Plant factories with artificial lighting are widely used in many areas (Hahn et al., 1996; Ikeda et al., 1992; Ioslovich and Gutman, 2000; Kato et al., 2010; McAvoy et al., 1989; Morimoto et al., 1995) as an ideal model for precision agriculture (Murase, 2000), in which artificial lights play an important role in the precise control of the light environment. However, the widespread use of plant factories with artificial lighting is limited by their high initial investment and operation costs, which are mainly attributable to the costs of electricity for artificial lighting (Ohyama et al., 2001). With the continuous expansion of plant factories, reducing the initial investment and electric-energy consumption of artificial lighting has become increasingly important.
Several possible solutions for reducing the electricity consumption of lighting have been studied. Nishimura et al. (2001) estimated reductions of 50% in the electricity consumption of lamps by improving the efficiency of the lighting. Yamada et al. (2000) found that stepwise photosynthetic photon flux (PPF) control was a useful method for reducing the electricity consumption of lighting and increasing the electricity utilization efficiency. Bao et al. (2008) integrated a photovoltaic power generating system into a plant factory to reduce dependence on the commercial grid. However, these solutions either have limited effects on electricity savings or are associated with high costs of equipment construction.
As an attractive alternative to traditional light sources, LEDs have been widely used in potential energy saving applications such as rooms (Ryckaert et al., 2012), supermarkets (Elsevier Science, 2006), tunnels (Zeng et al., 2011), and even in coastal fishing boats (Matsushita et al., 2012).
Because of their tailorable spectral composition, wavelength specificity, and narrow bandwidth, LEDs have been widely used to examine the effects of different environmental light parameters on plant growth, phytochemical processes, or both. Kitaya et al. (1998) found that the leaf number of lettuce increased with increasing PPF. The phenolic concentration of lettuce has been reported to increase by 6% under supplemental red light, while supplemental far-red light decreased anthocyanin, carotenoid and, chlorophyll (Chl) concentrations by 40%, 11%, and 14%, respectively (Li and Kubota, 2009). Green-light supplementation has been reported to stimulate a rapid increase in the growth rate of etiolated arabidopsis seedlings [Arabidopsis thaliana (Folta, 2004)], ascorbic acid accumulation (Samuolienè et al., 2012), and lettuce growth (Kim et al., 2004), but showed a negative effect on biomass production (Folta and Maruhnich, 2007). Previous studies have also indicated that blue light is essential for leaf expansion and biomass production (Hogewoning et al., 2010; Johkan et al., 2012; Li et al., 2010) and also plays an important role in Chl synthesis (Kurilčik et al., 2008; Li et al., 2012; Poudel et al., 2008; Senger, 1982), but the much higher blue fraction in fluorescent lamps (FL), compared with high-pressure sodium lamps, incandescent lamps, and LEDs, may not effectively interact with the plant (Dougher and Bugbee, 2001). Li and Kubota (2009) have shown that the ascorbic acid concentration in lettuce is sensitive to none of the abovementioned spectra of light, reaching the opposite conclusion of Ohashi-Kaneko et al. (2007), who reported that the ascorbic acid content in leaf lettuce increased under irradiation by blue or red-blue light. However, few studies have focused on the electricity savings of LEDs.
A movable system was built for this study, and its benefits were evaluated by analyzing the electricity consumption of the lighting as well as the plant growth, plant physiology, and phytochemical accumulation of the lettuce grown under different lighting modes. The objective of this research was to develop a lighting system for lettuce production in a plant factory by halving the electric input of the original light sources while providing high PPF with low electricity consumption and maintaining lettuce yield and quality.
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