Annual bedding plant sales for the 15 top-producing states were over $1.4 billion in 2012, the highest of any sector of the U.S. commercial floriculture industry (U.S. Dept. of Agriculture, 2013). Advancements in production of bedding plant seedlings, also known as young plants or plugs, have led to a large increase in finish plant quality and profitability (Armitage and Kaczperski, 1994; Kuehny et al., 2001). Young plant production occurs in late winter and early spring when the integrated photosynthetic photon flux (PPF), or daily light integral (DLI), can be 1 to 5 mol·m−2·d−1 or lower during cloudy weather in northern latitudes (Lopez and Runkle, 2008). Previous studies indicate that young and finished plant growth and quality are diminished by low DLI (Currey et al., 2012; Faust et al., 2005; Hutchinson et al., 2012; Lopez and Runkle, 2008; Oh et al., 2010). A DLI of 10 to 12 mol·m−2·d−1 has been shown to be a desirable minimum recommendation for growing high-quality young plants (Currey et al., 2012; Lopez and Runkle, 2008; Oh et al., 2010).
Previously, the only way for young-plant producers to appreciably increase ambient greenhouse DLI was to provide supplemental lighting (SL) from high-intensity discharge (HID) lights. High-pressure sodium (HPS) lamps are the most commonly used HID light sources, and several characteristics contribute to their use. However, HPS lamps primarily emit light in the spectral range of 565 to 700 nm, which is primarily yellow (565 to 590 nm), orange (590 to 625 nm), and red (625 to 700 nm), and have a peak at 589 nm. HPS lamps are ≈25% to 30% efficient with a lifespan of 10,000 luminous hours or more. Up to 75% of the energy not converted to light is emitted as radiant heat energy causing the surface of HPS lamps to reach temperatures as high as 450 °C and requires separation of lamps from plants to prevent leaf scorch (Fisher and Both, 2004; Nelson, 2012; Sherrard, 2003; Spaargaren, 2001).
LEDs are solid-state, semiconducting diodes that can emit narrow spectra of light from ≈250 nm to 1000 nm or greater and have been considered for use as sole source and SL (Barta et al., 1992; Bourget, 2008; Bula et al., 1991; Massa et al., 2008). The peak wavelengths of greatest interest for studies of plant growth and development include blue (450 nm), red (660 nm), and far-red (730 nm). Recently, LEDs have achieved an efficiency of 38% (red) to 50% (blue) converting electrical energy to photons (Philips Lumileds, 2011) and have an estimated lifespan of 50,000 h or greater (Bourget, 2008). Light-emitting diodes offer the ability to test wavelength combinations to manipulate plant morphology and control plant stature (Folta and Childers, 2008; Stutte, 2009).
Light quality has been shown to have a significant effect on plant growth, development, and physiology (Brown et al., 1995; Sage, 1992; Smith, 1982). Previous studies have focused on the use of LEDs as sole-source lighting in highly controlled and enclosed environments (Massa et al., 2008), as a SL source for intercanopy (Dueck et al., 2006; Hovi-Pekkanen et al., 2006; Trouwborst et al., 2010), or overhead (Dueck et al., 2012) lighting for greenhouse vegetable production, or propagation of ornamental cuttings (Currey and Lopez, 2013). Using LEDs requires determining the best light quality for each crop (Massa et al., 2008).
For example, when Zantedeschia jucunda K. Koch ‘Black Magic’ (calla lily) was grown in vitro under a total PPF of 80 μmol·m−2·s–1 of varying proportions of red and blue light from LEDs, stem elongation, but not dry mass, could be manipulated. As blue light increased from 0 to 32 μmol·m−2·s–1 and red light was reduced from 80 to 48 μmol·m−2·s–1 (red:blue ratio = 1.5), stem elongation decreased from 10.5 to 8.5 cm (Jao et al., 2005). In a separate study, van Ieperen et al. (2012) grew Cucumis sativus L. ‘Hoffman Giganta’ (cucumber) in growth chambers under LEDs providing a PPF of 100 μmol·m−2·s–1 of 100:0, 0:100, or 70:30 red:blue light over a 16-h photoperiod. Petiole length of plants grown under 70:30 red:blue LEDs was reduced by 1.0 cm, whereas stomatal density and net leaf photosynthesis increased by 248 mm−2 and 1.2 μmol CO2 per m−2·s−1, respectively, compared with plants grown under monochromatic red light. Hernández and Kubota (2012) demonstrated the benefits of greenhouse SL on the growth and development of Solanum lycopersicum L. ‘Komeett’ (tomato) seedlings grown under solar DLIs of 8.9 to 19.4 mol·m−2·d−1 and LED SL providing a PPF of 56 μmol·m−2·s–1. However, there were no significant differences in shoot dry mass, leaf count, stem diameter, hypocotyl length, leaf area, or chlorophyll concentration among the different LED SL treatments providing red:blue PPF ratios of 100:0, 96:4, or 84:16. Another study demonstrated no differences in productivity for greenhouse-grown tomato ‘Komeett’ and ‘Success’ grown under overhead HPS lamps or intracanopy LEDs towers providing 95:5 red:blue light (Gómez et al., 2013).
To our knowledge, no previous studies have quantified the effects of narrow-spectra high-intensity LEDs as a SL source for annual bedding plant seedlings. The objectives of this study were to: 1) quantify the effects of SL from three LED sources of different light quality and HPS lamps on seedling growth, morphology, and quality; and 2) determine whether there were any residual effects of SL source on subsequent growth and development after transplant in a common environment.
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