Stage-specific responses of Spergularia marina to salinity reveal strategies of tolerance and restoration potential.

Salinity is a major abiotic stress limiting plant productivity and threatening the integrity of saline ecosystems. The annual halophyte Spergularia marina plays a key ecological role in early successional stages of coastal and inland salt marshes due to its rapid colonization ability, trampling resilience, and tolerance to fluctuating salinity. However, its developmental responses to salinity remain poorly understood. This study evaluated the stage-specific physiological and biochemical responses of S. marina to NaCl stress (0-400 mM), revealing a biphasic tolerance pattern. Germination and early seedling growth were highly sensitive to salinity, while mature plants showed enhanced performance at 100 mM NaCl, including increased biomass, photosynthetic pigment content, osmolyte accumulation, and antioxidant enzyme activity. These adaptive responses minimize oxidative stress, as indicated by lower hydrogen peroxide and malondialdehyde levels. In contrast, concentrations above 150 mM NaCl led to pigment degradation, reduced antioxidant capacity, and growth inhibition. Multivariate analysis confirmed a physiological threshold near 100 mM NaCl (≈ 10 dS·m⁻¹), marking the point of maximal stress resilience. These findings highlight the potential of S. marina for restoring moderately salinized habitats, where its stage-specific plasticity, osmotic adjustment, and redox homeostasis contribute to ecosystem recovery under increasing environmental stress.