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Distribution of ¹⁵N Among Plant Parts of Nodulating and Nonnodulating Isolines of Soybeans ¹

² Department of Biology and Center for the Biology of Natural Systems, Washington University, St. Louis, Missouri; 63130, ³ USDA-SEA, Agricultural Research, University of Illinois, Urbana, Illinois 61801³

Differences among plant parts in the natural abundance of ¹⁵N are of interest from the point of view of developing a sampling strategy for using ¹⁵N measurements to estimate the contribution of symbiotically fixed N to N₂ fixing plants, and because they reflect isotopic fractionation associated with degradation, transport, and resynthesis of N-bearing molecules. This paper reports such differences in nodulating and nonnodulating isolines of soybeans (Glycine max [L] (Merrill, variety Harosoy)) grown under several different conditions.

Nodules were strikingly enriched in ¹⁵N compared to other plant parts (by an average of 8.3 excess ¹⁵N), and the enrichment increased with time during the growing season. ¹⁵N was much more uniformly distributed among other plant parts. Although there were significant differences among other plant parts, the maximum deviation of the ¹⁵N abundance of any plant part from that of the entire plant was about 2 ¹⁵N excess. The ¹⁵N abundance of the seed N was most representative of the whole plant.

There were significant differences between isolines in the distribution of ¹⁵N. The distribution of ¹⁵N within plants also varied with experimental conditions. The implications of these results for estimation of N₂ fixation from measurements of the natural abundance of ¹⁵N are discussed.

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