Annual bedding plants is the largest segment of floriculture crop production in the United States, with a reported wholesale value of $1.29 billion in 2015 for operations with >$100,000 in sales in the 15 states surveyed (USDA, 2016). To coordinate production cycles and have finished crops ready for spring markets, bedding plants (and other floriculture crops) are grown from seeds and cuttings in controlled-environment greenhouses at high densities during winter and spring. During this period, the mean daily light integral (DLI) received outdoors in northern latitudes (e.g., >35 °N lat.) is as low as 5 to 10 mol·m−2·d−1 (Korczynski et al., 2002). Inside a greenhouse, DLI can be reduced by 50% or more by the glazing, structural components, and other obstructions (Fisher and Runkle, 2004). During the propagation phase, increasing DLI when it is ≤10 to 12 mol·m−2·d−1 can increase shoot biomass, rate of development, rooting, and plant quality while reducing flowering time (Currey et al., 2012; Lopez and Runkle, 2008; Pramuk and Runkle, 2005; Torres and Lopez, 2011). DLI can be increased during periods of low DLI with SL, which is usually provided by HPS lamps.
LEDs have shown promise as SL in horticultural applications (Hernandez and Kubota, 2014; Randall and Lopez, 2014, 2015). Compared with traditional HPS lighting, LEDs potentially have a greater electrical efficacy and longer life span (Nelson and Bugbee, 2014). For conventional lamps, on/off cycles reduce the lifetime of filaments and igniters, and electronic ballasts must be periodically replaced (Morrow, 2008). Additionally, by emitting specific wavebands of light, LEDs have the potential to provide a light spectrum that maximizes light absorption for growth and development by targeting the absorption peaks of chlorophyll and other important photobiological pigments (Mitchell et al., 2015).
The addition of ancillary wavebands of light to monochromatic LEDs has been shown to elicit photosynthetic and morphological responses in sole-source lighting (SSL) experiments. Cucumber (Cucumis sativus ‘Hoffmann’s Giganta’) seedlings grown under red (R, 600–700 nm) light alone from LEDs developed leaves that had reduced carbon dioxide (CO2) assimilation mediated by decreased stomatal conductance and stomatal count compared with seedlings grown under 7% blue (B, 400–500 nm) + 93% R (Hogewoning et al., 2010). Similarly, Goins et al. (1997) reported wheat (Triticum aestivum ‘USU-Super Dwarf’) grown under R + 10% B (from B fluorescent lamps) had more than twice as much CO2 uptake, and dry weight was 153% greater than that of plants grown under R alone. Wollaeger and Runkle (2014) reported that partial substitution of R or green (G, 500–600 nm) light with B decreased seedling height and leaf expansion. Impatiens (Impatiens walleriana ‘SuperElfin XP Red’), tomato (Solanum lycopersicum ‘Early Girl’), and salvia (Salvia splendens ‘Vista Red’) seedlings were grown under LEDs at a PPFD of 160 μmol·m−2·s−1 and the light quality was 100% R or R with an increasing percentage of B. Those grown under at least 25% B were shorter and had decreased leaf area compared with seedlings grown under R alone (Wollaeger and Runkle, 2014).
Few studies have been published on how light quality of SL influences plant growth in greenhouses. Randall and Lopez (2014) reported decreased height of vinca (Catharanthus roseus ‘Titan Punch’), celosia (Celosia plumosa ‘Fresh Look Gold’), impatiens ‘Dazzler Pearl Blue’, petunia (Petunia ×hybrida ‘Plush Blue’), marigold (Tagetes patula ‘Bonanza Flame’), and viola (Viola ×wittrockiana ‘Mammoth Big Red’) seedlings grown under 15% B + 85% R light from LEDs compared with those grown under HPS SL at a PPFD of 160 μmol·m−2·s−1. However, there were no differences in height for the same species grown under 30% B + 70% R LED SL compared with those grown under 15% B + 85% R LED SL. In the production of vegetable transplants, the amount of B in SL for a desired growth habit remains unclear. Hernandez and Kubota (2014) measured growth and development responses of cucumber seedlings grown under increasing B:R ratios from LEDs under average DLIs of 5.2 and 16.2 mol·m−2·d−1. At the low DLI, chlorophyll concentration increased as the B:R ratio increased, but dry weight, leaf area, and leaf number decreased. In contrast, at the higher DLI, B:R treatments had no effect on the same metrics.
For LEDs to achieve their potential as a delivery method for SL, seedlings and finished plants must be of a quality equal to or greater than that of those produced under HPS SL and be at least as cost-effective. Our objective was to quantify the effects of SL from four different commercial LED fixtures and HPS lamps on growth and subsequent development of seedlings of popular bedding plant crops. We postulated that relatively small changes to the radiation spectrum of SL, regardless of lamp type, would have little or no effect on seedling growth and subsequent flowering.
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