The high temperature may cause difficult growth or bloom in the summer, which is the key problem limiting the cultivation and application of dahlia. The photosynthetic physiological mechanisms of dahlia under high temperature stress were studied to provide a theoretical basis for expanding the application range of cultivation and annual production. Two dahlia varieties, ‘Tampico’ and ‘Hypnotica Tropical Breeze’, were used as test materials and were treated for 1 d or 2 d at temperatures of 35/30 °C or 40/35 °C (day/night: 14 h/10 h) and then recovered at 25/20 °C for 7 d. A 25/20 °C treatment was used as the control. The results are as follows: (1) High-temperature stress resulted in the chlorophyll (Chl) content, Fv/Fm, transpiration rate (Tr), net photosynthetic rate (Pn), and water potential decreasing significantly, and the Chl content, Tr, and stomatal density of ‘Tampico’ were higher than those of ‘Hypnotica Tropical Breeze’ during the same period. (2) After the two dahlia varieties were treated with high-temperature stress and recovered at 25/20 °C for 7 d, the plant morphology and various physiological indices under the 35/30 °C treatment gradually returned to normal, with ‘Tampico’ in better condition than ‘Hypnotica Tropical Breeze’. (3) Both dahlia varieties could not withstand the stress of 40/35 °C for 2 days.

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.