We used 256 apple germplasm resources for a thorough examination of the genetic diversity associated with 26 phenotypic traits (i.e., genetic diversity analysis, cluster analysis, correlation analysis, principal component analysis, and membership function). The average coefficient of variation for 12 morphological traits was 66.39% (21.10–201.5%). The coefficient of variation was highest and lowest for the fruit arris and the width of the eye basin, respectively. Additionally, the diversity index ranged from 0.54 to 1.33. Moreover, the coefficient of variation for 14 numerical traits varied from 5.37% to 50%. The titratable acid content had the highest coefficient of variation, with a diversity index ranging from 2.01 to 2.08 (average of 2.045). A cluster analysis categorized 256 germplasms into four groups, among which Group I included germplasms with large fruits and the best comprehensive performance. Of the top 10 principal components revealed by the principal component analysis, principal component 1 was mainly related to fruit size and flavor. The top 10 germplasms were selected on the basis of comprehensive scores using the membership function method. Furthermore, a stepwise regression analysis identified 15 key traits for identifying apple germplasms, including the vegetative growth day, fruit weight, and the firmness of the fruit without skin. These results can serve as the foundation for future analyses of the phenotypic diversity of apple germplasms, while also providing a theoretical basis for screening, characterizing, and further improving excellent apple germplasms.

Apple (Malus Mill.) is one of the most important fruits in China, and it boasts the world’s largest cultivation area and yield. It needs to be grafted onto rootstocks to maintain a variety of characteristics. China has many apple rootstock resources that exhibit high resistance and strong adaptability; for these reasons, they are highly suited to China’s complex and diverse natural environment. In China, apple rootstock breeding began in the 1970s, and now, several rootstocks, such as the ‘GM256’ and ‘SH’ series, are widely used. However, domestic rootstock resources and varieties are difficult to root. This affects the selection, utilization, and promotion of apple rootstocks. Cutting is an important method of rooting for apple rootstocks. This study discusses the main factors that affect rooting in rootstock cutting propagation; it also summarizes the rooting ability of different apple rootstocks and presents analyses of the demand for rootstocks in the major areas of apple production in China. We present the apple rootstock resources that are suitable for the soil and climate conditions of this production. We also call for research on the cutting roots of these specific apple rootstock resources to be expanded and strengthened. It is hoped that cutting rootstocks suitable for major areas of apple production may be more easily screened and bred successfully in China in the near future.

‘Binzi’ (BZ) (Malus domestica subsp. chinensis var. binzi Li Y.N.) and ‘Xiangguo’ (XG) (Malus domestica subsp. chinensis var. xiangguo Li Y.N.) are the ancient cultivars in China. The BZ fruits have a low-fragrant flavor on harvest day but a high-fragrant flavor after storage at room temperature, while the XG fruits have a stronger flavor when mature. ‘Starking’ (SK) and ‘Golden Delicious’ (GD) fruits have a rich flavor and are recognized by all countries in the world. However, information on the differences between ancient Chinese cultivars and Western apple cultivars in aroma compounds remains unknown. The apple fruits were collected for continuous two years. Aroma compounds in the skin and pulp of the fruits were detected at room temperature (20 ± 1 °C) during storage. The dynamics of VOCs in BZ and SK fruits were more similarly reflected in esters, while those of XG and GD fruits were reflected in aldehydes and alcohols. Ethyl 2-methylbutyrate, with an extremely low odor threshold, was the main source of typical apple flavor in SK, BZ, and XG fruits, while hexyl acetate was the source of the banana flavor in GD fruits. 6-methyl-5-hepten-2-one and β-damascenone were the important ketones produced in the later stage of storage, derived from the carotenoid metabolism pathway and providing a citrus and rose flavor to the four apple cultivars. SK had the highest number of characteristic aroma components, which were mainly derived from the amino acid metabolism pathway, providing fruits with a sweet and fruity flavor. Although the characteristic aroma components of GD were derived from the fatty acid metabolic pathway, the number of volatile esters was lower. Ethyl butyrate, derived from the saturated fatty acid metabolism, had the highest content in BZ, providing a pineapple flavor; the flavor of XG was mainly derived from ethyl 2-methylbutyrate, 6-methyl-5-hepten-2-one, and β-damascenone. Therefore, we suggest BZ and XG apples as the aroma-breeding material with which to enrich new cultivars’ aroma components, derived from the fatty acid metabolism and carotenoid metabolism pathways, respectively.

In this study, we used genotyping to determine the genetic structure and molecular identities of apple landraces from six species of Malus Mill. in China, based on the fingerprints revealed by microsatellite markers with tailed primer M13. A total of 46 apple landraces of Malus Mill. selected from the National Apple and Pear Germplasm Repository in Xingcheng, China were genotyped with 14 SSR markers. The primers differentiated all the accessions. At least three SSR primers, CH04h02, CH01f07a and CH04g07, with a higher heterozygosity and Shannon’s information index than other combinations can distinguish all the accessions. All the alleles for these three primers were arranged in descending order, and they were assigned values beginning with 01. Character strings were constituted by combining all the codes of the three primers for every accession. By such means, separate and special molecular identities were obtained for every apple landrace, which could be expressed in the form of a bar code. Using such bar codes, trees can be labeled and scanned, which aids in the identification and tracking of genebank collections. The eight-step method for establishing the molecular identities of apple landraces reported here may serve as a reference when determining the molecular identities establishment of other apple germplasms of Malus Mill. This method might also be used for the establishment of a molecular database to aid the preservation of Malus Mill., which is in imminent danger in China. It may also be used to improve the gene bank management of Malus Mill.

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.

In this study, fruits of the red flesh Malus plant ‘Hongxun 2’ (Malus neidzwetzkyana (Dieck) Langenf.) and green flesh Malus plant ‘Xinye 13-11’ (Malus sieversii (Led.) Roem.) were used as experimental materials. Both of them came from Xinjiang, China, and Malus neidzwetzkyana (Dieck) Langenf. is believed to be a variant of Malus sieversii (Led.) Roem. The components and contents of polyphenols in the peel and pulp of the two kinds of fruit during the development period were detected, and the dynamic changes and differences in the polyphenols between the two kinds of fruit were discussed. The results showed that the total polyphenol content of ‘Xinye 13-11’ was higher in the peel and pulp than that of ‘Hongxun 2’, and the content of peel was higher than that of pulp in the two kinds of fruit. An analysis of five types of polyphenols showed that anthocyanins were only contained in the peel and pulp of ‘Hongxun-2’, and the peel had a higher content than the pulp. Cyanidin 3-O-galactoside was the main anthocyanin component. Four other types of substances, except hydroxycinnamics, were higher in ‘Hongxun-2’ than ‘Xinye 13-11’, while the contents of other substances in ‘Xinye 13-11’ were higher than those of ‘Hongxun 2’. The accumulation of major polyphenol components in the peel and flesh of ‘Hongxun 2’ and ‘Xinye 13-11’ apples was significant in the period before and after 65 days after flowering, and the contents of procyanidin B1 and procyanidin C1 were the highest in this period. In addition to the difference in anthocyanin content between ‘Hongxun 2’ and ‘Xinye 13-11’, the chlorogenic acid content in the peel and pulp of ‘Hongxun 2’ was significantly higher than that of ‘Xinye 13-11’, and the contents of other components were lower than those of ‘Xinye 13-11’. Moreover, based on the components and contents of polyphenol components, this paper supports the viewpoint that Malus neidzwetzkyana (Dieck) Langenf is a separate species to Malus sieversii (Led.) Roem.