In Canada, as in other northern regions, there is not enough natural light for production of many greenhouse commodities during the darker months of the year (i.e., October through February). In these regions, it is necessary for growers of year-round commodities, such as cut gerbera, to use SL to meet their crops’ economic minimum lighting requirements. Until recently, the only viable options for SL were high-intensity discharge (HID) systems such as HPS lamps. LED technology has improved significantly in recent years, with numerous proven horticultural applications in assimilation (both as SL in greenhouses and as sole-source lighting in growth chambers), photoperiodic, and photomorphogenic lighting (Nelson and Bugbee, 2014; Lopez and Runkle, 2017; Mitchell et al., 2015; Morrow, 2008). LEDs offer the promise of providing energy-efficient, wavelength-specific light in long-lasting fixtures (i.e., >50,000 h). Owing to their unique ability to modify the intensity of individual wavebands of light, LED fixtures can be customized to provide varying spectral recipes, potentially increasing quantum efficiency as well as providing greater plasticity for photoperiod and photomorphological control within a single fixture (Bourget, 2008). Morrow (2008), Pinho et al. (2012) and Currey and Lopez (2013) discussed many relevant horticultural considerations of both HPS and LED technologies.
Many commercially available horticultural LED systems are marketed as a direct replacement for conventional overhead HID greenhouse lighting systems. LED systems are often marketed as requiring 30% to 60% less electricity as HID systems to elicit the same photosynthetic effect on a crop. This is due to potentially higher efficacy (i.e., efficiency of converting electricity into light) and sole production of targeted wavelengths of blue (B, 400 to 500 nm) and red (R, 600 to 700 nm) light that closely match the maximum absorption bands for chlorophyll (McCree, 1972). Conversely, much of the radiation generated by HID systems falls in the green (G, 500 to 600 nm) region or outside of the PAR region altogether (Bergstrand et al., 2016). Therefore, LED-generated PAR may be more efficiently used in many horticultural production scenarios.
Most of the greenhouse-based LED scientific research has, thus far, focused on edible (Dueck et al., 2012; Gomez et al., 2013; Hernandez and Kubota, 2015; Martineau et al., 2012; Poel and Runkle, 2017) and potted floriculture commodities (Currey and Lopez, 2013; Poel and Runkle, 2017; Randall and Lopez, 2014). Many of these studies are over short time periods, either investigating a fast-growing crop (e.g., lettuce) or young plants (e.g., seedling development). Typically, these studies use consecutive replication strategies (i.e., treatments replicated over time), which can result in vastly different natural lighting conditions between replications. Moreover, many studies appear to have relied on fixed experimental plot locations for their treatments (i.e., treatment locations are not randomized between replications), which may not give proper consideration to varied environmental conditions that can occur within a greenhouse environment.
A largely untested application for horticultural LEDs is in the production of high-value cut flowers, where they could be used for assimilation, photoperiod, and photomorphological control depending on the crop, geographic region, and the time of year. The objective of this study was to determine whether LED SL can be used to replace HPS SL in cut gerbera production during the normal SL season in Ontario, Canada, by comparing harvest and postharvest metrics of crops growing under equivalent supplemental PPFD (µmol·m−2·s−1) in a concurrently replicated greenhouse trial.
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