Drought, Salinity, and Low Nitrogen Differentially Affect the Growth and Nitrogen Metabolism of

Abiotic stresses, such as salinity, drought, and nutrient deficiency adversely affect nitrogen (N) uptake and assimilation in plants. However, the regulation of N metabolism and N pathway genes in Sophora japonica under abiotic stresses is unclear. Sophora japonica seedlings were subjected to drought (5% polyethylene glycol 6,000), salinity (75mM NaCl), or low N (0.01mM NH4NO3) for 3weeks in a semi-hydroponic phenotyping platform. Salinity and low N negatively affected plant growth, while drought promoted root growth and inhibited aboveground growth. The NH4+/NO3− ratio increased under all three treatments with the exception of a reduction in leaves under salinity. Drought significantly increased leaf NO2− concentrations. Nitrate reductase (NR) activity was unaltered or increased under stresses with the exception of a reduction in leaves under salinity. Drought enhanced ammonium assimilation with increased glutamate synthase (GOGAT) activity, although glutamine synthetase (GS) activity remained unchanged, whereas salinity and low N inhibited ammonium assimilation with decreased GS activity under salt stress and decreased GOGAT activity under low N treatment. Glutamate dehydrogenase (GDH) activity also changed dramatically under different stresses. Additionally, expression changes of genes involved in N reduction and assimilation were generally consistent with related enzyme activities. In roots, ammonium transporters, especially SjAMT1.1 and SjAMT2.1a, showed higher transcription under all three stresses; however, most nitrate transporters (NRTs) were upregulated under salinity but unchanged under drought. SjNRT2.4, SjNRT2.5, and SjNRT3.1 were highly induced by low N. These results indicate that N uptake and metabolism processes respond differently to drought, salinity, and low N conditions in S. japonica seedlings, possibly playing key roles in plant resistance to environmental stress.