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Response of Soybean to Inoculation with Efficient and Inefficient Bradyrhizobium japonicum Variants

Dep. of Biology, Kennesaw State College, Marietta, GA 30061
Dep. of Biology, West Georgia College, Carrollton, GA 30118
USDA-ARS and Dep. of Soil Science, North Carolina State Univ., Raleigh, NC 27695-7169
USDA-ARS, Coastal Plains Res. Ctr., Florence, SC 29502-3039

^* Corresponding author.

Soybean [Glycine max (L.) Merr.] plants are grown in soils containing large indigenous bradyrhizobial populations. Individual strains within these populations differ in symbiotic efficiency and competitiveness for nodule occupancy. The purpose of this study was to determine the effect of mixed inocula of an efficient and an inefficient variant of Bradyrhizobim japonicum on symbiotic performance in soybean. Lee soybean was initially inoculated with either an efficient or an inefficient USDA II0 colony morphology variant. The opposite variant was then applied after 0, 2, 4, 8, or 13 d. Delayed inoculation with the efficient variant following the inefficient variant resulted in progressively decreased symbiotic performance. In a subsequent competition experiment, soybean cultivars Lee and Ransom were inoculated with efficient and inefficient variants at ratios of 1:1, 1:10 and 10:1. More nodules were formed by the efficient variant than were expected by chance. Significantly reduced dry weight and whole plant N contents were noted when the inefficient variant was present in more than 50% of the nodules. Split-root experiments were conducted and either type of variant was capable of inhibiting the other after a 7-d delay in inoculation. In contrast, when both sides of the split-root were simultaneously inoculated, nodule numbers were similar; however, nodules formed by the efficient variant were larger. This increase in size indicated a preferential partitioning of photosynthate to the nodules formed by the efficient variant. These results together indicate that N₂ fixation is enhanced with increased nodule occupancy by superior variants due to more effective strain-cultivar interactions.

Received for publication April 29, 1991.