Integrated assessment of excellent walnut strains (varieties) with timber-nut-dual-purpose

【Objective】To expedite the selection of the breeding of new walnut varieties suitable for both timber and fruit uses in China, this study initially screened over 5000 seedlings, followed by further selection to identify 15 excellent walnut strains (varieties). Through long-term monitoring and comprehensive evaluation of tree growth parameters and nut quality traits, excellent walnut strains (varieties) were identified, providing foundational germplasm and scientific support for timber-nut-dual-purpose walnut breeding.【Methods】This study established a 10-mu (approx. 0.67 hectares) experimental orchard on the Yellow River alluvial plain in Liaocheng municipality, Shandong Province. A randomized block design was employed. 15 walnut strains grafted on Jizhaomian rootstock were used as the test materials, and cultivars Qinglin (QL), Xiangling (XL) and Qingxiang (QX), served as the controls. Among the excellent walnut strains, HL-1, HL-2 and HL-3 were derived from common walnut (Juglans regia L.) seedlings, while HL-4 to HL-15 were selected from QL seedlings. Orchard management followed the established models and complied with relevant national standards. Measured parameters included tree structural characteristics (tree height, diameter at breast height, and basal diameter) and nut traits (external morphological features and internal quality indicators such as protein, fatty acid and ami-no acid contents). Single-trait comparisons and principal component analysis (PCA) were employed for comprehensive evaluation of the timber-nut-dual-purpose characteristics.【Results】The results showed that significant differences were observed among the walnut strains in tree height (10.41- 13.06 m), ground diameter (19.14-25.60 cm), and diameter at breast height (15.56-20.40 cm). Among these, HL- 1 and HL-10 exhibited significantly higher values across all these traits compared to the other walnut strains and the control. Regarding crown architecture, the east-west crown span (5.46-7.10 m) was consistently and significantly larger than north-south span (4.59-5.85 m), with 9 walnut strains surpassing the controls in crown dimensions. Similarly, crown expansion rates demonstrated a directional trend: east-west growth (0.61-0.88 m/year) exceeded north- south growth (0.32-0.67 m/year). All the tested walnut strains displayed typical smooth shell surfaces with non-exposed kernels and generally prominent, tight suture lines. 78.6% of the walnut strains produced elliptical nuts with pointed apexes, and 85.7% had flat bases. Kernel extraction was difficult for 64.3% of the strains, but relatively easier in HL-4, which had an exceptionally thin shell with a thickness of only 0.92 mm. In accordance with the requirements for selection of elite walnut cultivars specified in the Chinese Forestry Industry Standard (LY/T 3004.2—2018), the single nut weight of all the walnut strains (14.63-24.20 g) met the standard for elite cultivars. Notably, HL- 15 (24.20 g) exhibited a 16.3% higher fruit weight compared to QL. Based on the National Standard (GB/T 20398—2021), the kernel recovery of 10 walnut strains exceeded 40%, satisfying the quality requirements for premium walnut nuts. The highest kernel yield rate was 62.53% found in HL-4. Kernel weights across the walnut strains ranged from 6.43 g to 9.95 g. Kernel weight of HL-4 (9.95 g) was 24.38% larger compared to QX (8.00 g). The protein content of all the walnut strains met the standards for elite cultivars. Specifically, HL-11 exhibited a protein content 3.47% higher than XL (194.66 mg · g- 1 ); HL- 12 was 0.73% higher than QL (183.74 mg · g- 1 ); and HL- 15 1.73% higher compared to QX (179.97 mg · g-1 ). Five walnut strains, including HL-3, HL-10 and HL- 9, had a crude fat content exceeding 600 mg · g-1, meeting the elite cultivar standards. The highest level, 637.11 mg · g-1 , was found in HL-3, being 0.48% higher than that of QX (634.07 mg · g-1 ). The six walnut strains with high kernel recovery contained 17 amino acids. Total amino acid content ranged from 100.58 mg·g-1 (HL-1) to 140.22 mg·g-1 (HL-3), while essential amino acid content ranged from 27.11 mg·g-1 (HL-1) to 39.00 mg · g- 1 (HL-3). Among these strains, HL-3, HL-8 and HL-10 exhibited a higher total amino acid content and a higher essential amino acid content compared to XL. Linoleic acid (39.89%- 66.81%) was the predominant fat component, with the highest content (64.00%) found in HL-11. The oleic acid content in HL-4 (42.19%) was 2.7 times that of the QX control; HL-3 was rich in α-linolenic acid (9.71%). Comprehensive evaluation using Principal Component Analysis (PCA) on tree growth indicators identified two principal components accounting for a cumulative variance contribution rate of 86.00%, with HL-1, HL-10 and HL-9 achieving the highest comprehensive scores. For nut quality indicators, four principal components yielded a cumulative variance contribution rate of 93.86%, and HL-8, HL-3 and HL-10 attained the highest comprehensive scores. Considering its outstanding performance in both tree growth and nut quality, HL-10 was determined to be an optimal candidate walnut strain for timber-nut dual purposes.【Conclusion】Based on the analysis results, it is concluded that HL-10 can be considered as a candidate line for timber-nut-dual-purpose walnut. This study successfully selected excellent walnut strains combining rapid growth with high- quality nut production, providing valuable germplasm resources for industry upgrading.