Carotenoid components and their biosynthesis in a bud mutant of Shiranui mandarin (Citrus reticulata Blanco) with citrine flavedo.

Carotenoids are secondary metabolite responsible for colored pigments in plants and microbes (Li et al., 2022). They are a class of C40 tetraterpenoids consisting of eight isoprenoid units, and can be classified into carotenes and xanthophylls on the basis of their functional groups (Saini et al., 2015). Carotenes can be linear (phytoene, phytofluene, and ζ|-carotene) or branched (ß|-carotene and α|-carotene). Xanthophylls comprise ß,ß|-xanthophylls (ß|-cryptoxanthin, zeaxanthin, violaxanthins, and neoxanthin) and ß,ε|-xanthophylls (α-cryptoxanthin, α-carotene, and lutein). Citrus fruits are complex sources of carotenoids, which are the principal pigments responsible for the typical orange color of most types (Chen, 2020). The difference in total carotenoid content and the diversity of carotenoid isomer proportion also accounts for other colors of citrus fruits, such as yellow, red, and pink (Chen, 2020).