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Abstract
In this study, the effects of light-emitting diode (LED) panels with different illumination schedules and mounted above butterhead lettuce (Lactuca sativa var. capitata) seedlings on lettuce growth and photosynthesis were examined, and the performance of the vertical and horizontal movable system on energy savings was evaluated. The illumination schedules used were fixed LED [F-LED (four LED panels illuminated the area below)] and movable LED [M-LED (two LED panels moved left and right once per day to illuminate the same area as F-LED)] at distances of 10 and 30 cm above the seedlings. The plant yields were uniform in all LED treatments. The highest light utilization efficiencies and lowest electricity consumption were found for the treatments with irradiation from a shorter distance above the seedlings. The true leaf numbers and ascorbic acid concentrations were the highest in the M-LED and F-LED treatments at a distance above the seedlings of 10 cm, while the leaf lengths and sucrose concentrations in these groups were significantly lower than those in the 30-cm treatment. These results indicate that illumination with M-LED can halve the initial light source input while maintaining yield and that sustained illumination from a shorter distance above the seedlings is the main factor in electricity savings.
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...