S. Metabolism of nitrate and anunonium in seedlings of Norway spruce (Picea ahies) measured by in vivo "N and ' In vivo "N and '*N nuclear magnetic resonance spectroscopy was used to investigate the assimilation of nitrate and ammonium in seedlings of Norway spruce (Picea ahies [L.] Karst,), The main objective was to study accumulation of free NHJ and examine to what extent the nitrogen source affects the composition of the free amino acid pools in roots, stems and needles, NHJ concentrations in plants growing in the presence of 0,5-50 mM ammonium were quantified using ' "^N NMR. The NH:I values in tissues ranged from 6 to 46;(mol (g fresh weight)'', with highest concentrations in roots and needles. The tissue NHl peaked at 5,0 mAf NHJ in the medium, and failed to increase when NH4 in the medium was increased to 50 mM, indicating metabolic control of the concentration of this cation in tissues. The ^""N NMR spectra were used to estimate pH of the NH4 storage pools. Based on the pH sensitivity of the quintet of "NHJ resonance, we suggest that the pH of the ammonium storage compartments in the roots and stems should be 3,7-3.8, and in needles 3.4-3.5, representing extremely low pH values of the tissue, "N from nitrate or ammonium was first incorporated into the amide group of glutamine and then into a-amino groups, confirming that the glutamine synthetase/ glutamate synthase cycle is the major route of nitrogen assimilation into amino acids and thus plays a role in lowering the levels of NH* in the eytoplasm. NHJ can also be assimilated in roots in piants growing in darkness. The main "N-labelled amino acids were glutamine, arginine and alanine. Almost no ''N signals from needles were observed. Double labelling (dN + (o,a>N) of arginine is consistent with the operation of the omithine cycle, and enrichment indicates that this cycle is a major sink of newly assimilated nitrogen. Nitrogen assimilation in roots in the presence of added methionine sulphoximine and glutamate indicated the catabolic action of glutamate dehydrogenase. The ' •'N NMR spectra of plants grown on '^N-urea showed a marked increase in the labelling of ammonium and glutamine, indicating high urease activity. Amino acids were also quantified using high pressure liquid chromatography. Arginine was found to be an important transport form cf nitrogen in the stem.