Metabolic origin of δ15N values in nitrogenous compounds from Brassica napus L. leaves

Paul P.G. Gauthier*, Marlene Lamothe, Aline Mahé, Gemma Molero, Salvador Nogués, Michael Hodges, Guillaume Tcherkez

*Corresponding author for this work

    Research output: Contribution to journalArticlepeer-review

    42 Citations (Scopus)


    Nitrogen isotope composition (δ15N) in plant organic matter is currently used as a natural tracer of nitrogen acquisition efficiency. However, the δ15N value of whole leaf material does not properly reflect the way in which N is assimilated because isotope fractionations along metabolic reactions may cause substantial differences among leaf compounds. In other words, any change in metabolic composition or allocation pattern may cause undesirable variability in leaf δ15N. Here, we investigated the δ15N in different leaf fractions and individual metabolites from rapeseed (Brassica napus) leaves. We show that there were substantial differences in δ15N between nitrogenous compounds (up to 30‰) and the content in (15N enriched) nitrate had a clear influence on leaf δ15N. Using a simple steady-state model of day metabolism, we suggest that the δ15N value in major amino acids was mostly explained by isotope fractionation associated with isotope effects on enzyme-catalysed reactions in primary nitrogen metabolism. δ15N values were further influenced by light versus dark conditions and the probable occurrence of alternative biosynthetic pathways. We conclude that both biochemical pathways (that fractionate between isotopes) and nitrogen sources (used for amino acid production) should be considered when interpreting the δ15N value of leaf nitrogenous compounds. Still, little is known on the natural isotope composition (δ15N) in individual metabolites and on 14N/15N isotope fractionations in plant metabolism. This lack of knowledge impedes our current understanding of 15N-distribution among plant tissues and of relationships between environmental conditions and whole-plant δ15N values. In the present paper, we carried out δ15N analyses on several leaf fractions and individual metabolites by taking advantage of quite recent isotope ratio mass spectrometry techniques. Our study further compares δ15N values obtained in the dark with that in the light, and observed values with that predicted with a metabolic model.

    Original languageEnglish
    Pages (from-to)128-137
    Number of pages10
    JournalPlant, Cell and Environment
    Issue number1
    Publication statusPublished - Jan 2013


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