Evaluation of biochemical and physiological responses of purple coneflower (Echinacea purpurea L.) medicinal plant to low temperature stress

To find out the biochemical and physiological responses of purple coneflower (Echinacea purpurea L.) medicinal plant to low temperature stress، a compeletly randomized design with three replications was conducted at Sari Agricultural Sciences and Natural Resources University in 2013. Five months old of coneflower seedlings exposure to three temperature levels of 23 ºC (control)، 4 and 4 ºC for 6 h and then electrolyte leakage and malondialdehyde (MDA)، enzyme activities of superoxide dismutase (SOD)، peroxidase (POD)، catalase (CAT)، ascorbate peroxidase (APX) and polyphenol oxidase (PPO)، protein oxidation of the leaves، total phenols and flavonoids contents، percentage of DPPH free radical inhibition، as well as chlorophyll fluorescence parameters included F''m، F''v/F''m، qp، NPQ and ФPSII were determined. Results of experiment showed a significant increment of electrolyte leakage and malondialdehyde (MDA) content in leaves where the highest electrolyte leakage (55. 7%) and MDA (12. 3 nanomol/ g FW) were obtained at 4 ºC. In this study، the maximun activity of antioxidant enzymes (SOD، CAT and APX) were recorded when coneflower seedlings exposure to temperature lower than 4 ºC. Activity of POD and PPO enzymes، however، were decreased when temperature reduced to 4 ºC. The highest total protein content was calculated at 4 ºC that showes about 65% increment than control. Low temperature stress reduced total phenolic and flavonoid content of seedlings. Also، the highest antioxidant capacity of leaves (0. 93%) were measures in control seedlings. No significant differences were observed between the fluorescence parameters in dark-adapted leaves. In light-adapted leaves، F''m and F''v/F''m content decreased with increasing stress and the highest content of the NPQ and qP were recorded at 4 ºC. Meanwhile، the lowest ФPSII was obtained at 4 ºC. In conclusion، it seems that the coneflower plant relatively had good tolerance to low temperature stress to 4 ºC.