High-Blue/Low-Red Mixed Light Modulates Photoperiodic Flowering in Chrysanthemum via Photoreceptor and Sugar Pathways

Chrysanthemum (<i>Chrysanthemum morifolium</i> Ramat.), a typical short-day plant (SDP), relies on photoperiod and light quality signals to regulate flowering and growth. Red light interruptions inhibit its flowering, whereas supplemental blue light can counteract this inhibitory effect. To investigate how "high-blue/low-red" mixed light (RBL) regulates chrysanthemum flowering and growth, we treated 'Gaya Glory' plants with 4 h of supplemental or night-interruptional RBL (S-RBL4 or NI-RBL4, 0 or 30 ± 3 μmol m^-2 s^-1 PPFD) under 10 h short-day and 13 h long-day conditions (SD10 and LD13; white light, WL; 300 ± 5 μmol m^-2 s^-1 PPFD), recorded as SD10, SD10 + S-RBL4, SD10 + NI-RBL4, LD13, LD13 + S-RBL4, and LD13 + NI-RBL4, respectively. Under SD10 conditions, S-RBL4 promoted flowering and enhanced nutritional quality, whereas NI-RBL4 suppressed flowering. Under LD13 conditions, both treatments alleviated flowering inhibition, with S-RBL4 exhibiting a more pronounced inductive effect. Chrysanthemums displayed superior vegetative growth and physiological metabolism under LD13 compared to SD10, as evidenced by higher photosynthetic efficiency, greater carbohydrate accumulation, and more robust stem development. Furthermore, S-RBL4 exerted a stronger regulatory influence than NI-RBL4 on photosynthetic traits, the activities of sugar metabolism-related enzymes, and gene expression. The photoperiodic flowering of chrysanthemum was coordinately regulated by the photoreceptor-mediated and sugar-induced pathways: <i>CmCRY1</i> modulated the expression of florigenic genes (<i>CmFTLs</i>) and anti-florigenic gene (<i>CmAFT</i>) to transmit light signals, while S-RBL4 activated sucrose-responsive flowering genes <i>CmFTL1/2</i> through enhanced photosynthesis and carbohydrate accumulation, thereby jointly regulating floral initiation. The anti-florigenic gene <i>CmTFL1</i> exhibited dual functionality-its high expression inhibited flowering and promoted lateral branch and leaf growth, but only under sufficient sugar availability, indicating that carbohydrate status modulates its functional activity.