Variation in key flavonoid biosynthetic enzymes and phytochemicals in Rio Red grapefruit (Citrus paradisi Macf) during fruit development

In the current study, phytochemical contents and expression of genes involved in flavonoid biosynthesis in Rio Red grapefruit were studied at different developmental and maturity stages for the first time. Grapefruit were harvested in June, August, November, January, and April and analyzed for the levels of carotenoids, vitamin C, limonoids, flavonoids, and furocoumarins by HPLC. In addition, genes encoding for phenylalanine ammonia lyase (PAL), chalcone synthase (CHS), chalcone isomerase (CHI) and 1, 2-rhamnosyltransferase (2RT) were isolated and their expression in grapefruit juice vesicles was studied. Fruit maturity had significant influence on the expression of the genes, with PAL, CHS, and CHI having higher expression in immature fruits (June), while 2RT expression was higher in mature fruits (November and January). The levels of flavonoids (except naringin and poncirin), vitamin C and furocoumarins gradually decreased from June to April. Furthermore, limonin levels sharply decreased in January. Lycopene decreased while β-carotene gradually increased with fruit maturity. Naringin did not exactly follow the pattern of 2RT or of PAL, CHS, and CHI expression, indicating that the four genes may have complementary effects on the level of naringin. Nevertheless, out of the marketable fruit stages, early-season grapefruits harvested in November contained more beneficial phytochemicals as compare to mid- and late-season fruits harvested in January and April respectively.