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Partitioning of Symbiotically Fixed Nitrogen in Soybeans and Alfalfa¹

A growing body of evidence suggests a differential partitioning of soil N and symbolically fixed N₂ among organs during ontogeny of some legumes. This partitioning is poorly understood for soybeans [Glycine max (L.) Merr.] and only meager evidence of this phenomenon is available for perennial forage legumes such as alfalfa (Medicago saliva L.). Therefore, we compared the partitioning of fixed N, among organs of soybean and alfalfa grown on a silt loam (Typic Hapludoll) by periodically sampling throughout one growing season. Dinitrogen fixation and partitioning of fixed N₂ were measured with the ¹⁵N isotope dilution technique. Evidence of differential N partitioning was obtained for both species. The roots of alfalfa contained a higher proportion of fixed N₂ throughout the growing season compared with the stems, leaves, and the whole plant. The roots, stems, and young seeds of soybeans contained a lower proportion of fixed N2 throughout the season compared with the whole plant. In comparison with the N composition of the whole plant, the N in pod walls contained a higher proportion of fixed N₂ and that in green leaflets was unchanged. Senesced leaflets and petioles exhibited a reduced proportion of fixed N₂ coincident with an increased proportion of fixed N₂ in the whole plant. This suggests a preferential mobilization of fixed N₂ compared with soil N. The results suggest a differential allocation of symbiotic N₂ in alfalfa and soybean that is related to the availability of soil N and the composition of the mobile N reserves at specific developmental stages. The accumulation of symbiotic N₂ in pod walls of soybean might reflect a limitation of N transport to young seeds, and deserves further investigation.

Key Words: ¹⁵N isotope • Source-sink relations • Mobilization • Transport • N₂-fixation • Glycine max (L.) Merr. • Medicago sativa L.

² Former graduate research assistant, Dep. of Agronomy and Plant Genetics, Univ. of Minnesota (current address: EMBRAPA/CNPAF, Caixa Postal 179,74000 Goiania, Goias, Brazil); and plant physiologist, USDA-ARS, Dep. of Agronomy and Plant Genetics, Univ. of Minnesota, 1509 Gortner Ave., St. Paul, MN 55108.

Received for publication October 24, 1983.