Abstract: [Objective] With the influence of global climate change and human production activities, the scope of soil salinization in China is expanding, and the suitable growth environment of apple is weak acid or neutral soil. Soil salinization makes apple growth and fruit yield and quality face many difficulties, affecting the development of apple industry in China, which has become one of the important factors restricting the development of apple industry. In the actual production of apples, the salt tolerance of seedlings is weak, and rootstocks can provide roots for apples. The quality of rootstocks directly affects the absorption of nutrients and the adaptability of fruit trees to adversity. Therefore, rootstocks are the main determinants of salt tolerance in different rootstock-scion combinations. In the past, China's apple production model was arborization cultivation. With the continuous advancement of technology, arborization cultivation has exposed many problems and has gradually been replaced by dwarfing and close planting. However, most of the common dwarf rootstocks on the market are introduced from abroad. Due to the climatic limitations of the birthplace, there are certain limitations in introducing them into domestic production, such as poor adaptability and weak stress resistance. Therefore, it is urgent to breed excellent dwarf rootstock varieties in China. In this study, the salt tolerance of the hybrid offspring of Malus prunifolia and G41 was evaluated and its salt tolerance was comprehensively analyzed, which had certain reference significance for the breeding of dwarf rootstocks with strong salt tolerance. [Method] In this study, five kinds of tissue culture seedlings of asexual reproduction were used as experimental materials: the rootstock k15 and m2, which were selected from the hybrid offspring of apple rootstock Malus prunifolia× G41 by drought resistance, parent Malus prunifolia and the rootstocks M9-T337 and M26. After expanding to a sufficient number, they were transferred to the rooting medium for growth for 40 days, and transplanted into an 8 × 8 cm2 nutrient pot after opening the cover and refining the seedlings, and placed in a constant temperature light incubator. After the seedlings grew about 7-8 fully expanded true leaves, they were moved to a hydroponic pot with a 6.5 L 1/2 concentration of Hoagland nutrient solution, and the nutrient solution was changed every 3 days. After one week of pre-adaptation, the same size and healthy plants were selected for hydroponic salt treatment. The treatment was divided into two groups: (1) Control (CK), 1/2 Hoagland nutrient solution; (2) Salt treatment (ST), NaCl concentration was 150 mM/L. Nutrient solution was changed every 3 days for 15 days. During the treatment, the photosynthetic rate was measured every 3 days. At the end of the treatment, 10 strains of each strain in the control group and the treatment group were taken to determine the plant height, leaf number, dry and fresh weight. The relative conductivity, chlorophyll, chlorophyll fluorescence, NBT and DAB staining of fresh functional leaves were measured. Fresh roots were taken to determine root activity and analyze root architecture. The fully mature leaves were wrapped in tin paper, immediately frozen in liquid nitrogen and stored at -80 ℃. The MDA content, H2O2 content, O2- content, antioxidant enzyme activity and amino acid content of each strain were determined. The contents of Na+ and K+ in roots, stems and leaves were determined by dry samples after dry weight determination. [Results] 1. From the phenotypic point of view, there was no significant difference in the control group after the end of salt stress treatment; the M26 phenotype of the salt treatment group appeared first, and the leaves showed severe wilting and necrosis on the 9th day, followed by the m2 leaves showing brown spots on the 10th day, and then the Malus prunifolia, k15 and M9-T337 showed salt stress phenotype on the 12th day of treatment. From the root state, the roots of the salt treatment group were brown, and the root area decreased compared with the control group. From the point of view of plant height and leaf number, compared with the control group, each strain of the treatment group decreased. From the perspective of dry and fresh weight, compared with the control group, the lines in the treatment group were significantly reduced. By comparing the relative conductivity, MDA content and root activity, it was found that compared with the control group, the relative conductivity and MDA content of each strain in the salt treatment group increased, and the root activity decreased. 3. From the NBT, DAB staining and O2-and H2O2 content, after treatment, each strain accumulated a certain amount of reactive oxygen species; the results of antioxidant-related enzyme activity showed that the contents of SOD, POD and CAT in each strain increased. 4. When the salt stress was treated to the third day, the Pn, Tr, Gs and Ci of each line decreased significantly, and then the decrease slowed down. From the perspective of total chlorophyll content, compared with the control group, all lines in the salt stress group decreased, among which Malus prunifolia had the smallest decrease and M26 had the largest decrease. From the Fv/Fm value, there was almost no difference in the control group, while k15, Malus prunifolia and M9-T337 decreased less than the other two lines after salt treatment, and the degree of photosystem damage was smaller. 5. From the perspective of ion homeostasis, hydroponic salt stress will lead to more Na+ accumulation in plants. Compared with the control group, the Na+ content in roots, stems and leaves of M26 increased the most, and the increase of Malus prunifolia and k15 was relatively small; the K+ content in each part showed different degrees of decrease after salt treatment, among which M26 had the largest decrease, and Malus prunifolia and k15 had the smallest change. After treatment, the total Na+/K+ of each line increased significantly, and the increase of Malus prunifolia and k15 was relatively small. 6. The results showed that compared with the control group, the contents of Pro, Tyr and Phe increased significantly after salt treatment, while the contents of Gly, Leu and Asp decreased. 7. Comprehensive growth, physiological indicators, photosynthetic parameters, antioxidant enzyme activity, elements and amino acid content and other 16 related indicators, the average membership function value of each strain was calculated. The results showed that the average membership function value of k15 was the largest, indicating that the relative change degree of each index of k15 was the smallest under salt stress, and the salt tolerance was the strongest among the five apple lines. The average membership function value of M26 was the smallest, indicating that its salt tolerance was the weakest. [Conclusion] The results showed that k15 showed strong salt tolerance among the five lines under 150 mM hydroponic salt treatment, and M26 had poor salt tolerance. Under salt stress, the growth of each plant decreased, the growth was inhibited, the root area decreased, and the dry and fresh weight decreased. The relative conductivity and MDA content increased, and the root activity decreased. The accumulation of reactive oxygen species and the increase of antioxidant enzyme activity in plants; chlorophyll content decreased, photosynthesis was significantly inhibited ; Na+ content increased significantly, K+ content decreased, Na+/K+ imbalance; the contents of Pro, Tyr and Phe increased significantly, while the contents of Gly, Leu and Asp decreased. However, due to the differences in resistance of each strain, the rate of change of different indicators is different. Through the comprehensive analysis of membership function, the order of salt tolerance of each strain was obtained as follows: k15 ＞ Malus prunifolia ＞ M9-T337 ＞ m2 ＞ M26. Therefore, the hybrid k15 of apple rootstock Malus prunifolia × G41 has strong salt tolerance and is expected to become a new high-quality rootstock resource.
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