Temperature Management and Supplemental Lighting Strategy Effects on the Propagation of Petunia ×hybrida Part I: Growth and Morphology

In northern latitudes, commercial greenhouse growers use 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 mainly 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 objective of this study was to quantify how light intensity during callusing, ADT, RZT, and SL sources influences the morphology, rooting, and quality (part I) and leaf pigmentation (part II) of petunia to mitigate leaf purpling. 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 LED treatments (low blue or moderate blue light) at a photosynthetic photon flux density of 60 or 120 μmol·m −2 ·s −1 for the first 6 d and 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 a greater stem length and shoot dry mass than those at 21 °C. 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 similarly higher quality (shorter stems and greater root dry mass) than those grown under HPS lamps. These results indicate that growers may have to adjust other environmental parameters, such as light intensity, ADT, and RZT, to influence the morphology and quality of cuttings.