| Summary: | Carnation (<i>Dianthus caryophyllus</i> L.) is a major floricultural crop, cultivated widely for cut flowers. This study was conducted to determine the optimal supplementary light source for the cutting propagation of carnation ‘Dreambyul’ cuttings. Terminal cuttings were propagated in a glasshouse with an average of 260 µmol·m<sup>−2</sup>·s<sup>−1</sup> photosynthetic photon flux density (PPFD) coming from the sun (the control), supplemented with one of three artificial light sources: mixed (red: blue: white = 6:1:1) light-emitting diodes (LED-mix), metal halide (MH) lamps, or high-pressure sodium (HPS) lamps. The supplementary light was provided from 7:00 to 17:00 h at 100 µmol·m<sup>−2</sup>·s<sup>−1</sup> PPFD during propagation. The cuttings were kept on a fogged bench in a glasshouse for 25 days with 24/15 °C day/night temperatures and 88% relative humidity. Compared with the control, better root formation was observed from cuttings grown under the supplementary lights after 10 days. After 25 days of propagation, MH significantly increased the root length, root number, root fresh and dry weights, and shoot biomass, shoot length, and shoot fresh and dry weights. The best root ball formation and the highest root activities were also found in cuttings propagated with supplementary MH light. Supplementary light increased the plant temperature, quantum yield, stomatal conductance, and the contents of chlorophyll, soluble proteins, and carbohydrates. Overall, the root formation and development of carnation ‘Dreambyul’ cuttings were significantly promoted by the three supplementary light sources. Of the three, MH was identified as the optimal supplementary light source.
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