Effects of Diffuse Glass on Light Distribution, Canopy Photosynthetic Rate, and Growth of Lettuce (Lactuca sativa L.) in a LED Plant Factory

Abstract

LED is one of the most important light sources in plant factory. However, LED is not well penetrating into shaded volumes of plants grown in a small confined space due to the close distance to the plants and its small emission surface. The aims of this study were to determine the effect of diffuse glass on light distribution, canopy photosynthesis, growth of lettuce in a LED plant factory and to analyze the light absorption of the plants by 3D simulation. Horizontal and vertical light distributions were measured, and canopy photosynthetic rate was estimated by measuring the carbon dioxide consumption of the plants in sealed growth chambers. Diffuse glasses with haze factors of 80%, 40% and non-diffuse glass were set under the LEDs. The plants were grown at 150±15 μmol m-2 s-1 PPFD with 18/6 photoperiod, 1000 ppm CO2 concentration, 22/18 °C (day/night), and 70% relative humidity. Light absorptions of 3-D lettuce models with vertical level were estimated by using 3-D simulation software. Under the diffuse glasses, horizontal light distribution was more uniform with deeper light penetration into the middle layer of the plants, and the growth indexes such as canopy photosynthetic rate, fresh and dry weights, and leaf area, LAI and SPAD-value were greater than the non-diffuse glass. The 3-D simulation results confirmed that diffuse glasses improved the light absorption efficiency in the top and the middle layers by reduction of the fluctuation of spatial radiation distribution, canopy distribution, and differences among in leaves.