Effects of Salt Stress on the Physiology and Biochemistry of Six Poplar Germplasms and Evaluation of Their Salt Tolerance

Global soil salinization is accelerating. As the dominant fast-growing plantation genus, Populus spp. largely determines the success of coastal and inland saline-land restoration and the associated carbon-sequestration benefits. Yet most current studies rely on single indicators and lack a multidimensional physiological framework for ranking salt tolerance. Here, six elite poplar cultivars were exposed to 0% (CK), 0.2% (ST1), 0.3% (ST2) and 0.4% NaCl (ST3) for 30 d. We quantified membrane-lipid peroxidation, photosynthetic performance, osmotic adjustment and antioxidant enzymatics, then integrated the data with a principal-component–membership-function model. With increasing NaCl, MDA and REC either rose continuously or peaked slightly below ST3. ‘YX2’ reached the highest MDA (71.3 µmol g⁻¹ FW) and REC (80.3%) under ST2. Pn and SPAD declined overall, but ‘YX3’ retained the greatest photosynthetic stability (6.1 µmol m⁻² s⁻¹ at ST3). Osmolytes accumulated differentially: soluble sugar in ‘PZ2’ rose 52% above CK at ST3; soluble protein in ‘YX2’ peaked at 12.7 mg g⁻¹ FW; proline exceeded 110 µg g⁻¹ FW in ‘YX2’, ‘PZ1’ and ‘PZ2’. Antioxidant enzymes were up-regulated with stress; ‘YX3’ CAT peaked at 69.7 U g⁻¹ FW under ST2, while SOD and POD remained highly active. Correlation analysis revealed that photosynthetic decline is tightly linked to membrane oxidative damage, while the coordinated enhancement of antioxidant enzymes and concurrent accumulation of osmolytes form a synergistic protection mechanism. PCA showed that PC1 (57.1%) integrated photosynthetic capacity, membrane integrity and antioxidant synergy, whereas PC2 (14.3%) represented osmotic and enzymatic protection. The combined D-value ranked cultivars as ‘YX2’ > ‘YX3’ > ‘PZ2’ > ‘PZ1’ > ‘ZX1’ > ‘YX1’. This multi-trait platform provides both a theoretical reference and a germplasm basis for saline-site afforestation and salt-tolerant poplar breeding.