Acta Hortic. 1227. ISHS 2018. DOI 10.17660/ActaHortic.2018.1227.65 Proc. Int. Symp. on New Technologies for Environ. Control, Energy-Saving and Crop Production in Greenhouse and Plant Factory - GreenSys2017 Eds.: Qichang Yang and Tao Li 517 Morphology and growth of ornamental seedlings grown under supplemental light-emitting diode lighting and chemical plant-growth regulators C.E. Collado, B.E. Whipker and R. Hernández a Department of Horticultural Sciences, North Carolina State University, Raleigh, NC, USA. Abstract The use of supplemental light emitting diode (LED) lighting to promote plant compactness can be an alternative to chemical plant-growth regulators (PGRs). The objective of this experiment was to compare plant growth and morphology under different supplemental lighting (SL) treatments in combination with a PGR application. Petunia, dianthus, and geranium were grown in a greenhouse until the plug stage under 3.2 mol m -2 d -1 solar daily light integral (DLI), 23.2±2.0°C, and 41.5±16% relative humidity. Seven growing treatments were used. Two treatments consisted of LED SL with different percentages of blue (B), green (G), and red (R) photon flux (PF): B19:R81 and B6:G5:R89. There were two SL controls: high-pressure sodium (HPS) and no SL (No-SL). Three light treatments included a PGR application: No-SL+PGR, HPS+PGR, and B6:G5:R89+PGR. SL treatments received 5.76 mol m -2 d -1 additional DLI (100 μmol m -2 s -1 PF for 16 h). Morphological and growth parameters indicated the benefits of SL. Petunia seedlings under B6:G5:R89 had 23-32% greater plant compactness than plants under supplemental HPS and B19:R81. dianthus and geranium showed no significant differences in plant compactness among SL treatments. Petunia and geranium seedlings grown under B6:G5:R89 had 49 and 21%, respectively, greater plant compactness than those under HPS+PGR. SL by itself promoted plant compactness in all plant species compared to plants grown with no SL and with an application of chemical PGR. In addition, for petunia, using SL with B6:G5:R89 yielded plants with greater compactness than plants under HPS+PGR. For dianthus, SL by itself yielded the same plant compactness as seen in plants under HPS+PGR. For geranium plants, HPS yielded plants with similar compactness as HPS+PGR. Keywords: PGR, DLI, supplemental lighting, floriculture, light-emitting diode INTRODUCTION Plant compactness is one of the most important morphological characteristics for young plant production. Chemical plant-growth regulators (PGRs) are commonly used to enhance plant vigor; however, they are under high legislative regulation because of their potential environmental risks. For example, paclobutrazol, a widely used chemical PGR in the triazoles group (Davis et al., 1991), is considered a highly toxic compound to aquatic invertebrates such as Daphnia (MDAR, 2012; Liu et al., 2016). Light-emitting diodes (LEDs) can be designed with a customized spectrum providing the opportunity to manipulate plant morphology using light signals. The objective of the present experiment was to compare plant growth and morphology under different supplemental lighting treatments and under a PGR application in order to potentially reduce the use of PGRs in horticultural operations. a E-mail: ricardo_hernandez@ncsu.edu