Cold resistance evaluation of four apple varieties

Abstract:【Objective】The present experiment was undertaken to investigate the cold resistance of four apple varieties with parental genetic relationship and to identify the physiological indicators that can reflect the strength of cold resistance. The aim was to provide theoretical support for rapid screening of superior lines and cold-resistant cultivation management in the breeding process of apples in cold regions. 【Methods】In this study, Nagano Fuji No.2, Hanfu, Dongguang and Longshuai were top-grafted (more than one meter above the ground) onto 4- year- old Pingyitiancha rootstocks in spring, and 1- year- old branches were taken as test materials in mid-January in the second year. They were treated at low temperatures of -15 ℃, -20 ℃, -25 ℃, -30 ℃ and -35 ℃ for 24 h, and thawed at 4 ℃ for 24 h. The membrane structure stability, active oxygen metabolism and physiological indexes of osmotic adjustment substances were determined. The cold resistance of apples was comprehensively evaluated by principal component analysis, and the physiological factors affecting cold resistance were analyzed.【Results】 With the decrease of treatment temperature, the relative electrical conductivity of the branches from four apple varieties showed a trend of gradual increase and reached the maximum at -35 ℃. Moreover, the relative electrical conductivity of Nagano Fuji No.2 was still significantly higher than that of othervarieties, and the difference between Longshuai and Dongguang was not significant. With the aggravation of low-temperature stress, the MDA contents in branches of different varieties showed an upward trend, and the content of MDA in branches of Nagano Fuji No.2 was always higher than that of other varieties. The MDA contents in branches of Dongguang and Longshuai was low, and there was no significant difference in MDA content between the two varieties under -20 ℃, -25 ℃ and -30 ℃. The contents of O2 − and H2O2 in the branches of the varieties with weak cold resistance were higher, and the contents of O2 − in the branches of Nagano Fuji No.2 were significantly higher than those of other varieties from -30 ℃ to -35 ℃. Under low-temperature stress, the contents of H2O2 in the branches of Nagano Fuji No.2, Hanfu and Dongguang were higher than those of Longshuai, and the difference in H2O2 content in the branches of Dongguang and Longshuai was not significant at -15 ℃ and -20 ℃, but the difference in H2O2 content in the branches between Longshuai and other varieties were significantly lower than that with other low-temperature treatments. From -25 ℃ to -35 ℃, SOD and CAT activities of Longshuai branches were significantly higher than other varieties, but SOD activity decreased slightly, while CAT activity increased gradually. The APX activities of the branches of the four varieties reached the peak at -25 ℃ and the lowest at -35 ℃, with no significant difference. With the decrease in temperature, the changing trend of soluble protein content in branches of different varieties was different. The soluble protein content in Longshuai branches continued to increase, and reached the maximum at -35 ℃, which increased by 42.24 % compared with at -15 ℃. The free proline content of Nagano Fuji No.2 was always the lowest, and the free proline content of Dongguang was higher and reached the maximum at -35 ℃. With different low-temperature treatments, the soluble sugar content of Nagano Fuji No.2 was always the lowest, and Longshuai was the highest. At -15 ℃ and -35 ℃, the soluble sugar contents of Nagano Fuji No.2 and Longshuai decreased by 46.27% and 4.11%, respectively. With the decrease of temperature, the sucrose content in branches of Longshuai increased continuously and reached the maximum at -35 ℃, and there was no significant difference in sucrose contents among the four varieties. The contents of glucose and fructose in branches of Longshuai increased rapidly at -25 ℃ , increasing by 21.94% and 52.5% , respectively. The contents of glucose and fructose in branches of Longshuai were significantly higher than those of other varieties at -25 ℃. The variation trend of sorbitol content in different varieties of apple branches was relatively stable. Comprehensive evaluation of cold resistance by the principal component analysis from strong to weak was as follows: Longshuai＞Dongguang＞Hanfu＞Nagano Fuji No.2. It was found that the relative electronic conductivity value, SOD activity, soluble protein and soluble sugar contents had a great influence on the cold resistance of apple.【Conclusion】The relative conductivity value, SOD activity, soluble protein and soluble sugar contents in one-year-old branches of apple were sensitive to low temperature stress among many physiological indexes, which could be used for early rapid selection of superior cold- resistant lines among a hybrid offspring. Longshuai, as the offspring of Hanfu, had outstanding cold resistance and could be popularized and applied in cold apple producing areas.