Effects of red–blue light ratio on seedling growth and specialized metabolism in tissue-cultured Nauclea officinalis

Light quality plays an important role in plant growth and metabolism, yet its integrated effects on physiology and molecular regulation in Nauclea officinalis remain unclear. In this study, tissue-cultured seedlings were exposed to white (W), blue (B), red (R), and mixed red–blue light treatments (R1:B1, R1:B2, and R2:B1). The results showed that blue light and the R1:B2 treatment significantly upregulated the light-harvesting genes LHCB5 and CAB7 , as well as the chlorophyll biosynthesis-related gene HEMA1 . Photosynthetic pigment accumulation showed a clear temporal response, peaking at 20 d; blue light increased chlorophyll a, chlorophyll b, and total chlorophyll by 49.81%, 150.16%, and 80.48%, respectively, compared with the control, though these effects weakened by 30 d. Total flavonoid and total phenolic acid contents both increased with time, but they displayed distinct response patterns to light quality: At 30 d, total flavonoid accumulation was highest under R1:B2, while total phenolic acid content remained relatively high under W, R, B, and R2:B1 but was lower under R1:B1 and R1:B2. Transcriptomic analysis showed that core flavonoid biosynthetic genes, (including CHS , CHI , FLS/F3H , F3’H , DFR , ANS , LDOX , and ANR), exhibited clear treatment-specific expression patterns, and several genes showed relatively higher expression under R1:B2 and R2:B1, consistent with the flavonoid accumulation trend. Red light mainly suppressed gene expression, whereas mixed red–blue treatments promoted genes associated with photosynthesis and secondary metabolism. Seedling growth was also most strongly promoted under R1:B2, with significant increases in biomass, plant height, and root length ( P < 0.05). Overall, the R1:B2 red-blue ratio was the most favorable for coordinating growth, photosynthetic performance, and secondary metabolite accumulation in tissue-cultured N. officinalis seedlings under controlled conditions, providing a reference for seedling-stage light regulation during controlled cultivation.