Stem-tip cuttings of vegetatively propagated annual bedding plants are frequently propagated in late winter and early spring to meet the spring and early summer market demand for flowering bedding plants. However, this is also when seasonally low ambient outdoor DLIs (Korczynski et al., 2002) are further reduced inside a greenhouse by glazing material, interior structures, and hanging baskets suspended above benches (Faust, 2004; Hanan, 1998). Increasing DLI during propagation has been shown to enhance photosynthesis (Pn), growth, dry mass accumulation, and quality of rooted cuttings and reduce time to flower of transplanted cuttings (Currey et al., 2012; Currey and Lopez, 2012; Hutchinson et al., 2012; Lopez and Runkle, 2008). Whereas maintaining clean glazing material, minimizing superstructure, and reducing the density of hanging baskets may increase greenhouse DLI, the only way to appreciably increase DLI is to provide supplemental lighting.
There are several considerations when evaluating sources for providing supplemental light in a greenhouse, including light intensity, spectrum, electrical consumption, and uniformity of lighting patterns as well as financial considerations such as initial and ongoing maintenance costs measured with respect to return on investment (Sherrard, 2011). The most common supplemental light sources currently used in greenhouse production worldwide are high-intensity discharge (HID) HPS lamps, and several characteristics of HPS contribute to their popularity (Fisher and Both, 2004; Nelson, 2012; Sherrard, 2011; Spaargren, 2001). The majority of light emitted from HPS lamps is in the range of 565 to 700 nm, primarily yellow (565 to 590 nm) and orange (590 to 625 nm), with a peak at 589 nm. The efficiency for HPS lights is ≈25% to 30% and the lifespan for a lamp is 10,000 to 12,000 luminous h. However, up to 70% to 75% of the energy not converted to photosynthetic light is emitted as radiant heat, and the high surface temperature of HPS bulbs (up to 450 °C) necessitates adequate separation or distance between lamps and plants to avoid leaf scorch.
LEDs are solid-state, semiconducting diodes that can emit light from ≈250 nm to 1000 nm or greater (Bourget, 2008). Although low-intensity LEDs have been used for several decades for electronic applications, the development of LEDs with an output of 1 W or greater create the potential to use aggregates of LEDs (arrays) as supplemental photosynthetic light sources. There are several features of LEDs that make them attractive alternatives to HPS lamps. The most unique aspect of LEDs is the availability of narrow-spectrum light at wavebands of primary interest for plant growth and development, including blue (450 nm), red (660 nm), and far-red (730 nm). Additionally, LEDs currently have a luminous efficiency of 38% (red) to 50% (blue) in converting energy to light (M. Bourget, personal communication) and an estimated life of 50,000 luminous h or greater (Bourget, 2008) with increases in efficiency and lifespan occurring as technologies advance. The interest in the use of LEDs is increasing in commercial plant production, yet the impact of and science underlying the use of LEDs as a supplemental light source in ornamental plant propagation and production are not clear.
The narrow spectra of supplemental LED lighting have potential implications on morphology, development, and gas exchange of plants. First, although all photons between 400 and 700 nm (photosynthetically active radiation) are effective in driving photosynthesis, not all photons are equally efficient in their photosynthetic yield with two distinct broad peaks in the blue (400 to 500 nm) and red (600 to 700 nm) ranges (McCree, 1972). Therefore, the variation in relative quantum efficiency in the spectra of a supplemental light source may affect the efficacy of the light source in enhancing biomass accumulation. Additionally, there are several photoreceptors in higher plants such as phytochrome, cryptochrome, and phototropins that absorb specific wavelengths and mediate physiological responses (Briggs et al., 2006; Cashmore, 2006; Sharrock and Matthews, 2006). For example, the ratio of phytochrome in the physiologically active far red state (Pfr) to total phytochrome [Pfr + red phytochrome (Pr)] influences the magnitude of the shade avoidance response (i.e., stem elongation) for shade-intolerant species as well as flowering of photoperiodic plants with a long-day requirement for flowering (Glover, 2007; Thomas and Vince-Prue, 1997). Additionally, cryptochromes are blue light receptors that influence stem elongation.
Previous studies have focused on the use of LEDs as a sole light source in highly controlled and enclosed environments (Massa et al., 2008) or as a supplemental light source for overhead (Dueck et al., 2012) or intracanopy (Dueck et al., 2006; Hovi-Pekkanen et al., 2006; Trouwborst et al., 2010) lighting for greenhouse vegetable production. Furthermore, Currey et al. (2012) and Lopez and Runkle (2008) have reported the use of supplemental light to increase the DLI during root development of cuttings in propagation. However, little is known about the effects of supplemental light source and quality on growth, morphology, and gas exchange during vegetative propagation of herbaceous cuttings with special reference to narrow-spectra high-intensity LEDs. Our objective in the present study was to quantify the impact of supplemental light provided by LEDs or HPS lamps during root development of cuttings during propagation.
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