Acquisition of Freezing Tolerance in Non-Acclimated Alcea rosea L. ʻnigraʼ by Exposure to Different Duration of Cold Preconditioning

Lower winter temperatures may negatively affect winter survival by preventing maximum cold acclimation before freezing temperatures. The process of cold acclimation requires adaptation to both light and low temperatures, depending on the duration of exposure. Accordingly, research is needed to identify strategies to promote cold acclimation and increase freezing tolerance. Therefore, this experiment was conducted to investigate whether a shorter or more extended preconditioning cold treatment can improve the cold hardiness of hollyhock and its threshold of winter hardiness. The results showed that both 14 (CP1) and 28 days of Cold Preconditioning (CP2) decreased electrolyte loss, increased the activity of SOD, CAT, and APX enzymes, inhibited accumulation of Hydrogen peroxide (H2O2), and delayed the increase of Malondialdehyde (MDA) content, while non-acclimated plants experienced a decrease in MDA. No difference was observed in antioxidant activity and photosynthetic parameters between CP1 and CP2. Although proline and water-soluble sugar contents were higher in plants subjected to 28 days of cold preconditioning than in those treated for 14 days, no significant difference was found in survival percentage. Low temperatures decreased photosynthetic parameters, while increasing leaf contents of Abscisic Acid (ABA) and phenolic. The results suggested that 14-day cold preconditioning could be used to increase cold tolerance for non-acclimated hollyhocks to grow in the field at -4°C.