Fruit quality analysis and comprehensive evaluation of characteristic small-fruit type apple germplasm resources

【Objective】Apple is the most widely cultivated deciduous fruit species in China, possessing considerable adaptability and economic value. Since the reform and opening-up, the apple industry has developed rapidly. However, the homogenized variety structure and its monotonous flavour profile can no longer meet consumers' growing demand for greater diversity in fruit. The cultivation of diverse, differentiated and personalized new apple varieties holds significant importance, with the development of small-fruit type apples emerging as one of the new breeding objectives. Therefore, we systemically investigated fruit quality traits of characteristic small-fruit type apple germplasm resources and conducted a comprehensive evaluation, thereby providing valuable reference and a theoretical basis for the innovative utilization of superior germplasm resources.【Methods】This study utilized 5 characteristic smallfruit type apple cultivars, namely Huangtaiping, Jinhong, Hesetiaowen, Whitney, and Xinhong, as exper-imental materials to measure and analyze their fruit morphological characteristics and quality trait indicators. Apple fruit phenotypic traits were surveyed and recorded in accordance with the‘Apple Germplasm Resource Description Specifications and Data Standards’. The determination of soluble sugars and organic acids in apples was carried out using high-performance liquid chromatography. The determination of polyphenolic components and their contents was performed using ultra- high- performance liquid chromatography-mass spectrometry. Five fruit sections including cross sectional, longitudinal sectional, cheeks, stem cavity and calyx-end were selected. After 24-hour fixation at room temperature to ensure complete penetration of the fixative, paraffin sections were prepared. Cytological observations were conducted on five-part fruit sections of apple varieties using paraffin sections. Finally, a comprehensive evaluation of fruit quality traits across different varieties was performed through correlation analysis and fuzzy membership function methods.【Results】Five characteristic small- fruit type apple varieties exhibit significant differences in fruit morphological characteristics and quality indicators. The fruit color of Huangtaiping and Hesetiaowen is pink, while Jinhong, Whitney and Xinhong are turkey red, brown red, and strong red, respectively. The fruit surfaces of Jinhong and Xinhong show arris, while other apple varieties have no arris. The fruit dots of Huangtaiping and Hesetiaowen varieties exhibit small dots, while Whitney and Xinhong display large dots. The fruit cores of Jinhong and Xinhong are small, while the cores of Huangtaiping, Hesetiaowen, and Whitney are medium. In terms of fruit morphological characteristics, the stem depression of Jinhong fruit is shallow and narrow, while Huangtaiping, Whitney and Xinhong are all deep and medium. Additionally, the range of fruit weight across different apple varieties is 42.03- 82.55 g. Among these, Huangtaiping apples exhibit the lowest fruit weight, and both the transverse and longitudinal diameters of Huangtaiping apples are significantly smaller than those of other apple varieties. However, the titratable acidity, malic acid, citric acid, flavanols, phenolic acids, and total phenolic contents of Huangtaiping fruit are significantly higher than those of other apple varieties. Xinhong fruit exhibit the highest levels of fructose and sorbitol. Hesetiaowen fruit contain the highest levels of soluble solids, soluble sugars, sugar-acid ratio, sucrose, and dihydrochalcone. The Whitney apple variety exhibits the highest levels of glucose and flavonoids among all apple varieties, with a significantly higher content of phlorizine compared to other apple cultivars. Significant differences in cell morphology, size, and number also exist among different apple varieties. The fruit cells of Huangtaiping and Jinhong are arranged relatively densely, and exhibit consistent morphology across different parts. The fruit cells of Hesetiaowen are arranged in a relatively loose pattern. The cross section and longitudinal section of Whitney fruit show relatively dispersed cells with distinct intercellular spaces. The cell boundaries of Xinhong fruit exhibit an irregular shape. In terms of cell size and number in different fruit parts, the cross-sectional cell size and longitudinal cell number of Huangtaiping are significantly lower than those of other apple varieties, while there is no significant difference in cross-sectional cell size and longitudinal cell size, indicating that the volume difference between Huangtaiping fruit and other apple varieties mainly depends on the cross-sectional cell size and longitudinal cell number. There are significant differences in the number and size of cells in the stem cavity and calyx-end of various apple varieties, except for the number of cells in the cheeks. Significant differences exist in fruit size among varieties, with both the number of fruit cells and the volume of the fruit jointly influencing the final size of the fruit.【Conclusion】The comprehensive evaluation results based on the fuzzy membership function method indicate that the Hesetiaowen fruit exhibits the highest average membership function value, followed by the Huangtaiping, Whitney, Xinhong, and Jinhong fruits in a descending order. The evaluation results are consistent with the growth status and phenotypic characteristics of each apple germplasm resource. Based on the quality characteristics of each germplasm, specific traits such as high- quality fresh type, distinctive appearance type, and functional/processing type can be selected for germplasm breeding. The above research provides important references for the screening and comprehensive utilization of characteristic small- fruit type apple germplasm resources.