Population-specific physiological response profiles characterize salinity tolerance in the Antarctic extremophile

Introduction: Extremophile plant species provide valuable systems for understanding how salinity tolerance varies among populations originating from contrasting environments. The Antarctic vascular plant Colobanthus quitensis, is widely distributed from temperate South America to the maritime Antarctic and represents an useful model for investigating population-level physiological variation under environmental constraints. However, the physiological basis of variation in salinity tolerance among population remains poorly understood. Methods: Here, we evaluated morphophysiological and biochemical responses of five geographically and environmentally contrasting C. quitensis populations exposed for 60 days to a NaCl concentrations (0-200 mM) under controlled in vitro conditions. Dry biomass, tissue water content, root-to-shoot length ratio, proline, total soluble sugars, lipid peroxidation, and antioxidant enzyme activities were examined using univariate analyses, standardized response ratios, principal component analysis, an exploratory integrated response score, and observed-variable path analysis. Results: Population, NaCl concentration, and their interaction significantly affected most evaluated traits, demonstrating strongly population-dependent responses. Proline accumulation increased with salinity in most populations, whereas total soluble sugars and antioxidant enzyme activities showed contrasting trajectories. Tissue water content was generally maintained or slightly increased, indicating sustained tissue hydration during prolonged NaCl exposure. Multivariate and integrated analyses showed that populations differed in the magnitude and combination of growth-related, water-status, compatible-solute-related, antioxidant, and oxidative-damage responses. Under high salinity, averaged across 150 and 200 mM NaCl, the inland population pV showed the highest exploratory integrated response score, followed by pH, whereas pPA showed the lowest score. The observed-variable path analysis identified conditional associations among NaCl concentration, proline, antioxidant enzyme activity, MDA, root-to-shoot length ratio, and dry biomass, but did not support a simple linear relationship between antioxidant activation, oxidative damage, and biomass maintenance. Discussion: Overall, salinity tolerance in C. quitensis was associated with population-specific physiological configurations rather than a single common response profile. The absence of a simple correspondence between these profiles and broad present-day habitat categories further suggests that ecological and evolutionary histories may contribute to physiological diversification within the species.