Comprehensive evaluation of cold resistance of apple Longfeng, Yueyan and their two hybrid strains in cold region

Abstract: 【Objective】With global warming, the risk of late frost damage in flowering period of fruit trees is increasing. Therefore, in order to stabilize food security and ensure the red line of cultivated land, the main direction of apple breeding in cold areas is to select and breed middle and early cold-resistant varieties. The evaluation of cold resistance of existing apple varieties is extremely important for new varieties breeding, and cultivation. In this paper, apple Longfeng and Yueyan and their two superior strains in northeast cold region of Inner Mongolia were studied, in order to further verify the cold resistance of the two superior strains and to clarify the suitable cultivation areas of the two superior strains, and provide scientific basis for enriching the germplasm resources of apple in cold region andbreeding cold region apples with strong cold resistance.【Methods】Two commercial varieties, Longfeng and Yueyan, along with two superior lines, TL0092 and TL0099, were used as test materials. The physiological indicators of one-year-old branches under various low-temperature conditions, as well as the physiological responses of flower buds post low-temperature treatment, were assessed to evaluate the cold resistance of Longfeng, Yueyan, and the two superior lines. The one- year dormant branches were evenly divided into seven groups, subjected to seven different temperature treatments: 4 ℃, -10 ℃, -20 ℃, -25 ℃, -30 ℃, -35 ℃, and -40 ℃. The temperature was reduced to the setting temperature at a rate of 5 ℃·h-1 and and maintained for 24 hours, then the samples under low-temperature stress were thawed; 4 ℃ served as the control for physiological testing. The malondialdehyde content was measured using the thiobarbituric acid (TBA) method, superoxide dismutase (SOD) activity by the nitro blue tetrazolium reduction method, peroxidase (POD) activity by the guaiacol method, soluble protein (SP) content using Coomassie Brilliant Blue G-250 staining, and relative electrical conductivity (E) refered to the methods of Jin Mingli, Yang Xue, et al. The branches collected at the red-bud stage were hydroponically cultivated to bloom, and based on temperature changes during frost occurrence, six lowtemperature treatments (-1 ℃, -2 ℃, -3 ℃, -4 ℃, -5 ℃, -6 ℃) were set with room temperature as the control. The simulated frost box was pre-cooled to about 10 ℃, then cooled at a rate of 6 ℃/4 h to about 4 ℃, and slowly cooled to the set temperature at a rate of 1 ℃/0.5 h, maintained for 2 hours. The heating rate was set at 1 ℃/0.5 h back to room temperature. The ovarys and exocarps of the young fruits during full bloom and young fruit stages were used as the measurement sites to determine the supercooling point and freezing point of each test material, with continuous automatic data recording and analysis of surface temperature changes of the tissues.【Results】The electrical conductivity of all test materials increased with the temperature decreasing, showing an“S”-shaped change curve. The varieties with strong cold resistance had lower increase in relative electrical conductivity Among them, the line TL0099 exhibited a lower increase in the relative electrical conductivity, indicating that its cell membrane system suffered less damage than the two parents and the line TL0092, showed stronger cold resistance. Conversely, Yueyan showed a relatively higher leap in the relative electrical conductivity. The conductivity of the dormant branches of all test materials showe a peak under the -40 ℃ treatment. Among the four apple materials, the line TL0099 showed a sudden jump in the MDA content at -25 ℃, while Yueyan continuously increased until -25 ℃ before declining. Under low-temperature stress, the two parent varietiess showed an“N”-shaped change trend in the POD content, the line TL0092 showed a peak at -30 ℃, showing an initial increase followed by a decrease. The POD activity of TL0099 was significantly higher than that of the parents and TL0092. The SOD content of the branches of TL0099 and TL0092 under different low-temperature treatments was significantly higher than that of the parents Longfeng and Yueyan. The SP content of the all test materials showed a peak at -20 ℃ and did not increase further, the varieties with stronger cold resistance had higher SP content. The frost test during the flowering period revealed that the freezing temperature range for the ovarys and young fruits of the test materials was -2.4 ℃ to -5.6 ℃, with the supercooling point of young fruits higher than that of the ovarys, indicating that the young fruit's cold resistance was inferior to that of the ovarys.【Conclusion】By conducting the artificial low- temperature treatment experiments on one- year- old branches and frost tests during the flowering period, and analyzing with the membership function method, the cold resistance of the four apple varieties (strains) was ranked as follows: TL0099＞TL0092＞Longfeng＞Yueyan.The lines TL0092 and TL0099 were classified as Level Ⅱ cold- resistant strains, Longfeng as a Level Ⅲ moderately cold resistant variety, and Yueyan as a Level Ⅳ low-resistant variety.