Supplemental, end-of-day, and sole-source lighting from light-emitting diodes influences growth, morphology, and quality of annual bedding plant seedlings
Annual bedding plants make up the largest sector of the U.S. floriculture industry. High-quality annual bedding plant seedlings are compact, fully rooted transplants with a large stem caliper and high root dry mass. However, production usually occurs in late winter or early spring when the daily light integral (DLI) is as low as 1 to 5 mol·m–2·d –1 in northern latitudes. Therefore, supplemental lighting (SL) is often used to increase the DLI to a recommended 10 to 12 mol·m –2·d–1. The objectives of this study were to: 1) quantify the effects of SL from three light-emitting diode (LED) sources of different light qualities and high-pressure sodium (HPS) lamps; 2) quantify the effects of end-of-day (EOD) light from incandescent and compact fluorescent lamps, LEDs of low, medium, and high red:far-red light ratios, or mixed red:blue:far-red ratios (Expt. 2); and 3) quantify the effects of ambient light and SL from LEDs, HPS, and plasma (PL) lamps in a greenhouse, and the effects of sole-source light (SS) from LEDs with two different light qualities in a growth chamber vertical production system (Expt. 3) on seedling growth, morphology, quality, and subsequent flowering. Supplemental light proportions (%) from LEDs (Expt. 1 and 3) ranged from 100:0 to 70:30 red:blue light, and EOD (Expt. 2) LED light proportions ranged from 100:0 to 50:50 red:far-red or 100:0:0 to 0:100:0 and 62:33:5 red:blue:far-red light. In Expt. 1, stem elongation of Catharanthus, Celosia, Impatiens, Petunia, Tagetes, Salvia, and Viola seedlings was reduced 9%-55% under 85:15 red:blue LEDs compared to HPS lamps. In Expt. 2, stem elongation of Petunia seedlings after 21 d of EOD light was reduced up to 48% under LEDs providing a red:far-red light ratio ≥4.5 compared to ≈0.9; and 10%-11% under 100:0:0, 75:25:0, and 25:75:0 red:blue:far-red LEDs compared to 62:33:5 red:blue:far-red LEDs. In Expt. 3, stem elongation of Pelargonium, Petunia, and Tagetes was reduced up to 79% under SS compared to HPS lamps. Quality index, a quantitative measurement of quality, of Petunia was 44%–129% greater for seedlings grown under all light treatments compared to ambient solar light (control). Additionally, with the exception of Impatiens, time to flower was similar or reduced for seedlings propagated under light treatments compared to the control. Overall, the results obtained from these experiments indicate that SL, EOD lighting, and SS can be used to enhance seedling growth, morphology, and quality without substantially delaying subsequent flowering of the annual bedding plant species tested.^
Roberto Lopez, Purdue University.