Young plants are commonly produced from vegetative cuttings or seeds during late winter and early spring (Klopmeyer et al., 2003; Styer, 2003). However, during peak young plant production, the average greenhouse photosynthetic DLI can be as low as 1 to 5 mol·m−2·d−1 in northern latitudes leading to decreased quality (Lopez and Runkle, 2008; Pramuk and Runkle, 2005). Previous research has determined that a DLI of 10 to 12 mol·m−2·d−1 is necessary to produce high-quality young plants (Currey et al., 2012; Hutchinson et al., 2012; Lopez and Runkle, 2008; Oh et al., 2010; Pramuk and Runkle, 2005; Randall and Lopez, 2014). The only way to appreciably increase DLI during young plant production is through the use of overhead SL (Oh et al., 2010; Randall and Lopez, 2014; Sherrard, 2003).
High-intensity discharge lamps, such as HPS and metal halide lamps, have traditionally been used for SL to increase greenhouse DLI. High-pressure sodium lamps have long been the most efficient SL source, converting ≈25% to 30% of their electrical energy into photosynthetically active radiation [PAR (400 to 700 nm)] with an operational lifespan of 10,000 luminous hours or more (Spaargaren, 2001). However, as much as 72% of the PAR emitted by HPS lamps is in the 565 to 590 nm (yellow) and 590 to 625 nm (orange) wavebands. Moreover, up to 75% of the electrical energy used by HPS lamps is emitted as radiant heat, and the surface of the bulb can reach temperatures as high as 450 °C, thus requiring plants to be separated from the lamps to avoid leaf scorch (Fisher and Both, 2004; Sherrard, 2003; Spaargaren, 2001).
In recent years, some alternatives to HPS lamps have been introduced, including plasma lamps and high intensity LEDs. Light-emitting diodes are solid-state, single junction semiconductors that are capable of producing light wavelengths as short as 250 nm and up to greater than 1000 nm. Thus, they are useful for testing specific wavelength combinations for plant growth and morphology (Folta and Childers, 2008; Randall and Lopez, 2014; Stutte, 2009). They also radiate minimal heat toward the plant canopy, allowing lights to be placed close to crops. Until recently, LEDs were low power (<1 W) and impractical for SL (Bourget, 2008).
Due to their small size, wavelength specificity, high light output, and relatively low heat output, LEDs have been used in environmental chambers for SSL (Heo et al., 2006; Wollaeger and Runkle, 2013, 2014) or in greenhouses as overhead SL (Currey and Lopez, 2013; Randall and Lopez, 2014) for ornamental young plants. For example, Heo et al. (2006) grew seedlings of African marigold (Tagetes erecta ‘Orange Boy’), ageratum (Ageratum houstonianum ‘Blue Field’), and salvia (Salvia splendens ‘Red Vista’) for 28 d at 25 ± 2 °C under a 16-h photoperiod from SSL LEDs delivering a PPF of 90 ± 10 µmol·m−2·s–1 (DLI ≈5 mol·m−2·d−1) at a 1:1 ratio of red:blue, blue:far-red, red:far-red light, or under cool-white fluorescent lamps (CWFL). After 28 d, leaf area of ageratum and salvia grown under the red:blue LEDs increased by 100% to 122% and 42% to 66%, respectively, compared with the other LED treatments and was similar to plants under CWFL. In addition, height of ageratum, marigold, and salvia was reduced by 35% to 69%, 44% to 56%, and 57% to 64%, respectively, under the red:blue LEDs compared with the other LED treatments while remaining similar to the plants under the CWFL (Heo et al., 2006). Another study compared seedlings of impatiens ‘SuperElfin XP Red’, petunia ‘Wave Pink’, tomato (Solanum lycopersicum ‘Early Girl’), and African marigold ‘Deep Orange’ grown under SSL with an 18-h photoperiod and PPF of 160 µmol·m−2·s–1 (DLI ≈9 mol·m−2·d−1) delivered from LEDs providing 10% blue and 10% green light with the following combinations (%) of orange (peak = 596 nm), red (peak = 634 nm), and hyper-red (peak = 664 nm): 20:30:30, 0:80:0, 0:60:20, 0:40:40, 0:20:60, or 0:0:80. Height of tomato and marigold was 18% and 13% shorter under the 0:40:40 than the 0:80:0 orange:red:hyper-red LEDs, respectively, but was similar to the other light treatments; and shoot dry mass (SDM) of tomato was 25% to 40% greater under the 0:60:20 orange:red:hyper-red than under SSL providing 0:40:40, 0:20:60, or 0:0:80 orange:red:hyper-red light (Wollaeger and Runkle, 2013). Finally, Randall and Lopez (2014) compared seedlings of snapdragon (Antirrhinum majus ‘Rocket Pink’), vinca ‘Titan Punch’, celosia (Celosia argentea ‘Fresh Look Gold’), impatiens ‘Dazzler Blue Pearl’, geranium ‘Bullseye Scarlet’, petunia ‘Plush Blue’, salvia ‘Vista Red’, French marigold ‘Bonanza Flame’, and pansy (Viola ×wittrockiana ‘Mammoth Big Red’) grown under AL supplemented with a PPF of 100 µmol·m−2·s–1 for 16 h from either HPS or one of three LED arrays composed of (%) 100:0, 85:15, or 70:30 red:blue light. After 28 d, height of all species except snapdragon and geranium was reduced by 9% to 55% under 85:15 red:blue LEDs compared with those under HPS lamps; and stem diameter of geranium, marigold, and snapdragon was 8% to 16% greater under 85:15 red:blue LEDs compared with seedlings grown under HPS lamps.
To our knowledge, no studies have compared annual bedding plant seedlings grown under SSL to those grown under SL in a greenhouse providing the same DLI. The objectives of this study were to 1) quantify the effects of SL from HPS lamps and LED arrays compared with a SSL multilayer production system from LED arrays providing two different light qualities on seedling growth, morphology, and quality and 2) determine whether there were any residual effects of either SL or SSL on subsequent flowering.
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