PeerJ Reviewer Match - Calcium-based salt tolerance

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

Abstract

Background: Soil salinity is a major stressor limiting plant productivity, yet halophytes have evolved diverse mechanisms to cope with excess salt. Azima sarmentosa, a woody halophyte native to Southeast Asia, thrives in saline, calcium-poor soils, but its tolerance mechanisms remain poorly understood. This study examined anatomical, physiological, and biochemical traits of A. sarmentosa across a natural salinity gradient.
Methods: Soils, stems, mature leaves, and young leaves were analyzed using ion quantification, SEM–EDS/EDX, Synchrotron radiation X-ray tomographic microscopy (SRXTM), FT-IR spectroscopy, and multivariate analyses (correlation, PCA) to evaluate Ca, Na, and metabolic traits associated with stress tolerance.
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 uptake and biomineralization. SEM–EDS/EDX confirmed Ca-rich deposits and salt glands on both leaf surfaces, while SRXTM showed their three-dimensional distribution within tissues. Young leaves accumulated high levels of proline, phenolics, and flavonoids, reinforcing osmotic adjustment and antioxidant protection. FT-IR spectra confirmed phenolic groups. Correlation and PCA revealed antagonism: Ca-associated traits (pigments, proline, flavonoids) opposed Na/Cl and acetone-derived phenolics, highlighting 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, Ca-oxalate biomineralization, salt secretion, and metabolic defenses. Unlike halophytes that rely mainly on sodium sequestration, this species exhibits a distinctive Ca-based adaptation. Ca-oxalate formation not only immobilizes Ca ²⁺ but also sequesters CO₂-derived oxalate, linking ionic regulation with carbon cycling and extending the ecological significance of calcium-centered tolerance.

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