Analysis of Flavonoid Metabolism during the Process of Petal Discoloration in Three Malus Crabapple Cultivars

Abstract

Malus crabapple has high ornamental and ecological value. Here, the flavonoids in the petals of three pink Malus crabapple cultivars, Malus 'Strawberry Parfait' (GD), M. 'Pink Spire' (FY), and M. 'Hongyi' (HY), at the bud stage (flower buds are swollen, and the pistils and stamens are about to appear; L), full bloom stage (the flowers are fully open, and the stigma and anthers have recently appeared; S), and end bloom stage (the stigma and anthers are dry; M) were identified, and their abundances were determined. First, Kodak Color Control Patches were used to describe the colors of petals, and a colorimeter was used to determine the phenotypic values of flower colors. Flavonoids were determined using ultraperformance liquid chromatography-tandem mass spectrometry (UPLC-MS/MS). In all three crabapple cultivars, the red and yellow hues of the petals gradually disappeared, the color of the flowers changed from bright to dull, and the petals gradually faded. The extent of fading of the red hue of the petals was highest in GD, followed by FY and HY. A total of 302 metabolites were detected in the three cultivars. The content of total flavonoids in the three cultivars significantly differed, but there were no significant differences among species. The total flavonoid content of the three crabapple varieties was highest in HY, followed by FY and GD. The content of the anthocyanins delphinidin-3-O-sophoricoside-5-O-glucoside, pelargonidin-3-O-(6″-O-malonyl)glucoside, pelargonidin-3-O-glucoside, peonidin-3-O-glucoside, and cyanidin-3-O-arabinoside decreased significantly, which resulted in the discoloration of GD petals from L to M. The flavonoids and flavonols in FY might interact with anthocyanins in metabolic pathways. The content of these five anthocyanins decreased slowly, which resulted in the weaker discoloration of FY and HY compared with GD. The content of the five anthocyanins in HY did not decrease significantly, but the content of chalcone increased significantly, which might facilitate the production of anthocyanin auxiliary pigments and result in less pronounced fading of the petals. Cyanidin-3-O-arabinoside and pelargonidin-3-O-glucoside were the key flavonoids of the three crabapple cultivars. The total content and changes in anthocyanins were the key factors affecting petal color development and fading. Nonanthocyanin polyphenols, such as flavonoids, flavonols, and chalcone, are auxiliary pigments that affect petal fading. Overall, the results of this study provide new insights into the mechanism underlying the fading of the color of Malus crabapple flowers, and these new insights could aid the breeding of cultivars with different flower colors.

Figure 1

(a) Metabolites detected in Malus crabapple petals at different times. (b,c) PCA score graph of the mass spectrometry data of each group of samples and quality control samples. (d) Correlation graph between samples.

Figure 2

Metabolite volcano maps of petals in three different periods of Malus crabapple. (a–c) Metabolite volcano maps of petals in three different periods of GD; (d–f) metabolite volcano maps of petals in three different periods of FY; (g–i) metabolite volcano maps of petals in three different periods of HY. Note: Each point in the volcano plot represents a metabolite, and the abscissa represents the logarithm of the quantitative fold difference of a metabolite in two samples. The ordinate represents the VIP value. Green dots in the figure represent downregulated differentially expressed metabolites. Red dots represent upregulated differentially expressed metabolites, and gray dots represent metabolites detected but not significantly different.

Figure 3

Venn diagrams of differential metabolite numbers in the petals of GD, FY, and HY in three different periods. (a) Differential metabolite numbers of GD. (b) Differential metabolite numbers of FY. (c) Differential metabolite numbers of HY. (d) Differential metabolite numbers of L. (e) Differential metabolite numbers of S. (f) Differential metabolite numbers of M.

Figure 4

Clustering heat map of significantly different anthocyanins and differential abundance scores of petals of different cultivars of Malus crabapple in different periods. (a) Clustering heat map. (b–d) Differential abundance scores of GD, FY, and HY. Note: The vertical axis represents the differential pathway name, and the horizontal axis represents the differential abundance score (DA Score).

Figure 5

Flowering state diagram. (a) Open state diagram of GD. (b) Open state diagram of FY. (c) Open state diagram of HY.