Nitrate and Ammonium Contribute to the Distinct Nitrogen Metabolism of Populus simonii during Moderate Salt Stress

Soil salinity is a major abiotic stressor affecting plant growth. Salinity affects nitrification and ammonification in the soil, however, limited information is available on the influence of different N sources on N metabolism during salt stress. To understand the N metabolism changes in response to different N sources during moderate salt stress, we investigated N uptake, assimilation and the transcript abundance of associated genes in Populus simonii seedlings treated with moderate salt stress (75mM NaCl) under hydroponic culture conditions with nitrate (NO(3)(-)) or ammonium (NH(4)(+)). Salt stress negatively affected plant growth in both NH(4)(+)-fed and NO(3)(-)-fed plants. Both NH(4)(+) uptake and the total N concentration were significantly increased in the roots of the NH(4)(+)-fed plants during salt stress. However, the NO(3)(-) uptake and nitrate reductase (NR) and nitrite reductase (NiR) activity primarily depended on the NO(3)(-) supply and was not influenced by salt stress. Salt stress decreased glutamine synthetase (GS) and glutamate synthase (GOGAT) activity in the roots and leaves. Most genes associated with NO(3)(-)uptake, reduction and N metabolism were down-regulated or remained unchanged; while two NH(4)(+) transporter genes closely associated with NH(4)(+) uptake (AMT1;2 and AMT1;6) were up-regulated in response to salt stress in the NH(4)(+)-fed plants. The accumulation of different amino acid compounds was observed in the NH(4)(+)- and NO(3)(-)- fed plants during salt treatment. The results suggested that N metabolism in P. simonii plants exposed to salt enhanced salt resistance in the plants that were fed with NO(3)(-) instead of NH(4)(+) as the sole N source.