Early nodulation of a bean shown using haematoxylin staining (M.H. Chaverra, 1992). Used with permission.

Studies to date

Initial studies emphasized cultivar differences in plant traits affecting nodulation and N₂ fixation in soybean and common bean, showing variation in:

• Speed of nodulation (Chaverra and Graham, 1992),
• Preference in nodulation (Montealegre et al., 1995, Pazdernik et al., 1997b)
• Seasonal patterns of N₂ fixation
• Onset of nodule senescence (Espinosa et al., 1999)
• Differences between fixation and translocation efficiencies:
• "Chippewa" fixed 2.96g N plant^-1 but passed only 51% to the grain; "Parker" fixed only 1.70 g plant^-1, but had high harvest N index (79%) (Pazdernik et al., 1997a).

Often, traits shown to vary in one host, for example bean, can then be shown to exhibit similar variation in soybean

Once parents had been identified that differ in the traits each could contribute to improved N₂ fixation in bean and soybean, breeding programs were initiated.

Recurrent selection for enhances N₂ fixation in Phaseolus vulgaris (Elisondo Baron et al., 1999)

Study sought to improve yield and N accumulation under low N soil conditions, using parents selected for different traits important in enhanced N₂ fixation. A condition was that time to flower or mature not be changed.

*g per four plants

Three genotypes selected for high nitrogen fixation in these studies have now been crossed with the locally-important cultivar Montcalm, with backcross progeny being developed.