Differential Phenotypic and Ion–Hormone Homeostatic Responses of Two Thinopyrum ponticum Accessions Diverging in Salt Tolerance Under Salt Stress

Thinopyrum ponticum, a salt-tolerant grass widely used in the restoration of saline–alkali lands, was the focus of this study. Two accessions with contrasting salt tolerance—‘4–6’ (salt tolerant) and ‘5–22’ (salt sensitive)—were watered with 500 mM NaCl solution for 14 days, and seedling growth and physiological responses were assessed. Salt stress significantly inhibited the growth of both accessions, but ‘4–6’ was less impacted. Morphologically, ‘4–6’ adapted to stress by increasing the root-to-shoot ratio, increasing leaf length, and decreasing leaf width. In contrast, the growth of ‘5–22’ was comprehensively inhibited, with significant reductions in fresh weight, dry weight, leaf length, and leaf area. Physiologically, the contents of malondialdehyde and proline increased in both accessions, but ‘4–6’ exhibited stronger antioxidant capacity and more flexible regulation of sugar metabolism (with sucrose decreasing while fructose and glucose increased) to maintain osmotic balance. In comparison, ‘5–22’ showed dysregulated sugar metabolism, characterized by sucrose accumulation and a decrease in fructose, which exacerbated salt damage. Regarding hormones under salt stress, IAA content increased in leaves of ‘4–6’ but decreased in ‘5–22’. Jasmonate-related hormones decreased in both accessions; however, ‘4–6’ maintained higher basal levels and smaller reductions, indicating stronger hormonal regulation capacity. Correlation analysis confirmed that IAA- and JA-related hormones play important roles in salt tolerance of Thinopyrum ponticum. In terms of ion balance, ‘4–6’ maintained higher K+/Na+ and Ca2+/Na+ ratios, promoted beneficial cation transport to shoots, and restricted Cl− accumulation. In contrast, ‘5–22’ suffered from disrupted ion balance and excessive Cl− accumulation, resulting in severe growth inhibition. In addition, the key indicators screened by RDA provide an important reference for revealing the salt tolerance mechanism of Thinopyrum ponticum and for constructing a corresponding evaluation system. This study elucidates the mechanisms underlying differential salt tolerance among Thinopyrum ponticum accessions, highlighting the coordinated role of hormonal reprogramming and ion homeostasis. These findings offer both theoretical insights and practical guidance for breeding new salt-tolerant varieties of Thinopyrum ponticum and for the amelioration of saline–alkali lands.