Different Responses to NaCl vs. NaHCO₃ Stress in Three Limonium Species: Linking Seed Phenotype to Physiological Tolerance

Soil salinization severely restricts vegetation restoration in Northwest China. Native Limonium plants, capable of naturally colonizing saline-alkalisaline–alkali wasteland, are potential germplasm for low-cost ecological restoration. This study focused on three wild Limonium species (Limonium aureum, Limonium bicolor, Limonium gmelinii) in Gansu Province. In this study, we integrated seed phenotypic diversity with stress tolerance. We then investigated seed germination indices (e.g., germination rate, energy, vigor index) and seedling physiological–biochemical indices of three Limonium species under 0, 100, 200, 300 mM NaCl and NaHCO₃ stress. These indices included leaf and root Na⁺ and K⁺ contents, chlorophyll a and b and carotenoid contents, and malondialdehyde (MDA), proline, soluble sugar, and soluble protein contents, plus SOD and CAT activities. Results showed seed area and thickness were key to germination performance, with L. aureum having the largest and thickest seeds and strongest germination potential. The onset concentration of significant inhibition for salt/alkali was 200 mM. At the seedling stage, L. aureum performed best at 100–200 mM, while all three were damaged at 300 mM. Principal component analysis indicated that L. aureum had the highest comprehensive scores under both NaCl and NaHCO₃ stresses, while L. bicolor and L. gmelinii presented distinct stress-specific adaptation differences. Thus, L. bicolor is recommended for salt-dominated soils and L. gmelinii for alkaline environments, and L. aureum can be used for mildly heterogeneous habitats. This study clarifies inter-species differences under stress, providing a direct theoretical basis for ecological restoration in saline–alkali areas.