Effects Of Nitrogen Fertilization Levels On The Growth And Development Of Gala Apple Dwarfing Self-rootstock Seedlings

Good fertilizer and water management is the premise and guarantee of high-quality apple seedling breeding,and nitrogen has an important impact on the growth and development of apple seedlings.In China,the dwarfing self-rootstock of apple started late,and the precise fertilizer and water management system for dwarfing self-rootstock seedling breeding has not yet been established,and excessive fertilization and insufficient fertilization coexist.In view of this,this study used typical 2-year-old Gala/M9-T337 and Gala/G935 apple seedlings as experimental materials to study the effects of different nitrogen application rates on apple seedling growth indicators,biomass,leaf quality,photosynthetic characteristics,and root system.The membership function method was used to comprehensively evaluate the quality of seedlings,in order to provide theoretical and technical guidance for precise fertilization in high-quality apple seedling breeding.1.Screening of nitrogen application rate for Gala/M9-T337 apple seedlings.Using 2-year-old Gala/M9-T337 as the test material,five treatments were set up.The phosphorus and potassium fertilization rates were the same among different treatments,but the nitrogen fertilization rates were different.The growth indicators,biomass,leaf quality,photosynthetic characteristics,and root system indicators of Gala/M9-T337 apple seedlings were measured.The results showed that the nitrogen application level had a significant impact on the growth of Gala/M9-T337 apple seedlings.When the nitrogen application rate was N2,the height of Gala/M9-T337 apple seedlings,chlorophyll content,CAT enzyme activity,stomatal conductance（Gs）,and intercellular carbon dioxide concentration（Ci）in leaves were significantly higher than other nitrogen fertilizer treatments,with chlorophyll content as high as 1.52 mg·g^-1.When the nitrogen application rate is N3,the branch diameter,leaf area,SOD enzyme activity,net photosynthetic rate（Pn）,and transpiration rate（Tr）of the seedlings are significantly higher than other nitrogen fertilizer treatments,while MDA content is significantly lower than other treatments,with the seedling leaf area as high as 7427.11 mm².When the nitrogen application rate is N4,the fresh weight,dry weight,total root surface area,total root length,and total root tip number of the seedlings reach their maximum,with a fresh weight of 617.44 g and a dry weight of 330.78 g above ground of the seedlings.However,there were no significant differences in stem diameter,effective number of branches,branch length,and branch inclination among different treatments.In this study,based on the comprehensive score of the membership function of Gala/M9-T337 apple seedlings,it can be seen that when the nitrogen application rate is N3,the quality of 2-year-old Gala/M9-T337apple seedlings is the best.2.Screening of nitrogen application rate for Gala/G935 apple seedlings.Using 2-year-old Gala/G935 as the test material,five treatments were set up.The phosphorus and potassium fertilization rates were the same among different treatments,but the nitrogen fertilization rates were different.The growth indicators,biomass,leaf quality,photosynthetic characteristics,and root system indicators of Gala/G935 apple seedlings were measured.The results showed that the nitrogen application level had a significant impact on the growth of Gala/G935seedlings.When the nitrogen application rate is T2,the POD enzyme activity in the leaves of Gala/G935 apple seedlings is the highest,and the maximum root to shoot ratio is 0.59.When the nitrogen application rate is T3,the slope of seedling branches,soluble sugar content in leaves,CAT enzyme activity,net photosynthetic rate（Pn）,total root surface area,root volume,and total root length all reach the maximum,while MDA content is the lowest.The soluble sugar content in seedling leaves is 12.98 mg·g^-1,and the root volume is 7.94 cm³.When the nitrogen application rate was T4,the stem diameter,soluble protein content,and chlorophyll content of the seedlings were significantly higher than other nitrogen fertilizer treatments.The stem diameter of the seedlings was 13.91 mm,and the chlorophyll content reached 2.47 mg·g^-1.When the nitrogen application rate is T5,the effective branch number,branch length,and branch diameter of the seedlings are the highest.However,the overall differences in plant height,aboveground fresh weight,dry weight,underground fresh weight,dry weight,leaf area,root diameter,and total root tip number of Gala/G935 apple seedlings among different treatments were not significant.In this study,according to the comprehensive score of the membership function of Gala/G935 apple seedlings,it can be seen that when the nitrogen application rate is T3,the quality of 2-year-old Gala/G935 apple seedlings is the best.In summary,this study screened the optimal nitrogen application rates for 2-year-old Gala/M9-T337 and Gala/G935 apple seedlings,which were N3 and T3,respectively.There was a significant difference in nitrogen application rates between the two,which may be related to their own genetic characteristics.