Monochromatic light-emitting diode lamps (LEDs), emitting red and blue lights, revolutionized crop production in closed-system plant factories with artificial lighting in the early 1990s. The LED industry developed broad-spectrum white-LEDs by 2010, and many types of white-LEDs for home and office uses are now available for plant factory entrepreneurs. This paper tries to clarify whether these white-LEDs can be used as effective light sources in plant factories by examining what types of spectrum distribution are better suited for plant production. An experiment was conducted using seven LEDs, of which six were white-LEDs, to compare the performance in producing lettuce, and the results were compared with recent studies that used white-LEDs for growing lettuce under closed-system production conditions. Results showed that broad-spectrum white-LEDs performed significantly better than narrow-spectrum LEDs. Among lights in conventional color bands, red and blue lights give critical effects on plant growth, not in isolation but in combination; not too "cool" white LEDs perform better. Green and far-red lights also have some positive effects. Altogether, for a given light intensity, broad-spectrum white LEDs outperform narrow-spectrum LEDs. It is found that the spectrum distribution for white-LEDs to attain high productivity in lettuce production is such that the percentage share of photon flux density by conventional color band falls in the following ranges: 0% < blue < 30%, 0% < green < 50%, 30% < red < 70%, and 0% < far-red < 20%.
A plant factory with artificial lighting is a closed plant production system designed to produce high yield with good quality. At the high plant densities used in plant factories, outer leaves beneath the dense canopy get insufficient light and senesce fast, resulting in poor yield. Methods that retard the senescence of outer leaves would improve yield and profitability. Here, we examined the effect of supplemental upward lighting with different wavelengths (blue, green, red, and white) and intensities on the retardation of outer leaf senescence, photosynthesis, and plant growth in lettuce (Lactuca sativa L. var romana). Supplemental upward lighting of any type retarded senescence and decreased waste, improving the marketable shoot biomass. Blue and red were most effective, as both are efficiently absorbed by chlorophyll. White, blue, and red, but not green, significantly increased the ascorbic acid content in the outer leaves. These results indicate that supplemental upward lighting with blue, red, or white light-emitting diodes (LEDs) can achieve high yields of lettuce with low waste and a high ascorbic acid content.
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