Phenotypic Variation of Fruit Traits and Their Responses to Soil Nutrients in Natural Manchurian Walnut Germplasms

Manchurian walnut (Juglans mandshurica) is a rare woody oil tree and a key species for ecological restoration in northern China. The Wulingshan National Nature Reserve serves as the core germplasm habitat within its distribution range. However, the coupling mechanism between fruit traits of wild germplasm and soil environment in this region has not been systematically evaluated, restricting germplasm conservation and breeding utilization. In this study, 50 wild J. mandshurica germplasm accessions from the reserve were analyzed. Fruit phenotypic traits, kernel quality traits and rhizosphere soil nutrient indices were determined, and comprehensive germplasm evaluation was conducted via variation analysis, cluster analysis, principal component analysis, membership function method and correlation analysis. The coefficients of variation of 20 quantitative traits ranged from 6.58% to 45.17%, with quality traits showing higher variability than phenotypic traits. The average kernel oil content reached 59.46%. The 50 accessions were clustered into four distinct groups with significantly divergent trait profiles. With increasing elevation, soil properties such as soil hydrolyzable nitrogen, available phosphorus, and electrical conductivity all showed a consistent decreasing trend. For J. mandshurica, green fruit length, green fruit width, the three principal nut diameters, and nut starch content, together with the aforementioned soil hydrolyzable nitrogen and electrical conductivity, were all negatively correlated with elevation. Six germplasm accessions (A32, A42, A34, A21, A47, A15) with excellent comprehensive traits were finally selected. This study clarifies the germplasm variation characteristics of J. mandshurica in Wulingshan and the regulatory effects of soil environmental factors, providing a scientific basis for its resource conservation and targeted breeding.