Integrated Assessment of Photosynthesis, Chlorophyll Fluorescence, and Leaf Anatomy to Evaluate the Adaptive Range of Xanthoceras Sorbifolium Bunge to Salt Stress

Salinity critically constrains plant productivity, necessitating precise determination of species-specific tolerance thresholds. Xanthoceras sorbifolium Bunge is a valuable woody oil plant species native to northern China. This study elucidates salt adaptation mechanisms and tolerance limits in X. sorbifolium through integrated analyses of growth, photosynthesis, chlorophyll fluorescence, and leaf anatomy under five NaCl gradients (0–320 mM). Results demonstrate dose-dependent biomass inhibition, with root/stem elongation showing acute sensitivity to low salinity (80 mM NaCl), while photosynthetic pigment degradation (Chl a: 58.2%; Chl b: 65.8%) and chloroplast dysfunction intensified at 80–160 mM NaCl. Chlorophyll fluorescence revealed biphasic responses: NPQ-mediated photoprotection persisted at ≤160 mM NaCl, but collapsed at 320 mM NaCl, coinciding with Fv/Fm reduction below 0.6 and a 137.7% surge in Y(NO), indicating irreversible photodamage. Gas exchange shifts implicated stomatal limitation (≤160 mM NaCl) transitioning to non-stomatal constraints (≥240 mM NaCl), corroborated by CO2 response curve alterations. Anatomically, low salinity (80 mM NaCl) induced adaptive palisade tissue thickening (+21.9%) and epidermal reinforcement, whereas ≥160 mM NaCl caused spongy tissue atrophy (−32.6%), impairing gas exchange. Tolerance thresholds were delineated: 80–160 mM NaCl represents the critical range for photosynthetic maintenance via anatomical buffering and NPQ regulation; 240 mM NaCl triggers systemic collapse through Rubisco inactivation; 320 mM NaCl induces mortality via photorespiration failure and energy dissipation breakdown.