Exogenous glycine betaine (GB) application has been reported to improve plant tolerance to various abiotic stresses, but its effect on freezing tolerance has not been well studied. We investigated the effect of exogenous GB on freezing tolerance of cabbage (Brassica oleracea L.) leaves. Seedlings fed with 30 mM GB via sub-irrigation showed effectively assimilated GB as evident by higher GB concentration. Exogenous GB did not retard leaf-growth (fresh weight, dry weight, and leaf area) rather slightly promoted it. Temperature controlled freeze-thaw tests proved GB-fed plants were more freeze-tolerant as indicated by lower electrolyte leakage (i.e., indication of less membrane damage) and alleviating oxidative stress (less accumulation of O₂^•− and H₂O₂, as well as of malondialdehyde (MDA)) following a relatively moderate or severe freeze-thaw stress, i.e., −2.5 and −3.5 °C. Improved freezing tolerance induced by exogenous GB application may be associated with accumulation of compatible solute (proline) and antioxidant (glutathione). GB-fed leaves also had higher activity of antioxidant enzymes, catalase (CAT), ascorbate peroxidase (APX), and superoxide dismutase (SOD). These changes, together, may improve freezing tolerance through membrane protection from freeze-desiccation and alleviation of freeze-induced oxidative stress.

Freezing tolerance and physiological/biochemical changes were investigated for cabbage (Brassica oleracea L. ‘Myeong-Sung’) leaves treated with 0.5 mM salicylic acid (SA) by sub-irrigation. SA treatment did not interfere with leaf-growth (fresh/dry weight, and leaf-area), rather promoted growth (leaf-area) as compared to the control. Temperature-controlled, laboratory-based freeze-thaw assays revealed that SA-treated leaves were more freeze-tolerant than controls as evident by less ion-leakage as well as malondialdehyde content after freeze-thaw stress treatments (−2.5 and −3.5 °C). SA treatment also significantly alleviated freeze-induced oxidative stress as evidenced by the lower accumulation of O₂^•− and H₂O₂, concomitant with higher activities of antioxidant enzymes (ascorbate peroxidase and superoxide dismutase) relative to the control. Specifically, SA-treated leaves had a greater abundance of compatible solute (proline) and secondary metabolites (phenolic/flavonoid contents). These changes, together, may improve freezing tolerance through protecting membranes against freeze-desiccation and mitigating freeze-induced oxidative stress.

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.