Peer-review: Yes|Yes - Open Access: Yes|http://science.thomsonreuters.com/cgi-bin/jrnlst/jlresults.cgi?PC=MASTER&ISSN=0140-7791

Nitrogen isotope composition (ä15N) in plant organic matter is currently used as a natural tracer of nitrogen acquisition efficiency. However, the ä15N 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 ä15N. Here, we investigated the ä15N in different leaf fractions and individual metabolites from rapeseed (Brassica napus) leaves. We show that there were substantial differences in ä15N between nitrogenous compounds (up to 30?) and the content in (15N enriched) nitrate had a clear influence on leaf ä15N. Using a simple steady-state model of day metabolism, we suggest that the ä15N value in major amino acids was mostly explained by isotope fractionation associated with isotope effects on enzyme-catalysed reactions in primary nitrogen metabolism. ä15N 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 ä15N value of leaf nitrogenous compounds.

Global Wheat Program

English

No CIMMYT affiliation|John Wiley

Carelia Juarez

INT3193

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