Pigments in Citrus Fruit: Mutants, Compounds, Genes, and Beyond

Abstract

Pigments in citrus fruit include chlorophylls, carotenoids, and flavonoids. While chlorophylls color green citrus fruit, carotenoids and flavonoids are synthesized predominantly in citrus fruit after the color breaker stage. Various carotenoids are responsible for a spectrum of characteristic red, orange, and yellow colors for mature citrus fruit. Lycopene is the carotenoid producing pink to red in some sweet orange and grapefruit mutants while other carotenes and xanthophylls contribute to commonly seen orange to yellow colors. Anthocyanins, a subgroup of colored flavonoids, yield additional blood red colors only in blood oranges. Citrus fruit mutants rich in red lycopene or anthocyanins that generate additional organoleptic attraction and marketing appeal have been extensively used in various pigment studies and comparisons. Most biosynthetic genes and some regulatory genes in the carotenoid and anthocyanin pathways have been cloned and characterized. Pigmentation and gene expression changes during citrus fruit postharvest degreening and storage were also gradually uncovered. Genetic knowledge of citrus fruit pigmentation facilitates continuing research on and variety improvement for beneficial pigment compounds. Dietetics of some citrus pigments has been studied from both nutritional and preventive medicinal perspectives, with emphasis on lycopene and anthocyanins due to the predominant content in popular red-fleshed cultivars, special marketing appeal, and potential health benefits. Nutraceutical and medicinal benefits of consumption of citrus fruit (juice) include substantially improved health biomarkers for lipid profiles, low risk of obesity and cardiovascular disease, and potential suppression of some cancers. Regular moderate consumption of fresh citrus fruit and/or orange juice should be encouraged as part of a daily healthy diet.