The Physiological and Biochemical Responses of Cycas bifida and C. panzhihuaensis to Freezing Stress

Seedlings of Cycas bifida and C. panzhihuaensis were used to compare the freezing tolerance of the 2 species and explore the underlying physiological and biochemical mechanisms. The results showed that the semilethal low temperature of C. bifida and C. panzhihuaensis were −0.15 ℃ and −2.05 ℃, respectively. After freezing treatment, the chlorophyll content decreased in C. bifida but increased in C. panzhihuaensis; the photosynthetic efficiency decreased and malondialdehyde increased for both the species with a greater extent in C. bifida; the activity of different antioxidant enzymes showed different change patterns in both the species; the soluble sugar content decreased in C. bifida and increased in C. panzhihuaensis, but the proline content of both the species increased compared to those of the control.