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Nitrogen Metabolism of Normal and High-Seed-Protein Soybean

USDA-ARS, Soybean and Alfalfa Res. Lab., Plant Sciences Institute Bldg. 011, HH 19, Beltsville Agric. Res. Ctr.-West, Beltsville, MD 20705

^* Corresponding author.

Nitrogen metabolism differences, including N₂ fixation, between high-and normal-seed-protein soybean [Glycine max (L.) Merr.] lines of equivalent plant maturity and seed yielding ability have not been determined in field tests. A replicated field experiment was conducted in 1985 and 1986 to determine such differences on a Mattapex silt loam (fine, silty, mixed, mesic Aqualfic Hapludult) with low soil N. The study used the nodulating-nonnodulating N difference method to estimate N₂ fixation of two soybean lines with contrasting seed protein levels. Eight sequential harvests were conducted from R5 (beginning seed) to combine harvest for the dry weights and N contents of vegetative and reproductive portions of the plants. There was no significant difference between the 2-yr mean seed yields of the high- and normal-protein lines, but the high-protein line was 2 d later in maturity. The high-protein line accumulated more N, fixed more N₂, and remobilized more N to the seed than did the normal-protein line (231 vs. 194, 114 vs. 77, and 87 vs. 76 kg ha^–1, respectively). The greater N accumulation by the high-protein line resulted from N₂ fixation being sustained until the late R6 (full seed) stage. Nodule occupancy by strains of Bradyrhizobium japonicum was similar for the high- and normal-seed-protein lines. Thus, the ability to sustain N₂ fixation until later stages of reproduction is a host plant attribute that may contribute to the greater N metabolism of a high-seed-protein soybean.

Received for publication April 1, 1991.

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