The physiological and biochemical responses of eastern purple coneflower to freezing stress

The freezing hardiness (expressed as LT50) as well as changes in the antioxidant enzymes activity, total protein and lipid peroxidation (MDA content), total phenolic and flavonoid content, antioxidant capacity, chlorophyll fluorescence (Fv/Fm) of Echinacea purpurea (L.) Moench were investigated. Five-month-old purple coneflower seedlings were kept at 4°C for two weeks to induce cold acclimation. The acclimated seedlings were treated with freezing temperatures (0, −4, −8, −12, −16, and −20°C) for 6 h. The unfrozen control plants were kept at 4°C. The results with lowering freezing temperatures showed a sharp increase of ion leakage and MDA content at −20°C as compared to the nonfreezing temperature. Exposing seedlings to freezing temperatures were accompanied by decreasing dark-adapted chlorophyll fluorescence (Fv/Fm). Freezing stress significantly reduced superoxide dismutase (SOD), ascorbate peroxidase (APX), and catalase (CAT) activity of seedling leaf except at 0°C. With lowering freezing temperature, peroxidase (POD) and polyphenol oxidase (PPO) activity showed a sharp decline to −20°C. Furthermore, total protein and antioxidant capacity of Echinacea leaves were declined significantly after exposure to freezing temperature, and thereafter reached to the highest at −8°C. Total phenolic content of freezing-treated seedlings was significantly lower than that of the nonfreezing seedlings. Total flavonoid content increased significantly with lowering freezing temperatures. It was found that percentage of freezing injury closely correlated to antioxidant enzymes activity (POD and PPO; r = −0.93) and Fv/Fm ratio (r = −0.77). Based on our results, the freezing tolerance (LT50) of Echinacea seedlings under artificially simulated freezing stress in the laboratory was −7°C.