End-of-day Manipulation of Phytochrome and Chryptochrome Using Light-Emitting Diodes Has Limited Effect on Bedding Plant Seedling Stem Elongation

High-quality bedding plant seedlings or plugs are compact, fully rooted transplants with a large stem caliper and high root dry mass. Controlling stem elongation is essential to producing a high-quality plug. End-of-day lighting (EOD) has been shown to promote or suppress stem elongation in certain plants. A high red:far-red ratio (R:FR) or blue (B) light has been used to suppress stem elongation while a low R:FR increases stem elongation in EOD treatments. Two experiments were conducted to study the effects of EOD light treatments on seedling stem elongation. Seedlings of Cosmos, Impatiens, Pelargonium, and Petunia (Expt. 1) and Capsicum, Petunia, Solanum, and Tagetes (Expt. 2) were grown at 21 °C under an 8.5-h photoperiod of ambient solar light and supplemental light from high pressure sodium lamps (HPS) that provided a PPF of 50 µmol·m^‒2·s^‒1 at canopy level. In Expt. 1, seedlings were placed under one of five EOD light treatments that delivered 30 min of light at the end of the photoperiod from either incandescent lamps (R:FR ≈0.78), compact fluorescent lamps [(CFL) R:FR ≈8.4], or LEDs that provided a photosynthetic photo flux (PPF) of 20 µmol∙m⁻²∙s⁻¹ of R:FR ≈212, 4.5, or 0.9, or a no EOD control. Similarly, in Expt. 2, seedlings were placed under one of five EOD light treatments that delivered a PPF of 20 µmol∙m⁻²∙s⁻¹ of varying proportions (%) R:B:FR light (100:0:0, 0:100:0, 75:25:0, 25:75:0, or 50:45:5) or a no EOD control. Only Petunia was significantly affected by light treatment for both experiments. In Expt. 1, height of Petunia was greatest under LEDs providing a R:FR ≈0.9 compared to other treatments in experiment. In Expt. 2, stem elongation of Petunia was reduced by 11%, 11%, and 10% for plants grown under 100:0:0, 75:25:0 and 25:75:0 R:B:FR LEDs, respectively, compared to 50:45:5 R:B:FR LEDs. Seedling height and quality were generally unaffected by EOD treatments.