The vegetable greenhouse industry in Quebec, Canada, was valued at approximately U.S. $50 million in 1999 with 12 ha dedicated to lettuce, representing ≈12% of the total greenhouse area that year (Carrier, 1999). Greenhouse production expanded greatly in the province in 1987 with the construction of 3.9 ha for tomato (Solanum lycopersicum), cucumber (Cucumis sativus) and pepper (Capsicum annuum) production under artificial lighting (Papadopoulos and Demers, 2000). The current situation for vegetable growers in Canada is challenging because of increased heating costs and costs of supplemental lighting required for year-round production.
The established practice for greenhouse growers interested in supplemental lighting technologies is to install HPS lamps and use them to extend the photoperiod of the crops to increase yields (McAvoy, 1984). However, this practice can be onerous for large installations, both in equipment and energy costs. Some other disadvantages of HPS lamps include heat generation and suboptimal spectrum for photosynthesis. LED lamps are a promising technology that has the potential to improve irradiance efficiency above HPS. Sustained developments in LED technology have brought their irradiance to a suitable level for being considered as a replacement to HPS lamps in hydroponics growth environments. LED lamps are anticipated to replace HPS lamps in most applications as a result of their reduced electricity consumption, improved quality of light, and the possibility for customization of the light spectrum for increased yields (Bourget, 2008; Morrow, 2008; Tennessen et al., 1994). Although equipment costs are still high, as is the case with most new technologies, growers across the world stand to substantially decrease their energy use, which directly translates into reduced costs for the greenhouse growers and reduced carbon emissions from the energy standpoint.
Earlier research reported that changes in irradiance wavelengths can result in changes in biomass production and morphology of plants because of changes in the ratio of the red/far-red spectrum (Brown et al., 1995; Heraut-Bron et al., 2001; Hoenecke et al., 1992; Taiz and Zeiger, 1998) and the effect of blue light (Hoenecke et al., 1992; Taiz and Zeiger, 1998). Moreover, changes in irradiance wavelengths can cause changes in the chlorophyll a/b ratio (Walters and Horton, 1995), chlorophyll biosynthesis and action spectrum (Anstis and Northcot, 1974; French, 1991; Koski et al., 1951; Ogawa et al., 1973; Virgin, 1993), β-carotene biosynthesis (Ogawa et al., 1973), and decreases in chlorophyll b under ultraviolet B light (Taiz and Zeiger, 1998).
Research has been performed to test the impact of light from LED lamps in several specific wavelengths, notably far-red, red, blue, and ultraviolet (Dougher and Bugbee, 2001; Okamoto et al., 1997; Yanagi et al., 1996). More recently, brighter diodes enabled their use as a potential replacement for traditional HPS systems in the 600- to 1000-W category of lamps (Steranka et al., 2002). Claims of 50% energy savings for similar biomass yields are now common in the marketplace (Craford, 2005). The aim of this study was to examine if LED lamps can produce similar biomass and phytochemical levels compared with HPS lamps at reduced energy cost for lettuce grown in a hydroponics setup.
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