Nitrogen metabolism and translocation in soybean plants subjected to root oxygen deficiency

Although nitrate (NO3−) but not ammonium (NH4+) improves plant tolerance to oxygen deficiency, the mechanisms involved in this phenomenon are just beginning to be understood. By using gas chromatography–mass spectrometry, we investigated the metabolic fate of 15NO3− and 15NH4+ in soybean plants (Glycine max L. Merril cv. IAC-23) subjected to root hypoxia. This stress reduced the uptake of 15NO3− and 15NH4+ from the medium and decreased the overall assimilation of these nitrogen sources into amino acids in roots and leaves. Root 15NO3− assimilation was more affected by hypoxia than that of 15NH4+, resulting in enhanced nitrite and nitric oxide release in the solution. However, 15NO3− was translocated in substantial amounts by xylem sap and considerable 15NO3− assimilation into amino acids also occurred in the leaves, both under hypoxia and normoxia. By contrast, 15NH4+ assimilation occurred predominantly in roots, resulting in accumulation of mainly 15N–alanine in this tissue during hypoxia. Analysis of lactate levels suggested higher fermentation in roots from NH4+-treated plants compared to the NO3− treatment. Thus, foliar NO3− assimilation may be relevant to plant tolerance to oxygen deficiency, since it would economize energy expenditure by hypoxic roots. Additionally, the involvement of nitric oxide synthesis from nitrite in the beneficial effect of NO3− is discussed.