Red–Blue and White Supplementary Light Increase Carotenoid Accumulation and Improve Fruit Quality in Satsuma Mandarin: Associations with Chlorophyll Content and Carotenogenic Gene Expression

Light regulates plant growth and secondary metabolism. To improve the nutritional quality and economic benefits of satsuma mandarin (Citrus unshiu Marc.) under greenhouse conditions with reduced natural light, we investigated the effects of supplementary lighting with different light qualities and intensities on fruit coloration, carotenoid accumulation, and fruit quality. Supplementary lighting was applied from 156 days after flowering using red, blue, red–blue combinations, and white light. All treatments significantly increased the color contribution index and total carotenoid content in both the rind and flesh compared with the control, with the red–blue light RB14 and white light W1 treatments being the most effective. β-Cryptoxanthin, the major carotenoid in satsuma mandarin, increased by 120.3% and 97.6% under RB14 and W1, respectively. Supplementary lighting also significantly increased the soluble sugar, total soluble solid, ascorbic acid, amino acid, flavonoid, and phenolic acid contents at most sampling time points. The leaf chlorophyll content increased, and chlorophyll degradation was delayed. qRT-PCR showed that RB14 and W1 upregulated the key carotenoid biosynthesis genes CitPSY, CitPDS, CitZDS, CitCRTISO, CitLCYE, CitLCYB, CitBCH2, CitZEP, and CitVDE and downregulated CitCCD4. These results suggest that supplementary lighting may enhance carotenoid accumulation and improve fruit quality by increasing the chlorophyll content and modulating carotenogenic gene expression, thereby providing a theoretical basis for citrus quality improvement.