Genetic Diversity Analysis of Apple Germplasm Resources Based on Phenotypic Traits

Apple germplasm resources can be divided into 35 species (taxonomy) and further categorized as wild germplasms, landraces, and cultivars (germplasm type-based classification). Analyses of the variations in apple fruit phenotypic traits among species and germplasm resources revealed the underlying genetic diversity in apple germplasm resources. The study data provide a foundation for the research on apple fruit quality and for promoting the efficient use of apple germplasm resources. Six fruit phenotypic traits of 185 apple germplasm resources preserved in the National Repository of Pear and Apple Germplasm Resources (Xingcheng) were recoded, organized, and analyzed. The six fruit traits of apple germplasm resources varied. The rank order of fruit traits in terms of the degree of variation was as follows (highest to lowest): fruit weight, vitamin C, titratable acidity, fruit hardness, soluble solids, and soluble sugar. Correlation and principal component analyses indicated that fruit firmness, soluble solids, and soluble sugar were major traits that can represent the six fruit phenotypic traits. The six fruit traits differed between and within groups. A hierarchical cluster analysis based on the average coefficient of variation (CV) for each phenotype divided the 185 apple germplasm resources into five groups. The examined apple germplasm resources had diverse fruit phenotypic traits, with the highest CV for fruit weight, indicative of abundant genetic variations underlying the differences between germplasm types. Variations in fruit weight, fruit firmness, soluble solids, and soluble sugar were greater for wild germplasms than for landraces and cultivars. Because fruit firmness and soluble sugar were continuously targeted during long-term artificial selection, the CVs for these fruit traits were significantly lower for cultivars than for wild germplasms.