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Exogenous Salicylic Acid Alleviates Freeze-Thaw Injury of Cabbage (Brassica oleracea L.) Leaves

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.