Abstract
Flavonoids are widely distributed in plants and play important roles in many biological processes. Citrus fruits are rich dietary sources of flavonoids. However, there have been very few reports about the comprehensive metabolic profile and natural diversity of flavonoids in different tissues of various Citrus cultivars. In this study, based on the 7416 metabolic signals detected with non-targeted metabolomics approach, Principal Component Analysis revealed the flavedo has the largest differences from other tissues in metabolite levels; as many as 198 flavonoid signals were then detected in 62 Citrus germplasms from 5 species mainly cultivated worldwide, while 117 flavonoids were identified, including 39 polymethoxylated flavonoids (PMFs), 7 flavones, 10 C-O-glycosylflavonoids, 44 O-glycosylflavonoids, 10 C-glycosylflavonoids and 7 newly annotated O-glycosylpolymethoxylated flavonoids. Tissue-specific accumulations were observed: O-glycosylated flavonoids were abundant in all fruit tissues, while PMFs were accumulated preferentially in the flavedo. Among different species, mandarins had the highest levels of PMFs and O-glycosylpolymethoxylated flavonoids, followed by sweet oranges. Based on the flavonoid profiles, 62 germplasms could be clearly grouped into five distinct clusters via hierarchical clustering analysis, which were perfectly matched with their species, with sweet oranges and mandarins clustering closely and being further away from other three species.
Highlights
As one of the most widely cultivated fruit crops in the world, citrus fruits are rich in flavonoids, vitamin C, folate, dietary fiber and carotenoids[5]
Many studies have shown that polymethoxylated flavonoids (PMFs) and flavone O-glycosides are the most common flavones in citrus fruit, and their aglycones are apigenin, diosmetin and
PMFs glycosides were identified in Murraya paniculata leaves, which was the first time that the presence of PMFs glycosides in the genus was reported[25]
Summary
As one of the most widely cultivated fruit crops in the world, citrus fruits are rich in flavonoids, vitamin C, folate, dietary fiber and carotenoids[5]. The most widely cultivated Citrus are mandarins, oranges, pummelos, grapefruits and lemons[11]. Citrus fruits are one of the most important dietary sources of flavonoids, especially poly-methoxyflavones (PMFs)[17]. PMFs, one special group of flavonoids in which almost all hydroxyls are capped by methylation, are predominantly present in Citrus genus, especially in the peels of sweet oranges and wild mandarins[23, 24]. The phenolic compounds in citrus juices from Spanish cultivars were comprehensively characterized with the same method[31]. Despite this progress, the flavonoid metabolism is not systematically studied, and their naturally occurring variation in citrus germplasm remains elusive
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