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Nitrogen (N) is taken up by most plant species in the form of nitrate (NO ₃⁻ ) or ammonium (NH ₄⁺ ). Plant response to continuous NH ₄⁺ nutrition is species‐dependent. In this study, we compare the responses of tomato ( Solanum lycopersicum L. cv. Rio Grande) plants to N source (NO ₃⁻ or NH ₄⁺ ). To this end, early plant growth, photosynthesis, chlorophyll, carbohydrate, and N‐compound concentrations as well as the activities of main enzymes involved in N metabolism (nitrate reductase, nitrite reductase, glutamine synthetase, and glutamate dehydrogenase) were analyzed. Early plant growth was remarkably ameliorated under NH ₄⁺ ‐ in comparison to NO ₃⁻ ‐based nutrition. Concomitantly, photosynthetic activity, total chlorophyll, and carbohydrate concentrations were significantly increased. With increasing external NH ₄⁺ concentration, NH ₄⁺ accumulated mainly in roots. In addition, root protein concentration was significantly increased, reflecting high NH ₄⁺ incorporation into organic nitrogen. Root glutamine synthetase (GS) activity was enhanced by NH ₄⁺ for concentrations below 5 mM, whereas root glutamate dehydrogenase (GDH) activity increased in parallel to NH ₄⁺ availability. Together with the positive effect of NH ₄⁺ on tomato plant cv. Rio Grande growth, these results reveal that GDH could have, in addition to GS, a possible role in NH ₄⁺ detoxification and tolerance of NH ₄⁺ ‐based nutrition.
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Horchani et al. (2010) studied this question.
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