A. Oraee - Department of Horticultural Science and Landscape, Faculty of Agriculture, Ferdowsi University of Mashhad, Mashhad, Islamic Republic of Iran.
A. Tehranifar - Department of Horticultural Science and Landscape, Faculty of Agriculture, Ferdowsi University of Mashhad, Mashhad, Islamic Republic of Iran.

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 ۱۴ (CP۱) and ۲۸ days of Cold Preconditioning (CP۲) decreased electrolyte loss, increased the activity of SOD, CAT, and APX enzymes, inhibited accumulation of Hydrogen peroxide (H۲O۲), 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 CP۱ and CP۲. Although proline and water-soluble sugar contents were higher in plants subjected to ۲۸ days of cold preconditioning than in those treated for ۱۴ 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 ۱۴-day cold preconditioning could be used to increase cold tolerance for non-acclimated hollyhocks to grow in the field at -۴°C.

Antioxidant activity, Hollyhock, Cold tolerance, Freezing resistance, Hormonal changes.