Calcium dominance, ion regulation, and metabolic defenses underlie salt tolerance in the halophyte Azima sarmentosa

Background Soil salinity is a major constraint to plant productivity, yet halophytes have evolved diverse strategies to tolerate excess salt. Azima sarmentosa , a woody halophyte native to Southeast Asia, thrives in saline, calcium-poor soils, but its underlying tolerance mechanisms remain insufficiently described. This study investigated anatomical, physiological, and biochemical traits associated with salinity tolerance across a natural salinity gradient. Methods Soils, stems, mature leaves, and young leaves were analyzed using ion quantification, scanning electron microscopy Energy Dispersive X-ray Spectrometer (SEM-EDS/EDX), synchrotron radiation X-ray tomographic microscopy (SRXTM), Fourier transform infrared spectroscopy (FT-IR) spectroscopy, and multivariate analyses (correlation and Principal Component Analysis (PCA)) to characterize Ca–, Na–, and metabolic-related responses. Results Despite low soil Ca 2+ , plants maintained high Ca 2+ /Na + ratios and produced abundant Ca-oxalate (CaOx) crystals in leaves and stems, indicating selective calcium uptake and biomineralization. SEM–EDS/EDX confirmed Ca-rich deposits and bicellular salt glands on both leaf surfaces, while SRXTM visualized their three-dimensional distribution within tissues. Young leaves accumulated high levels of proline, phenolics, and flavonoids, supporting osmotic adjustment and antioxidant protection. FT-IR spectra corroborated the presence of phenolic functional groups. Correlation analysis and PCA revealed a strong antagonism in trait associations: Ca-related variables clustered with pigments, proline, flavonoids, whereas Na + /Cl − grouped with EtOH-derived phenolics, highlighting a divergence between Ca-driven protection and Na-linked stress. Conclusion A. sarmentosa withstands salinity through an integrative, calcium-centered strategy involving selective Ca 2+ uptake, CaOx biomineralization, salt secretion, and metabolic defenses. Unlike halophytes that rely mainly on sodium sequestration, this species exhibits a distinctive Ca-based adaptation. CaOx formation not only immobilizes Ca 2+ but also incorporates CO 2 -derived oxalate, linking ionic regulation with carbon cycling and broadening the ecological significance of calcium-mediated salt tolerance.