The production of young plants from seed (plugs) for spring bedding plant markets commonly begins during late winter and early spring (Styer, 2003). For high-quality plug production, the recommended DLI is 10 to 12 mol·m−2·d−1 (Pramuk and Runkle, 2005; Randall and Lopez, 2014). However, in greenhouses located in northern latitudes, the DLI is often insufficient during this time of the year, with DLIs as low as 1 to 5 mol·m−2·d−1 commonly reported (Fausey et al., 2005; Pramuk and Runkle, 2005). SL refers to the practice of increasing the amount of photosynthetically active radiation (PAR) available to plants in addition to what is supplied naturally through ambient solar radiation. Thus, through the provision of SL, high-quality young plants can be grown during times of the year when a lack of solar radiation may limit uniform and consistent production (Hernández and Kubota, 2012).
Numerous studies have reported that increasing the DLI with SL from HPS lamps improves young plant quality and reduces subsequent time to flower (TTF) for many bedding plant species (Hutchinson et al., 2012; Lopez and Runkle, 2008; Oh et al., 2010; Pramuk and Runkle, 2005). For example, Oh et al. (2010) observed increased seedling quality as the DLI increased from 7.6 to 17.2 mol·m−2·d−1 for petunia (Petunia ×hybrida ‘Madness Red’) and pansy (Viola ×wittrockiana ‘Delta Premium Yellow’). Seedling shoot dry mass (SDM) increased linearly as the propagation DLI increased and TTF was hastened for both species (Oh et al., 2010). Albright et al. (2000) documented a similar linear relationship between SDM and total accumulated radiation, from seeding to final harvest (35 d), for butterhead leaf lettuce (Lactuca sativa ‘Ostinata’). Likewise, Graper and Healy (1992) found that an increased DLI led to increased growth rate and partitioning of carbohydrates into sugars for petunia ‘Red Flash’ seedlings.
HPS lamps are the current industry standard for SL in greenhouses, commonly providing a PPFD (400–700 nm) of 70 to 90 µmol·m−2·s−1 to the plant canopy (Lopez et al., 2017). LEDs are a promising alternative to more traditional lighting sources, such as fluorescent, incandescent, and high-intensity discharge lamps, because of their energy-efficiency and long lifespans (Mitchell et al., 2012). However, advancements such as electronic ballasts and double-ended lamps have led to a competitive environment regarding the most efficient and cost-effective source for greenhouse SL. For example, recent studies have reported that commercially available LED fixtures are similar or have become more energy-efficient than double-ended HPS lamps (Nelson and Bugbee, 2014; Wallace and Both, 2016).
LEDs are solid-state semiconductor devices that are able to produce radiation with a very narrow spectrum (Stutte, 2009). Thus, one of the novel benefits from the use of LEDs is the ability to select wavelengths that elicit specific morphological or physiological plant responses (Morrow, 2008). For example, blue wavelengths of radiation (400–500 nm) serve a direct role in mediating stem extension and providing growth inhibition in a variety of crops (Cosgrove, 1981; Kigel and Cosgrove, 1991; Runkle and Heins, 2001).
Previous research found the use of experimental LED fixtures to be a viable SL method for the production of bedding plant seedlings and cuttings (Currey and Lopez, 2013; Randall and Lopez, 2014). For example, Currey and Lopez (2013) found little difference in the growth, morphology, and post-transplant performance of New Guinea impatiens (Impatiens hawkeri ‘Celebrette Frost’), geranium (Pelargonium ×hortorum ‘Designer Bright Red’), and petunia ‘Suncatcher Midnight Blue’ cuttings produced under SL providing a PPFD of 70 µmol·m−2·s−1 from either HPS lamps or experimental LED arrays with red:blue (R:B) radiation ratios (%) of 100:0, 85:15, or 70:30. Similarly, Randall and Lopez (2014) found the quality of snapdragon (Antirrhinum majus ‘Rocket Pink’), vinca (Catharanthus roseus ‘Titan Punch’), impatiens (Impatiens walleriana ‘Dazzler Blue Pearl’), geranium ‘Bullseye Scarlet’, petunia ‘Plush Blue’, salvia (Salvia splendens ‘Vista Red’), French marigold (Tagetes patula ‘Bonanza Flame’), and pansy ‘Mammoth Big Red’ seedlings grown under experimental LED arrays with R:B radiation ratios of 100:0, 85:15, and 70:30 providing a PPFD of 100 µmol·m−2·s−1 was similar to or greater than those produced under HPS lamps. Randall and Lopez (2014) determined seedling quality using the quality index (QI), an objective, integrated, and quantitative measurement by which to evaluate seedlings (Currey et al., 2013).
To our knowledge, no published research has evaluated the use of LED SL in a commercial setting. Therefore, the purpose of the study was to assess the use of LED fixtures manufactured to provide SL as an alternative to traditional HPS lamps for the production of common and specialty bedding plants in a commercial greenhouse. Specifically, the objectives of this study were to 1) evaluate the effect of SL source on the morphology and nutrient concentration of bedding plant seedlings; and 2) determine whether SL source during propagation or finishing influences finished plant quality or flowering.
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