The warm gaps during cold weather will increase the risk of freezing damage to plants. The successful winter survival of hollyhock (Alcea rosea ’nigra’) can be limited by climate warming. This study exposed the cold-acclimated and nonacclimated plants to warm and cold temperatures and determined their physiological and biochemical traits. The results showed that after acclimation, cold temperature effectively delayed the chilling injury, reduced the electrolyte leakage, significantly increased the activity of SOD, APX, and CAT, inhibited the content of hydrogen peroxide (H2O2), and retarded the increase in malondialdehyde (MDA) content, whereas they were increased in the other treatments. When noncold and cold-acclimated plants were exposed to warm temperatures, a rapid decrease in antioxidant activities and osmolyte regulators occurred, resulting in a plant that no longer possesses an elevated level of freezing tolerance. Cold temperatures in non-acclimated plants did not induce substantial acclimation and reduced frost resistance primarily by limiting the water-soluble sugars and proline availability. The freezing tolerance of acclimated plants maintained a lower LT50 and exhibited higher net photosynthesis and stomatal conductance under freezing temperatures. Freezing tolerance in hollyhock plants (–6 C) induced by suboptimal temperature acclimation is associated with enhanced photosynthetic adaptability, osmoregulation ability, and antioxidant activities.

Keywords