Physiological and Biochemical Responses of Nitraria tangutorum to Long-Term Saline Irrigation in an Arid Coal-Mining Restoration Area

Saline irrigation is a major constraint on vegetation establishment in arid coal-mining landscapes. Nitraria tangutorum Bobrov is a xerohalophytic shrub established in the Dananhu mine-restoration area of Hami, Xinjiang, but its physiological responses to long-term saline irrigation remain insufficiently characterized. In this field study, approximately two-year-old plants were maintained for approximately two growing seasons under freshwater drip irrigation or irrigation water containing 8 or 12 g L−1 total dissolved solids. Unlike short-term controlled salinity assays that evaluate individual response pathways, the present field study integrates root and shoot responses across osmolyte accumulation, oxidative injury, redox regulation, nitrogen metabolism, phytohormone signaling, and lipid remodeling under long-term mixed-salt irrigation. Increasing salinity significantly reduced shoot total chlorophyll content, whereas shoot carotenoid content showed a nonsignificant numerical increase. Soluble sugars, proline, and soluble proteins increased, while total free amino acids declined. Superoxide anion (O2−), hydrogen peroxide (H2O2), and malondialdehyde (MDA) increased progressively, demonstrating oxidative injury. The protein concentrations of superoxide dismutase (SOD), catalase (CAT), peroxidase (POD), and ascorbate peroxidase (APX) increased, whereas glutathione peroxidase (GSH-PX) and glutathione reductase (GR) declined. Concurrent decreases in ascorbic acid/ascorbate (AsA) and reduced glutathione (GSH), together with increases in dehydroascorbic acid (DHA) and oxidized glutathione (GSSG), indicated progressive oxidation of the cellular redox environment. Nitrate reductase (NR) declined under salinity, while glutamine synthetase (GS) and glutamate synthase (GOGAT) increased at 8 g L−1 but decreased at 12 g L−1. Growth-associated hormones declined, whereas stress-associated hormones increased. Together, these responses reveal a dose-dependent transition from co-occurring biochemical adjustment and oxidative injury at 8 g L−1 to broader redox and metabolic disruption at 12 g L−1 under long-term field irrigation. These variables provide candidate indicators of downstream salinity response, but ion homeostasis, plant water status, growth, survival, and long-term performance must be evaluated before salt tolerance or irrigation thresholds can be established.