Solid-state 15N NMR analysis of highly 15N-enriched plant materials

Solid-state N-15 NMR spectroscopy was used to determine the chemical nature of nitrogen in N-15-enriched material from the roots and stems of wheat (Triticum aesitivum), field pea (Pisum sativum) and kikuyu grass (Pennisetum clandestinum) and from the roots, stems and leaves of a eucalyptus species (Eucalyptus globulus). Nitrogen-15 cross polarization (CP) spectra of the materials were all very similar, with 64-75% of total signal assigned to amide N. Spin counting analysis indicated that 37-80% of potential signal was accounted for in the CP spectra, and that NMR observability using the CP technique (N-obs-CP) was higher for stems and leaves than for roots, and higher for wheat and eucalyptus than for peas and kikuyu. The N-15 direct polarization (DP) spectra contained higher proportions of signal assigned to amine (up to 22%) and nitrate (up to 17%), and less assigned to amide N (50-72%) than the corresponding CP spectra. Spin counting analysis indicated that 68-93% of potential signal was accounted for in the DP spectra, confirming the DP technique to be more quantitatively reliable than CP.