Nitrogen form differently modulates growth, metabolite profile, and antioxidant and nitrogen metabolism activities in roots of Spartina alterniflora in response to increasing salinity.

Sodium tolerance and nitrogen-source preferences are two of the most fascinating and ecologically important areas in plant physiology. Spartina alterniflora is a highly salt-tolerant species and appears to prefer ammonium (NH ₄ ⁺ ) over nitrate (NO ₃ ^- ) as an inorganic N source, presenting a suite of aboveground physiological and biochemical mechanisms that allows growth in saline environments. Here, we tested the interactive effects of salinity (0, 200, 500 mM NaCl) and nitrogen source (NO ₃ ^- , NH ₄ ⁺ , NH ₄ NO ₃ ) on some physiological and biochemical parameters of S. alterniflora at the root level. After three months of treatments, plants were harvested to determine root growth parameters and total amino acids, proline, total soluble sugars, sucrose, and root enzyme activity. The control (0 mM NaCl) had the highest root growth rate in the medium containing only ammonium and the lowest in the medium containing only nitrate. Except for NO ₃ ^- -fed plants, the 200 mM NaCl treatment generally had less root growth than the control. Under high salinity, NH ₄ ⁺ -fed plants had better root growth than NO ₃ ^- -fed plants. In the absence of salinity, NH ₄ ⁺ -fed plants had higher superoxide dismutase, ascorbate peroxidase, glutathione reductase, and guaiacol peroxidase activities than NO ₃ ^- -fed plants. Salinity generally promoted the activity of the principal antioxidant enzymes, more so in NH ₄ ⁺ -fed plants. Nitrogen metabolism was characterized by higher constitutive levels of glutamate dehydrogenase (GDH) activity under ammonia nutrition, accompanied by elevated total amino acids levels in roots. The advantage of ammonium nutrition for S. alterniflora under salinity was connected to high amino acid accumulation and antioxidant enzyme activities, together with low H ₂ O ₂ concentration and increased GDH activity. Ammonium improved root performance of S. alterniflora, especially under saline conditions, and may improve root antioxidant capacity and N-assimilating enzyme activities, and adjust osmotically to salinity by accumulating amino acids.
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