A ^{14}\text{N}$ and 15N Nuclear Magnetic Resonance Study of Nitrogen Metabolism in Shoot-Forming Cultures of White Spruce (Picea glauca) Buds

Nitrogen-14 and nitrogen-15 nuclear magnetic resonance (NMR) spectra were recorded for freshly dissected buds of Picea glauca and for buds grown for 3, 6 and 9 weeks on shoot-forming medium. Resonances for Glu (and other $\alpha \text{NH}_{2}$ groups), Pro, Ala, and the side chain groups in Gln, Arg, Orn, and γ-aminobutyric acid could be detected in in vivo 15N NMR spectra. Peaks for α-amino groups, Pro, NO3 - and NH4 + could also be identified in ^{14}\text{N}$ NMR spectra. Perfusion experiments performed for up to 20 hours in the NMR spectrometer showed that 15N-labeled NH4 + and NO3 - are first incorporated into the amide group of Gln and then in the $\alpha \text{NH}_{2}$ pool. Subsequently, it also emerges in Ala and Arg. These data suggest that the glutamine synthetase/ glutamate synthase pathway functions under these conditions. The assimilation of NH4 + is much faster than that of NO3 -. Consequently after 10 days of growth more than 70% of the newly synthesized internal free amino acid pool derives its nitrogen from NH4 + rather than NO3 -. If NH4 + is omitted from the medium, no NO3 - is taken up during 9 weeks and the buds support limited growth by utilizing their endogenous amino acid pools. It is concluded that NH4 + and NO3 - are both required for the induction of nitrate- and nitrite reductase.