Abstract:To obtain new germplasm with good cold resistance, nineteen regenerated lines of perennial ryegrass from ‘Derby’ and ‘Pinnacle’ with good cold resistance were used in this research. After two winters‘ observation, five regenerated lines with good performance were obtained. Then some physiology and biochemistry indexes relating to cold resistance were measured under natural low temperature. The aim was to assess cold resistance of five lines comprehensively, screen new germplasm with excellent hardiness, and find out indexes relating to freezing tolerance. The survey of overwintering showed that CK14-3 and G16-1 behaved well in green period, tillering and overwinter rate. Under natural low temperature, leaf water content reduced and cytomembrane was injured. Proline, soluble sugar, and reducing sugar content increased, while soluble protein content decreased. The activity of superoxide dismutase increased, while the activities of peroxidase and catalase decreased. Moreover, photosynthetic system was impaired, both of chlorophyll content and potential photochemical efficiency decreased. By principal component analysis and membership function calculating, the cold-resistance ranking of perennial ryegrass was ‘Pinnacle’, CK14-3, G16-1, CK9-3, G3-2, G10-5 from strong to weak. The experiment materials could be classified into three types: ‘Pinnacle’ and CK14-3 were strong in cold-tolerance, G16-1 and CK9-3 were medium, and G3-2 and G10-5 were poor. The study indicates that the more cold-resistance the perennial ryegrass is, the less cytomembrane is damaged; the less proline content and reducing sugar content it has, the quicker increasing rate of soluble sugar content and superoxide dismutase activity and reducing rate of peroxidase activity is, the slower reducing rate of catalase activity, chlorophyll content and potential photochemical efficiency change is. CK14-3 and G16-1 perform well in cold hardiness and ranked in top, so they were probable to breed as new germplasm with good cold resistance. This study not only provides a reference for germplasm breeding of mutation with good resistance, but also offers theoretical basis for cold resistance research of perennial ryegrass.