Exogenous abscisic acid enhances freeze-thaw stress tolerance in Antarctic moss Sanionia uncinata through coordinated antioxidant defense and osmoprotectant accumulation

Abscisic acid (ABA) is known to improve plant freeze-thaw stress tolerance (FTST), but its role in Antarctic mosses remains largely unexplored. Accordingly, this study investigated the effects of exogenous ABA application on FTST of gametophore Sanionia uncinata, one of the most widespread moss species in maritime Antarctica. Samples were treated with 10 μM ABA for 72 h under controlled culture conditions, then subjected to freezing stress at -13 and -16 °C. Various physiological parameters were measured to assess ABA-induced FTST, including ion-leakage, photosynthetic efficiency (Fv/Fm), malondialdehyde (MDA) content, compatible solutes/osmolytes (proline and total sugars), and antioxidant enzyme activities, e.g., superoxide dismutase (SOD), catalase (CAT), and ascorbate peroxidase (APX). Gametophores treated with ABA efficiently assimilated ABA, evident by ∼20-fold higher than controls. ABA application did not impede gametophore growth but slightly increased dry weight/fresh weight ratio (5 % increase). ABA significantly improved freezing tolerance, reducing freeze-induced membrane injury by ∼20 % at both stress temperatures, alleviating oxidative stress with 25-34 % lower MDA accumulation, and enhancing PSII maximum quantum yield (Fv/Fm) by 6-7 % following freeze-thaw stress. Improved FTST was paralleled by enhanced activities of antioxidant enzymes, CAT and APX increasing by ∼11 % and 56 %, respectively, while SOD remained unchanged. ABA treatment also promoted osmoprotectant accumulation, with proline content increasing by 33.5 % and total soluble sugars nearly doubling (99 % increase). These findings provide the first quantitative evidence that exogenous ABA markedly improves FTST in an Antarctic moss through coordinated antioxidant defense and osmoprotectant accumulation, highlighting ABA as a critical regulator of stress resilience in S. uncinata.