In Northern latitudes, 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 have reported changes in crop morphology when cuttings of some species, especially petunia (Petunia ×hybrida), were grown under LEDs. The objectives of this study were to quantify how temperature and light influence the growth, nutrient content, and coloration of petunia. 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 two types of LED fixtures proving different light qualities each at a photosynthetic photon flux density (PPFD) 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 daily temperature (ADT) of 23 °C often had greater stem length and shoot dry mass than cuttings grown at an air ADT of 21 °C. Cuttings of both cultivars grown with a root-zone temperature (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 had horticulturally-desirable attributes of shorter stems, greater root dry mass than those grown under HPS lamps. Additionally, cuttings of ‘Dark Blue’ grown under LEDs emitting a moderate amount of blue light had lower phosphorus content than those grown under HPS lamps. Overall, cuttings of both ‘Dark Blue’ propagated under LEDs had higher anthocyanin content in their leaves and were more red and blue than those grown under HPS lamps. The coloration and anthocyanin content of ‘White’ were generally unaffected. To determine how far-red (FR) light influences petunia morphology and coloration, and additional study was conducted. Shoot-tip cuttings of petunia SureShot ‘Dark Blue’ and ‘White’ were propagated in 72-cell trays inside glass-glazed greenhouses with a root-zone temperature (RZT) of 21 or 25 °C. Cuttings were grown under sunlight supplemented by LEDs emitting a blue:green:red:far-red (B:G:R:FR) light ratio of either 10:7:82:1 (low G and FR) or 10:18:59:13 (moderate G and FR) at a total photon flux density (TPFD) of 70 or 120 μmol·m‒2·s‒1. Cuttings of both cultivars grown under LEDs emitting a higher R:FR ratio were generally more compact. Additionally, cuttings of both cultivars grown under a supplemental PFD of 70 μmol·m‒2·s‒1 were typically taller with thinner stem diameters than those grown under 120 μmol·m‒2·s‒1. Cuttings of ‘Dark Blue’ grown under a higher PFD were overall more red, more blue, and had higher total anthocyanin concentrations in their leaves than those under the lower PFD. Cuttings of ‘White’ grown with an RZT of 25 °C were generally taller than those with an RZT of 21 °C, whereas cuttings of ‘Dark Blue’ grown with higher RZT had overall lower anthocyanin concentrations in their leaves. Furthermore, cuttings of both cultivars had overall higher root-dry masses when grown with a higher RZT. These results indicate that suggest that the severity of foliage purpling developed during propagation under LEDs, while being cultivar dependent, is not related to the R:FR ratio of SL. In addition, pigment accumulation may be mitigated by root-zone heating or limiting the supplemental PFD (<120 μmol·m‒2·s‒1) during periods of low solar irradiance.
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