Observations on the Composition and Metabolism of the Nitrogen-Fixing Root Nodules ofAlnus

Abstract

Nodules of Alnus glutinosa (Alder) were exposed to excess ¹⁵N either before or after detachment from the plant. The soluble nitrogen compounds were extracted from the nodules and the extracts fractionated by chromatography on an ion exchange resin. The amino-acid composition of the extracts was thus determined, and it was confirmed that citrulline is the predominant amino-acid present, being accompanied by smaller amounts of aspartic, glutaniic, and γ-aminobutyric acids, arginine, and other constituents. The highest atom per cent, excess ¹⁵N always found in glutarnic acid and the next highest in citrulline or aspartic acid. Ammonia contained a smaller proportion of ¹⁵N than these compounds and arginine showed only very small enrichment. When the citrulline was degraded to ammonia and ornithine it was found that the ammonia liberated was richer in ¹⁵N than even glutamic acid. The significance of these findings in relation to the fixation and further metabolism of nitrogen by the alder nodule is discussed.