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Metabolism of nitrate and ammonium in seedlings of Norway spruce ( Picea abies ) measured by in vivo 14 N and 15 N NMR spectroscopy
Author(s) -
Aarnes Halvor,
Eriksen Aud B.,
Southon Timothy E.
Publication year - 1995
Publication title -
physiologia plantarum
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.351
H-Index - 146
eISSN - 1399-3054
pISSN - 0031-9317
DOI - 10.1111/j.1399-3054.1995.tb00943.x
Subject(s) - ammonium , picea abies , chemistry , glutamine , nitrogen , glutamine synthetase , metabolism , ammonia , nitrate , nuclear magnetic resonance spectroscopy , nuclear chemistry , amino acid , biochemistry , botany , stereochemistry , biology , organic chemistry
In vivo 15 N and 14 N nuclear magnetic resonance spectroscopy was used to investigate the assimilation of nitrate and ammonium in seedlings of Norway spruce ( Picea abies [L.] Karst.). The main objective was to study accumulation of free NH+ 4 and examine to what extent the nitrogen source affects the composition of the free amino acid pools in roots, stems and needles. NH+ 4 concentrations in plants growing in the presence of 0.5–50 m M ammonium were quantified using 14 N NMR. The NH+ 4 values in tissues ranged from 6 to 46 μmol (g fresh weight) −1 . with highest concentrations in roots and needles. The tissue NH+ 4 peaked at 5.0 m M NH+ 4 in the medium. and failed to increase when NH+ 4 in the medium was increased to 50 m M , indicating metabolic control of the concentration of this cation in tissues. The 14 N NMR spectra were used to estimate pH of the NH+ 4 storage pools. Based on the pH sensitivity of the quintet of 14 NH+ 4 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. 15 N from nitrate or ammonium was first incorporated into the amide group of glutamine and then into α‐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+ 4 in the cytoplasm. NH+ 4 can also be assimilated in roots in plants growing in darkness. The main 15 N‐labelled amino acids were glutamine. arginine and alanine. Almost no 15 N signals from needles were observed. Double labelling (δN + w, wN) of arginine is consistent with the operation of the ornithine 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 15 N NMR spectra of plants grown on 15 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 of nitrogen in the stem.