A comprehensive and intuitive evaluation system for the quality of crabapples (Malus spp.) was established. In this study, 14 varieties of crabapples were tested. The core quality indexes were selected through variability analysis, correlation analysis, and principal component analysis (PCA) to establish the comprehensive quality evaluation model. The reliability of the model was verified by sensory evaluation and cluster analysis. In addition, the characteristics and utilization values of these varieties were investigated by cluster analysis. The results showed that there were significant differences in 18 quality indexes among the varieties and significant correlations among these quality indexes. PCA showed that the cumulative contribution rate of the first 5 principal components was 89.253%. Combining this result with those of correlation analysis, we found that single fruit mass, solid/acid ratio, sugar/acid ratio, and total flavanol content were the major quality indexes of crabapples. According to the comprehensive evaluation model, ‘123 Xiaopingguo’, ‘Longfeng’, ‘Cuifeng’ and ‘Huangtaiping’ had better fruit quality than the other varieties. The antioxidant activity of crabapple polyphenols showed a positive linear correlation with the contents of polyphenols and flavonoids with correlation coefficients of R²=0.837 and 0.728 for 2,2'-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid) (ABTS) cation radical scavenging capacity, R²=0.829 and 0.768 for 1, 1-diphenyl-2-trinitrophenylhydrazine (DPPH) radical scavenging capacity, and R²=0.575 and 0.969 for ferric ion reducing antioxidant power (FRAP), respectively. Likewise, the sensory evaluation scores were linearly correlated with the PCA scores (R²=0.862), indicating the reliability of the comprehensive evaluation model. The results of cluster analysis showed that the crabapple varieties could be divided into 4 categories. Category Ⅰ included M. orbusta and ‘Haihong’, which could be processed into health products with high VC content. Category Ⅱ included ‘Jixinguo’, ‘Longfeng’, ‘Cuifeng’ and ‘123 Xiaopingguo’, with good taste and being suitable for fresh consumption. Category Ⅲ included ‘Adam’, ‘Profusion’, M. micromalus, ‘Brandywine’, and ‘Red Splender’, which are suitable for making polyphenols-rich health foods. Category Ⅳ included ‘K9 Qingshaguo’, ‘Kelsey’ and ‘Huangtaiping’, which can be processed into preserved fruit, juice, wine and other products. The results were consistent with those of PCA, corroborating the reliability of the model. The comprehensive quality evaluation system can provide a theoretical basis for the development and utilization of different varieties of crabapples and also provide a good scheme for the quality evaluation of crabapples.

Anthocyanins are the most widely produced secondary metabolites in plants, and they play an important role in plant growth and reproduction. The nitrogen source is an important factor affecting anthocyanin production, but the nitrogen concentrations on metabolism and the underlying genetic basis remain unclear. In this study, in vitro anthocyanin induction was conducted on Malus spectabilis. The leaf explants were cultivated in media containing different nitrogen concentrations. The results suggested that when the nitrogen contents decreased in limit, the color of leaf explants turned from green to red, and increased anthocyanin accumulation led to a change in phenotype. Furthermore, the content of other flavonoids, such as dihydroquercetin, epicatechin, and catechin, increased under low nitrogen conditions. The transcription levels of the general flavonoid pathway genes, from phenylalanine ammonia lyase (PAL) to anthocyanidin synthase (ANS), were associated with the concentration of corresponding flavonoid compounds and phenotype changes. In particular, the expression level of ANS increased substantially under a low nitrogen treatment, which was significantly and positively correlated with the anthocyanin levels (R² = 0.72, P < 0.05). The increased expression patterns of anthocyanin pathway genes were similar to that of the transcription factor MYB10. We further verified MYB10 played an important role in the anthocyanin pathway in leaves of Malus spectabilis. These results suggested that we can improve the desirable ornamental plant phenotypes by controlling nitrogen content. This process may offer clues to further development of new agricultural practices.

To explore the potential of crabapples as functional food, polyphenols in crabapples and ‘Fuji’ apples were extracted, and the phenolic profile, total polyphenols, antioxidant activity and anti-proliferative activity against several human cancer cells were determined. The results indicated that crabapple extracts have more abundant phenols and higher total polyphenols (from 4.46 to 46.63 mg GAE·g⁻¹ DW) compared to ‘Fuji’ apples. Crabapple extracts possessed higher antioxidant activity than apple by DPPH and ABTS analysis. All fruit extracts exhibited inhibitory effects on proliferation in different cancer cells; however, crabapple extracts performed significantly better, with half inhibitory concentration (IC₅₀) values varied from 48.34 µg·mL⁻¹ to 974.81 µg·mL⁻¹ for colon cancer cells SW480, 64.67–1466.35 µg·mL⁻¹ for stomach cancer cells BGC-803, 78.88–910.64 µg·mL⁻¹ for esophageal cancer cells CaEs-17. Besides, the red crabapples had higher antioxidant activity and anti-proliferative activity than yellow fruits. These results showed that crabapples, especially red crabapples, have great potential as a healthy food, as they are rich in phenolic compounds with high antioxidant and anti-proliferative activities to cancer cells.