Development of spatial-close lighting device based on lettuce canopy shading characteristics

Severe shading on lower canopy leaves has limited growth potential in leafy vegetables in modern agriculture. This study aims to investigate the effects of light and shade conditions on individual leaves at different growth stages, according to the light interception characteristics of lettuce. A spatial-close lighting system (SCL) was also developed to identify shading patterns. Light sources were arranged in proximity around the lettuce canopy from the top and lateral sides, enabling supplemental lighting in shaded leaf regions and dynamic beam activation. The results showed that the lettuce had only developed 6 leaves at 10 day after transplanting, with a small leaf area and uniform spatial distribution. Minimal mutual shading was achieved with a shading rate of only 12%, and the shaded leaves were distributed at a height of 1.5-3.0 cm above the cultivation board. At 11-20 days after transplanting, the lettuce entered a rapid growth stage, during which 9 new leaves emerged. Leading to a substantial rise in the shading rate to 57%, and the shaded leaves were distributed at a height of 1.5-5.0 cm above the cultivation board. During 21-25 days after transplanting, the rapid growth continued to simultaneously expand the shaded and lighted area. The 6 additional leaves with small sizes were produced to concentrate at the top of the canopy. The shading rate only slightly increased to 62%, and the shaded leaves were distributed at a height of 7.0-13.0 cm above the cultivation board. Furthermore, light distribution within the canopy was extremely uneven. At 25 days after transplanting, the average photosynthetic photon flux density (PPFD) of top layer leaves ranged from 160.7 μmol/(m²·s) to 179.5 μmol/(m²·s), whereas that of bottom layer leaves was only 52.0 μmol/(m²·s) to 55.5 μmol/(m²·s). The lower, middle, and upper LED modules of SCL were installed at the central heights of major functional leaves at different growth stages (3, 8, and 13 cm above the cultivation panel), particularly with dynamic power switching, according to plant height. Cultivation experiments demonstrated that the energy-saving lighting reduced the leaf shading rate by 39.6%, whereas the light intensity increased by 43.3% and 11.0%, respectively, in the lower and middle canopy. Aboveground fresh weight of lettuce, energy use efficiency, and total leaf area increased by 23.1%, 193.0%, and 13.0%, respectively, whereas energy consumption per unit yield decreased by 65.8%. Light exposure on shaded lettuce leaves was alleviated for the optimal light interception on the entire plant canopy. Favorable light conditions can be expected for efficient and high-quality lettuce production in plant factories.