Individual free amino acid delta N-15 values in plant tissue reflect the metabolic pathways involved in their biosynthesis and catabolism and could thus aid understanding of environmental stress and anthropogenic effects on plant metabolism. In this study, compound-specific nitrogen isotope analysis of amino acid by gas chromatography-combustion-isotope ratio mass spectrometry (GC-C-IRMS) was carried out to determine individual free amino acid delta N-15 values. High correlations were observed between the delta N-15 values obtained by GC-C-IRMS and elemental analyzer-isotope ratio mass spectrometry (EA-IRMS) determinations, and the mean precision measured was better than 1 aEuro degrees. Cation-exchange chromatography was employed to purify the sample, and the difference between prior to and following passage through the resin was within 1 aEuro degrees. The amino acid delta N-15 values of plant leave samples following incubation in N-15-nitrate at different time points were determined. A typical foliar free amino acid N-15-enrichment pattern was found, and glutamine was the most rapidly labeled amino acid; other amino acids derived from the GS-GOGAT cycle were also enriched. The pyruvate family amino acids were labeled less quickly followed by the aromatic amino acids. This study highlighted that amino acid metabolism pathways had a major effect on the delta N-15 values. With the known amino acid metabolism pathways and delta N-15 values determined by the presented method, the influence of various external factors on the metabolic cycling of amino acid can be understood well.

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