In high latitudes (≥40°), commercial greenhouse growers utilize supplemental lighting (SL) and heating to offset low solar radiation, air average daily temperature (ADT), and root-zone temperature (RZT) during peak young-plant production. Growers have historically used high-pressure sodium (HPS) lamps to deliver SL but are transitioning to light-emitting diode (LED) fixtures, mostly because of their improved energy efficacy. However, many growers report changes in crop morphology and undesirable purple leaf pigmentation when cuttings of some species, especially petunia (Petunia ×hybrida), were grown under LEDs. The objectives of this study were to 1) quantify how light intensity during callusing, ADT, RZT, and SL sources influence the morphology, rooting, leaf pigmentation, and quality of petunia and to 2) develop strategies to mitigate the purpling of leaves. Shoot-tip cuttings of petunia SureShot ‘Dark Blue’ and ‘White’ were inserted into 72-cell trays and propagated inside a greenhouse at an air ADT of 21 or 23 °C and with an RZT of 21 or 25 °C. Cuttings were grown under SL delivered by HPS lamps or LED fixtures proving different light qualities (low blue or moderate blue) at a photosynthetic photon flux density of 60 or 120 µmol·m–2·s–1 for the first 6 d, then 120 µmol·m–2·s–1 for the remaining 16 d. Cuttings of both cultivars grown at an air ADT of 23 °C often had greater stem lengths and shoot dry masses than cuttings grown at 21 °C, as well as lower concentrations of anthocyanins. Cuttings of both cultivars grown with an RZT of 25 °C typically had longer stems than those grown with an RZT of 21 °C. Overall, cuttings of both cultivars propagated under LEDs were of greater quality (shorter stems, greater root dry mass) than those grown under HPS lamps. The color of cuttings grown under LEDs were more red and blue than those grown under HPS lamps, especially at low ADT and RZT. Additionally, the anthocyanin content of ‘Dark Blue’ cuttings grown under LEDs was greater than those grown under HPS lamps. Little differences were observed between cuttings grown under either LED fixture. These results indicate that growers using LEDs may have to adjust other environmental parameters, such as light intensity, ADT, and RZT, to produce cuttings of similar morphology and quality to those grown under HPS lamps.
