Evaluation of cold hardiness of several apple interstocks

Abstract:【Objective】China is the world’s largest apple producer and consumer, with more than 50% of the world’s total apple planting area and output. Apple industry plays an important role in promoting farmers’poverty alleviation, rural revitalization and agricultural supply-side structural reform in China. Severe freezing injury will damage the tree, affect the yield and quality, and cause serious economic losses. Low temperature freezing injury has become a main limiting factor for the development of apple industry in China. In order to avoid or reduce the freezing injury, it is necessary to improve the cold resistance of apple trees, and rootstocks have a great influence on the cold resistance. In this study, through the determination of semilethal temperature under low-temperature conditions, cold resistance related physiological indexes and morphological index, as well as the cold resistance of several apple rootstocks were comprehensively evaluated by principal component analysis, the aim was to screen out rootstocks with strong cold resistance and provide a theoretical basis for the cultivation, production, application, introduction and breeding of apple rootstocks with strong cold resistance.【Methods】The annual dormant branches of 9 types of apple interstocks (5 newly selected rootstocks with unknown cold resistance, i.e., 100, 147, 9-3, 1-8 and 22#46, and 4 types of rootstocks with known cold resistance such as GM256, SH40, M9 and MM106 as the control) were used as test materials. All rootstocks were grafted on apomictic Malus hupehensis Rehd.. During natural overwintering stress, strong and consistent annual dormant branches of each rootstock were sampled. The branches of various rootstocks were treated in two parts: One part of the branches was treated at low temperature after sampled, the relative electro-lyte conductivity (REC) was measured, and the semilethal temperature was calculated; another part of the branches were frozen in liquid nitrogen after sampled and stored in the refrigerator at -80 ℃ for measuring the physiological and biochemical indexes such as superoxide dismutase (SOD) and peroxidase (POD) activities, the contents of malondialdehyde (MDA), abscisic acid (ABA), superoxide anion (O2 - ), soluble protein, soluble sugar, starch and proline of rootstock branches. The cold resistance of apple rootstocks was comprehensively evaluated by principal component analysis.【Results】During the whole process of natural overwintering stress, the physiological characteristics showed dynamic changes of all apple rootstocks. The morphological indicators (freezing injury index) and physiological indexes such as SOD and POD enzyme activities, the contents of MDA, ABA, superoxide anion, soluble sugar, soluble protein and proline were negatively correlated with the natural overwintering temperature, and the semilethal temperature and starch content were positively correlated with the natural overwintering temperature, and all these physiological indexes reached the maximum or minimum value when the temperature achieved the lowest in January, but may have slight difference among some rootstock species due to different variety characteristics. The rootstocks with higher cold resistance had higher SOD and POD activities, as well as ABA, soluble sugar, soluble protein, starch and proline contents, and lower MDA and superoxide anion production rate. The rootstocks with poorer cold resistance had lower SOD and POD enzyme activities, as well as ABA, soluble sugar, soluble protein, starch and proline contents, and higher MDA content, freezing injure index and superoxide anion production rate.【Conclusion】The order of cold resistance of these apple interstocks was as following in a descending order: 100 > 147 > 9-3 > GM256 >SH40 > 1-8 >22#46> M9>MM106. GM256 was an excellent apple rootstock with strong cold resistance. In this study, the apple rootstocks with better cold resistance than GM256 were 100, 147 and 9-3. It is expected to provide valuable reference for the cultivation, production, application, introduction and breeding of apple rootstocks with strong cold resistance.