Seed propagation of bedding plants for spring markets commonly begins during the late winter months, when the greenhouse photosynthetic daily light integral (DLI) in northern latitudes can be as low as 1 to 5 mol·m–2·d–1 (Pramuk and Runkle, 2005; Styer, 2003). This low DLI can be detrimental to seedling quality, as previous research has determined that a DLI of 10 to 12 mol·m–2·d–1 is required for the production of high-quality seedlings (Pramuk and Runkle, 2005; Randall and Lopez, 2014). To remedy this issue, greenhouse operations use high-intensity electric lamps to provide SL, with a standard target PPFD of 70 to 90 µmol·m–2·s–1 (Lopez et al., 2017). Although HPS lamps are the current industry standard, LEDs have emerged as a competing SL source with regard to irradiance and efficacy (Nelson and Bugbee, 2014; Wallace and Both, 2016).
Another desirable attribute of LEDs is an increased life span. Although on/off cycles can reduce the life span of filaments and ignitors for conventional lamps, this issue is not present with LEDs (Morrow, 2008; Poel and Runkle, 2017). In addition, turning LED lamps on and off is instantaneous, and devices can be integrated easily into digital control systems for the manipulation of lighting duration and intensity (Morrow, 2008). For SL applications, the capability to control on/off cycles via quantum sensors and an established intensity threshold provides growers with greater electrical energy savings by not supplying SL to a crop when ambient levels in the greenhouse are deemed sufficient for plant growth. Thus, LED technology may provide substantial benefits over conventional lamps for SL, because advancements in sensors and control systems allow for more precise management of the light environment.
Because LEDs can be designed to provide a variety of narrow and broad wave bands, specific morphological or physiological responses in a crop can be targeted by adjusting the spectrum emitted by the lamps (Both et al., 2017; Massa et al., 2008). For example, Hernández and Kubota (2016) found that as the percentage of blue light provided by red:blue LEDs increased (up to 75%) at a light intensity of 100 µmol·m–2·s–1, seedlings of cucumber (Cucumis sativus) ‘Cumlaude’ displayed shorter hypocotyls and a smaller leaf area. Similar responses have been observed with bedding plant seedlings produced under sole-source LED lighting; seedlings grown under sufficient intensities of blue light (≥10 µmol·m–2·s–1) were often more compact and of higher quality (Randall and Lopez, 2014, 2015; Wollaeger and Runkle, 2015). Although many such responses have been observed under sole-source lighting environments, the benefit of spectrum manipulation for SL in the greenhouse remains uncertain.
When natural day lengths are short (e.g., <13 h), PL is commonly used in greenhouse environments to initiate or accelerate flowering for species with a long-day photoperiodic response (Craig and Runkle, 2012; Mattson and Erwin, 2005). Conversely, PL can be used for species with a short-day photoperiodic response to maintain a vegetative state. Traditionally, PL has been delivered by incandescent, halogen, or compact fluorescent lamps at a low intensity (1–2 µmol·m–2·s–1) primarily consisting of red (600–700 nm) and far-red (700–800 nm) wavelengths (Runkle and Both, 2017). More recently, low-intensity LED lamps have become an alternative because of their increased electrical efficacy, longer life span, and ability to manipulate spectral quality (Craig and Runkle, 2012; Morrow, 2008).
In recent years, some young plant propagators in northern latitudes have installed low-intensity LED lamps to extend the photoperiod to 16 to 24 h in an attempt to improve growth and development. Although this low-intensity photoperiodic lighting contributes very little to the cumulative DLI, some growers have reported that timing of both rooted cuttings (liners) and seedlings (plugs) is reduced compared with those provided no electric lighting (Sparks, 2016). Although the perceived increase in seedling quality is likely unrelated to increased biomass resulting from the low contribution of PL to DLI, impacts to seedling morphology may be present. For example, FR wavelengths are often emitted from PL lamps as a result of their role in flowering for species with a long-day photoperiodic response. However, FR light has also been shown to manipulate shade avoidance symptoms in many species, often characterized by increased stem elongation and leaf area expansion (Franklin and Whitelam, 2005; Park and Runkle, 2017). Therefore, although biomass may be unaffected, responses such increased leaf area under PL may be perceived as increased growth.
Although previous research has assessed the use of LEDs for greenhouse SL, to our knowledge, no detailed research has been conducted to evaluate the benefits of cycling LEDs using an instantaneous intensity threshold. In addition, with the release of new PL lamps, confusion still exists among growers regarding the benefits of PL beyond initiating, accelerating, or delaying flowering responses in photoperiod-sensitive species. Therefore, the objectives of our study were 1) to quantify seedling quality and production time of five commercially important species under continuous 16-h or instantaneous threshold SL, continuous low-intensity LED PL for 16 or 24 h with and without far-red light, or no electric lighting; and 2) to determine whether the described lighting treatments during propagation affect finished plant quality or flowering in a common finishing environment.
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