Effects of Cold Acclimation on Several Enzyme Activities in Euonymus radicans ‘Emorald & Gold’ and Its Relation to Semi-lethal Temperature

The changes in activities of superoxide dismutase (SOD), peroxidase (POD) and ATPase in the leaves of Euonymus radicans were studied when seedlings were cold-acclimated (at 4℃) for 1 week, 2 weeks, 3 weeks and then treated for 1d under low temperature stress (at-5℃).The semi-lethal temperatures of acclimated and unacclimated seedlings were also investigated.The results indicated that the activities of the three enzymes in the leaves of the seedlings treated at 4℃ for 1, 2 and 3 weeks were all higher than those of unacclimated seedings (treated at 22℃ as controls). The activities of SOD and POD increased continuously with the prolongation of the time of cold acclimation, but stepped up to summits then down to the levels of the controls. The activities of SOD culminated at the first week, and the activities of POD at the second week. When acclimated and unacclimated seedlings were both treated at -5℃ for 1d, the activities of the three enzymes in the leaves of acclimated seedlings were a little lower than those before stress, but higher than those of the controls. Moreover, the decrease rate of enzyme activities was greatly lower than that of the controls. The results showed that cold acclimation could enhance the stability of the three enzymes in the leaves of seedlings under low temperature stress; the semi-lethal temperature was -19.1℃ when the seedlings were treated at 4℃ for 3 weeks, but it was -5.4℃ when the seedlings were treated at 22℃. The decline of the semi-lethal temperature caused by the adaptive changes of enzyme activities was one of the foundations of enhancing the cold tolerance.