LED Supplemental Light Intensity and Root Zone Heating, but Not Far-red Light, Influence Morphology, Leaf Color, and Anthocyanin Concentration of Petunia×hybrida during Rooting

In high-latitude regions (e.g., ≥40°), commercial greenhouse growers use supplemental lighting (SL) and root zone heating to decrease rooting time and increase the quality of young plants during periods of low solar irradiance. Light-emitting diode (LED) fixtures are increasing in popularity, but some growers have reported the development of purple foliage of select vegetatively propagated crops under LEDs, especially petunia ( Petunia × hybrida ), during rooting. The objective of this study was to mitigate petunia leaf purpling by modifying the photon spectrum and flux density (light quality and quantity) of SL and delivering root zone heating. Shoot-tip cuttings of SureShot ‘Dark Blue’ and ‘White’ petunia 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 (FR low ) or 10:18:59:13 (FR high ) at a total photon flux density (TPFD; 400–800 nm) of 70 or 120 µmol·m −2 ·s −1 . Cuttings of both cultivars grown under the FR low were generally more compact. Additionally, cuttings of both cultivars grown under a supplemental TPFD of 70 µmol·m −2 ·s −1 were typically taller with thinner stems than those grown under 120 µmol·m −2 ·s −1 . Cuttings of ‘Dark Blue’ grown under the higher TPFD featured more positive chromametric a* values and less positive b* values or displayed leaves that were overall more red and blue in color and also had higher total anthocyanin concentrations in their leaves compared with those under the lower TPFD. Cuttings of ‘White’ grown with a RZT of 25 °C were generally taller than those with a RZT of 21 °C, whereas cuttings of ‘Dark Blue’ grown with the higher RZT had relatively low anthocyanin concentrations in their leaves. Furthermore, cuttings of both cultivars generally had higher root dry masses when grown with the higher RZT. These results indicate that the severity of foliage purpling developed during propagation under LEDs, although cultivar-dependent, is not related to the percentage of FR in the SL. In addition, pigment accumulation can be mitigated by root zone heating or limiting the TPFD (<120 µmol·m −2 ·s −1 ) of SL during periods of low solar irradiance.