Phenotypic Variation and Classification of Melon Germplasm Resources Based on DUS Test Characteristics

Melon (Cucumis melo L.) germplasm resources are vital for seed-industry innovation and cultivar breeding. To systematically characterize their phenotypic variation, classification relationships, and breeding potential, 403 melon accessions were evaluated using nine quantitative characteristics specified in the guidelines for tests of distinctness, uniformity, and stability. These characteristics included fruit longitudinal diameter, fruit transverse diameter, fruit peduncle length, internode length, flesh thickness, petiole length, rind thickness, soluble solids content, and seed cavity diameter. The evaluated population exhibited extensive phenotypic variation, with coefficients of variation ranging from 14.0% to 57.4%. Rind thickness showed the greatest variation, while fruit transverse diameter, flesh thickness, fruit peduncle length, and fruit longitudinal diameter also displayed considerable differentiation. The Shannon–Weaver diversity index ranged from 1.74 to 1.96, indicating substantial diversity in fruit morphology, vegetative growth, internal fruit structure, and quality-related characteristics. Spearman rank correlation analysis revealed close developmental relationships among fruit-related characteristics, with rind thickness showing the strongest positive correlation with flesh thickness. Principal component analysis extracted three components with eigenvalues greater than one, which together explained 65.22% of the total phenotypic variation and primarily represented fruit morphological structure, vegetative growth, and fruit quality. Hierarchical cluster analysis classified the 403 accessions into four quantitative phenotypic groups, with highly significant differences among the groups for all nine characteristics. Furthermore, a weighted comprehensive selection index identified several accessions with superior combined agronomic performance, among which accession 20225110454A ranked first. These findings provide a quantitative phenotypic basis for melon germplasm conservation, core collection development, parental selection, and breeding-oriented resource utilization.