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N₂ Fixation (C₂H₂) and Ureide Content of Soybeans: Ureides as an Index of Fixation¹

The ureide contents of soybeans [Glycine max (L.) Merr.] were measured to determine if ureide concentration is a useful indicator of N₂ fixation. Acetylene reduction activity (ARA) excised roots and the concentrations of ureides in stem segments were measured in field grown soybeans at weekly intervals. Ureides occurred in every soybean cultivar tested. Their concentration in the shoot varied over the growing season. The concentration droppeduring early vegetative growth, then rose and fell during reproductive growth. The plant age when maximum ureide concentration occurred was similar in three seasons. Within maturity groups there was little difference in ureide concentration. Stem nitrate levels decreased continuously over the season and were negatively correlated with ARA and ureides. A relative abundance of ureides was calculated as the fraction of ureide-N in the ureide-N + NO₃-N pool. The relative abundance correlated well with ARA, and gave a correction for the effect of soil nitrate on ureide concentration. Ureide concentration and ARA, measured at the same time, were significantly correlated. Dinitrogen fixation over the season was estimated by integrating ARA. The concentration of ureides in the young stem at the R3 stage showed the highest significant correlation with integrated seasonal ARA. The single ureide sampling was not as closely correlated with the estimated seasonal fixation as a single ARA determination at the R2 or R3 stage.

Key Words: Dinitrogen fixation • Glycine max (L.) Merr.

² Postdoctoral associate and plant biochemist, Boyce Thompson Inst. for Plant Research, Tower Road, Ithaca NY 14853, USA. Present address of T. G. P.: National Research Council of Canada Prairie Regional Lab., Saskatoon, Sask. S7N0W9 Canada.

Received for publication April 23, 1982.