Development of nitrogen-assimilating enzymes in sunflower cotyledons during germination as affected by the exogenous nitrogen source

Activities of nitrate reductase (NR; EC 1.6.6.1), nitrite reductase (NiR; EC 1.7.7.1), glutamine synthetase (GS; EC 6.3.1.2) and glutamate dehydrogenase (GDH; EC 1.4.1.3) were measured in cotyledons of sunflower (Helianthus annuus L. cv Peredovic) seedlings during germination and early growth under various external nitrogen sources. The presence of NO ₃ ^- in the medium promoted a gradual increase in the levels of NR and NiR activities during the first 7 d of germination. Neither NR nor NiR activities were increased in a nitrogen-free medium or in media with either NH ₄ ⁺ or urea as nitrogen sources. Moreover, the presence of NH ₄ ⁺ did not abolish the NO ₃ ^- -dependent appearance of NR and NiR activities. The increase of NR activity was impaired both by cycloheximide and chloramphenicol, which indicates that both cytoplasmic 80S and plastidic 70S ribosomes are involved in the synthesis of the NR molecule. By contrast, the appearance of NiR activity was only inhibited by cycloheximide, indicating that NiR seems to be exclusively synthesized on the cytoplasmic 80S ribosomes. Glutamine-synthetase activity was also strongly increased by external NO ₃ ^- but not by NH ₄ ⁺ or urea. The appearance of GS activity was more efficiently suppressed by cycloheximide than chloramphenicol. This indicates that GS is mostly synthesized in the cytoplasm. The cotyledons of the dry seed contain high levels of GDH activity which decline during germination independently of the presence or absence of a nitrogen source. Cycloheximide, but not chloramphenicol, greatly prevented the decrease of GDH activity.Abbreviations GDH glutamate dehydrogenase - GS glutamine synthetase - NiR nitrite reductase - NR nitrate reductase     

Amino acids as a nitrogen source for tomato seedlings: The use of dual-labeled (C, N) glycine to test for direct uptake by tomato seedlings

Direct uptake of organic nitrogen (ON) compounds, rather than inorganic N, by plant roots has been hypothesized to constitute a significant pathway for plant nutrition. The aim of this study was to test whether tomatoes (Solanum lycopersicum cv. Huying932) can take up ON directly from the soil by using ¹⁵NH₄Cl, K¹⁵NO₃, 1, 2-¹³C₂¹⁵N-glycine labeling techniques. The ¹³C and ¹⁵N in the plants increased significantly indicating that a portion of the glycine-N was taken up in the form of intact amino acids by the tomatoes within 48 h after injection into the soil. Regression analysis of excess ¹³C against excess ¹⁵N showed that approximately 21% of the supplied glycine-N was taken up intact by the tomatoes. Atom% excesses of ¹⁵N and ¹³C in the roots were higher than in any shoots. Results also indicated rapid turnover of amino acids (e.g., glycine) by soil microorganisms, and the poor competitive ability of tomatoes in absorbing amino acids from the soil solution. This implies that tomatoes can take up ON in an intact form from the soil despite the rapid turnover of organic N usually found under such conditions. Given the influence of climatic change and N pollution, further studies investigating the functional ecological implications of ON in horticultural ecosystems are warranted.