Day/Night Temperature Treatments Induced Alterations in the Process of Flower Bud Differentiation and Floral Coloration in Dahlia

Temperature is a key determinant of flowering time and floral pigmentation in plants, and dahlia (Dahlia pinnata) is particularly sensitive to thermal changes in its flower color. This study investigated how day/night temperature regimes regulate flower bud differentiation and coloration in the potted cultivar ‘Hypnotica Tropical Breeze’. Five temperature treatments (15/5, 20/10, 25/15, 30/20, and 35 °C/25 °C) were applied, and flower bud differentiation progression, colorimetric traits, and pigmentation-related differentially expressed genes were assessed. The 15/5, 20/10, and 25 °C/15 °C treatments all allowed successful bud differentiation and flowering, yet the duration varied considerably—46 days at 20 °C/10 °C and 41 days at 25 °C/15 °C versus 100 days at 15 °C/5 °C. Among these three regimes, 20 °C/10 °C gave the highest a* (red–green axis, positive = red) and lowest b* (yellow–blue axis, positive = yellow) at the ray floret tips. Moreover, the relative flavonoid and anthocyanin indices, as well as soluble sugar and starch contents, were all highest at 20 °C/10 °C and lowest at 15 °C/5 °C, with significant differences across treatments. Transcriptomic analysis revealed pronounced differential expression of anthocyanin-biosynthetic genes (CHI2, F3H, DFR, ANS, UFGT) and transcription factors (bHLH, MYB) among the three temperature groups. In particular, UFGT transcript abundance was lowest at 15 °C/5 °C, and FNSII expression was also reduced under this regime relative to 20 °C/10 °C. Together, these findings indicate that day/night temperature regimes are linked to bud differentiation and color development in dahlia, together with shifts in sugar metabolism and pigment-related gene expression.