The Main Traits And Molecular Basis Of Xinjiang Precocious Walnut

Walnut（Juglans regia L.）is a kind of important economic forest tree species.Xinjiang is the main producing area of high-quality commercial walnuts in China.In the late 1950s,walnut seedling population characterized by very short juvenile period and side floral bud fruiting was first discovered in Tarim Basin.Because this population has the characteristics of flowering and fruiting in the year or next year after sowing,it was called the"precocious walnuts",while the walnuts which took over 5 years before fruiting were called the"late-bearing walnuts".Currently,the features of"short juvenile period"and"bearing early and plenty"have been widely applied in cross-breeding,resulting in more than 90%of the superior varieties in northern China.The precocious varieties represented by’Wen 185’and’Xinxin 2’have also become the main varieties in Xinjiang.However,the research on the related traits of precocious walnuts is still insufficient.In this study,a total of 88 materials containing precocious and late-bearing tree bodies and potted seedlings were selected as the research objects from the Xinjiang Walnut Resource Conservation Bank.The nut nutrient composition,along with the tree phase,physiology,resistance,gene expression pattern and metabolite profiles,were compared to reveal the important agronomic characteristics and molecular and metabolic differences of precocious walnuts,which could provide a theoretical basis for the commercial production,large-scale and intensive cultivation and molecular-assisted breeding of precocious walnuts.The main findings are as follows:（1）The analysis of the nutrient composition found that the total fat content and total protein content of precocious walnuts were slightly higher than those of late-bearing walnuts,while the sugar content was slightly lower than that of late-bearing walnuts.Geography and climate had significant effect on the nutrient compositions of walnuts.The protein contents of precocious and late-bearing walnuts in the Yarkand River Basin were significantly higher than those in the Toshigan River and the Kumarak River Basin;（2）According to the survey of’Wen 185’and’Xinxin 2’growing for 3 to 19 years,the tree structure and yield tend to be stable when the ground diameter was about 16 cm.For the trees with stable yield,the ratio of the total diameter of first-level lateral branch to ground diameter was about 3.5,while the ratio of the total diameter of bearing branch to the diameter of the growing branches was above 4.This indicator could be used as an indicator of high yield and stable yield.In order to achieve early high-yield,the number of lateral branches of precocious walnut seedlings in the second year should be more than 7 branches,and the ratio of lateral branch biomass to total above-ground biomass should be less than 0.16;（3）Studies on the photosynthetic characteristics of precocious and late-bearing walnut trees at juvenile and adult stages showed that:compared with the late-bearing walnuts,the precocious walnuts showed higher photosynthetic rate at both juvenile and adult stages.The average daily net photosynthetic rate of precocious walnuts at adult stage could reach 16.91μmol·m^-2·s^-1,and the light saturation point was 2121.30μmol·m^-2·s^-1.（4）The potted seedlings of precocious and late-bearing walnuts were treated with water and salt stress.The results showed that:compared with the late-bearing walnuts,the stems,roots and total biomass of precocious walnuts significantly decreased after salt stress and combination stress treatments.Further,the rate of photosynthesis and carbon assimilation ability decreased more significantly,as well as the soluble sugar content of leaves and roots and the starch content of roots.Also,the content of proline,free amino acids,peroxidase,and superoxide dismutase significantly decreased.However,the contents of malondialdehyde and hydrogen peroxide increased significantly.The precocious walnuts showed lower salt tolerance and drought tolerance.（5）The distribution of carbon and nitrogen of precocious walnut and late-bearing walnut appeared differentiation at seedling stage,The carbon content in each organ of precocious plant was lower than that of late-bearing plant.The phloem of branch was the main difference component of nitrogen distribution between precocious plant and late-bearing plant,The nitrogen content in phloem of late-bearing plant was significantly higher than that of precocious plant.（6）Using transcriptome and metabolomics techniques,the gene expression patterns and metabolite characteristics of precocious and late-bearing walnuts were analyzed.Some genes,including ERF5、MYB41,were screened out,which were highly expressed in all tissues of precocious walnuts.They were expected to become important molecular markers for precocious walnuts,since these genes were important for the formation of precocious trait.Three metabolites,including 3-hdroxydodecanoic acid,atemetin and doxycytidine,which were significantly highly expressed in precocious walnuts,were screened out.They were expected to become characteristic substances for identifying precocious walnuts.The clear of the main traits such as the characteristics of nutrition components,photosynthetic characteristics,tree phase structure,drought and salt stress response of precocious walnut are helpful to the utilization of germplasm resources in the breeding of walnut in Xinjiang,and provide theoretical support for the cultivation and management of precocious Improved varieties.The purpose of this study was to explore the relationship between precociousness,photosynthetic,drought and salt stress characteristics and carbon and nitrogen metabolism and molecular mechanism of precocious Walnut,It lays a foundation for the study of molecular mechanism of precociousness.