Magnesium enhances peel coloration in citrus by regulating carotenoid and chlorophyll contents and biosynthetic gene expression

• Mg application led to an increase in the Mg content in leaf, peel, and pulp of satsuma mandarin and navel orange. • Mg promoted citrus peel coloring in satsuma mandarin and navel orange by facilitating chlorophyll degradation and β, β-xanthophylls accumulation in the flavedo. • Mg improves pulp coloration in satsuma mandarin but has no significant impact on navel orange pulp. Magnesium (Mg) is an essential nutrient for crops, playing a crucial role in their growth and development. Previous research demonstrated that Mg enhances fruit coloration and improves the quality of Satsuma mandarin ( Citrus unshiu Marc . ). This study further explores the effects of Mg on fruit coloration in two citrus cultivars, Satsuma mandarin, and navel orange ( Citrus sinensis Osb.), and investigates the underlying mechanisms. Mg treatment significantly increased Mg content in the leaves, peel, and pulp of both cultivars. Concurrently, it reduced chlorophyll content in the flavedo of Satsuma mandarin (165–195 days after flowering, DAF) and navel orange (150–195 DAF) by downregulating chlorophyll biosynthesis genes ( GGDR, CHLH, CHLM, CHL27, PORA ) and upregulating chlorophyll degradation genes ( NYC, PPH, PAO, RCCR ). Chlorophyll content decreased by 9.23%–38.08% in Satsuma mandarin and 17.90%–44.64% in navel orange. Furthermore, Mg treatment enhanced peel coloration by promoting the accumulation of β, β-xanthophylls and ABA in the flavedo, mediated by the upregulation of carotenoid biosynthesis genes ( PSY, PDS, ZDS, LCYb1, LCYb2, ZEP ) and ABA metabolism-related genes ( NCED2 and NCED3 ). Notably, these effects exhibited significant variation between cultivars and developmental stages. While Mg improved pulp coloration in Satsuma mandarin, it had no significant effect on navel orange pulp. These findings underscore the critical role of Mg in regulating citrus fruit coloration and provide a scientific basis for optimizing Mg fertilization strategies to improve fruit quality and economic value.