Radiation intensity and quality from sole-source light-emitting diodes affect seedling quality and subsequent flowering of long-day bedding plant species

Authors: CRAVER, JOSHUA - Purdue University; Boldt, Jennifer; LOPEZ, ROBERTO - Michigan State University

Submitted to: HortScience
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/27/2018
Publication Date: 10/1/2018
Citation: Craver, J.K., Boldt, J.K., Lopez, R.G. 2018. Radiation intensity and quality from sole-source light-emitting diodes affect seedling quality and subsequent flowering of long-day bedding plant species. HortScience. 53:1407-1415. https://doi.org/10.21273/HORTSCI13228-18.
DOI: https://doi.org/10.21273/HORTSCI13228-18

Interpretive Summary: Light-emitting-diodes (LEDs) have become an increasingly popular alternative to traditional lighting sources in controlled environments such as greenhouses and indoor production facilities. Benefits include higher energy efficiency, low radiant heat output, and the ability to target specific wavelengths of light. One successful production scenario for annual bedding plant seedlings is to start them indoors under sole-source lighting, then transplant and finish them in the greenhouse. However, when LEDs are used and only red and blue light is provided, a delay in flowering can occur in some photoperiodic long-day species. We evaluated the effects of light quality and light intensity on growth and nutrient uptake of annual bedding plant seedlings grown in a sole-source LED environment. Seedlings were grown at three light intensities (105, 210, or 315 µmol·m–2·s–1) under three light qualities (red:blue 87:13, red:far-red:blue 84:7:9, or red:green:blue 74:18:8) for four weeks. Next, they were transplanted and grown in the greenhouse until flowering. Light intensity was the dominant factor influencing seedling quality. Stem caliper, root dry mass, and shoot dry mass increased as light intensity increased, and stem length decreased as light intensity increased. All of these are metrics that yield a sturdy seedling plug better able to withstand shipping and transplant, and ultimately, a higher-quality finished plant. In terms of light quality, the addition of far-red light reduced time to flower in pansy. For growers looking to start bedding plant seedlings in an indoor, sole-source light environment, increasing the light intensity will improve seedling quality and inclusion of far-red light will minimize delays in flowering for some photoperiodic long-day species.

Technical Abstract: Light-emitting-diodes (LEDs) have become an increasingly popular alternative to traditional lighting sources due to their energy efficiency, low output of radiant heat, and ability to target specific wavelengths of radiation. Previous research has shown high-quality annual bedding plant seedlings can be produced using LED sole-source lighting (SSL). However, when only red and blue radiation are used, a delay in time to flower was reported when seedlings of some long-day species were subsequently finished in a greenhouse. Thus, our objectives were to 1) evaluate the effects of light quality and intensity in a SSL environment on the morphology and nutrient uptake of annual bedding plant seedlings, and 2) determine whether an increase in light intensity or the inclusion of far-red or green radiation in a SSL environment would promote earlier flowering of long-day plants at finish. Coreopsis (Coreopsis grandiflora ‘Sunfire’), pansy (Viola ×wittrockiana ‘MatrixTM Yellow’), and petunia (Petunia ×hybrida ‘Purple Wave’) seedlings were grown at light intensities of 105, 210, or 315 µmol·m–2·s–1, achieved from LED arrays with light ratios (%) of red:blue 87:13 (R87:B13), red:far-red:blue 84:7:9 (R84:FR7:B9), or red:green:blue 74:18:8 (R74:G18:B8). Four-week old seedlings were subsequently transplanted and grown in a common greenhouse environment. Regardless of light quality, stem caliper, root dry mass, and shoot dry mass of seedlings generally increased for all three species as the light intensity increased from 105 to 315 µmol·m–2·s–1. Similarly, stem length of all three species generally decreased as the light intensity increased. Regardless of light intensity, pansy seedlings grown under a light quality of R84:FR7:B9 flowered an average of 7 and 5 d earlier than those under R87:B13 and R74:G18:B8, respectively. These results provide information regarding the specific light parameters from commercially-available LEDs necessary to produce high-quality seedlings under SSL, with light intensity appearing to be the dominant factor in determining seedling quality. Furthermore, the addition of far-red wavelengths can significantly reduce time to flower after transplant and allow for a faster greenhouse turnover of some crops with a long-day photoperiodic response.