Different Distribution of Carotenoid Content in Periderm-Phloem-Xylem of Red Carrot Taproots

The color of carrot taproots is mainly determined by the types and contents of carotenoids and anthocyanins. Consistency in taproot color from the outside to the inside is an important indicator of carrot quality. However, most carrot cultivars exhibit inconsistency in taproot color across these tissues. This difference is primarily due to variations in carotenoid content and types in non-purple carrot, though the underlying mechanism remains unclear. In this research, four red carrot cultivars 'Benhongjinshi' (BHJS), 'Dayumeirenzhi' (DY), 'Betafruit' (BT), and 'Meiguihong' (MGH) were analyzed for carotenoid content using high-performance liquid chromatography. The expression levels of genes involved in carotenoid biosynthesis were also examined. The results showed that the total carotenoid content varied among the tissues: in the periderm, 'BHJS' had 0.4244 mg/g, 'DY' 0.4955 mg/g, 'BT' 0.5229 mg/g, and 'MGH' 0.5925 mg/g; in the phloem, 'BHJS' had 0.5757 mg/g, 'DY' 0.6234 mg/g, 'BT' 0.2744 mg/g, and 'MGH' 0.3948 mg/g; in the xylem, 'BHJS' had 0.2917 mg/g, 'DY' 0.0807 mg/g, BT 0.1748 mg/g, and 'MGH' 0.1073 mg/g. The total carotenoid content was lowest in the xylem across all four red carrot cultivars. The expression levels of genes DcLCYB1, DcLCYB2, and DcCCD4, which regulate carotenoid metabolism, differed in the periderm, phloem, and xylem of carrot taproots. These differences in carotenoid type and content across the periderm, phloem, and xylem contribute to the inconsistent color of red carrot taproots from outside to inside. This phenomenon may result from the differential expression of structural genes regulating carotenoid metabolism in these tissues.