Multiomics Insights into Flavonoid-Mediated Light-Induced Yellowing of the Curd in Loose-Curd Cauliflower ( <i>Brassica oleracea</i> var. <i>botrytis</i> )

China cultivates over 400,000 ha of cauliflower (<i>Brassica oleracea</i> var. <i>botrytis</i>), with loose-curd types accounting for 90% of production. Due to large curd size, light exposure during late growth leads to curd yellowing, reducing commercial value. Manual curd covering is widely used but is labor-intensive. This study used integrated multiomics to reveal that light-induced flavonoid accumulation is the primary cause of curd yellowing. Phenotypic and metabolite analyses showed higher flavonoid levels in uncovered curds, with baimaside (quercetin-3-<i>O</i>-sophoroside) identified as the key yellowing biomarker by UPLC-MS/MS. Transcriptomic analysis revealed 2976 differentially expressed genes enriched in flavonoid biosynthesis and light-responsive pathways. Dynamic clustering showed early gene repression under covering, correlating with deyellowing. Promoter and transcription factor analyses demonstrated that light-responsive regulatory networks mediate this process via cis-elements. Overall, the findings clarify that light-triggered flavonoid biosynthesis drives curd yellowing and offer a theoretical and technical foundation for breeding yellowing-resistant loose-curd cauliflower varieties.