Effects of Supplemental LED Interlighting Irradiance Position on Photosynthesis and Aboveground Dry-Matter Weight Accumulation in Tomatoes Under Dense Planting

Supplemental interlighting is a technique to improve horticultural light conditions. However, optimal methods for energy-efficient supplemental lighting are not yet established. Therefore, this study investigated the influence of supplemental light canopy position during the tomato fruit enlargement stage on photosynthetic function and aboveground dry-matter weight. A supplementary interlighting module was fixed at the initial irradiation position, then the irradiation position for three other modules were raised to 10 cm above each fruit truss at different developmental stages. These stages were the early enlargement stage (ES), from flowering until the first fruit reached a diameter of 10 mm, then the vigorously enlarging stage (VES), with tomato fruit diameter from 10 to 30 mm, and the late enlargement stage (LS) with a tomato fruit diameter greater than 30 mm. Cultivation was carried out using a D-tray system with a planting density of 5.5 plants center dot m-2. The LED supplemental interlighting reduced specific leaf area (SLA), altering the plant canopy structure. This increased the canopy light transmittance from 40% to 70% at 20 cm from the canopy and from 20% to 40% at 40 cm from the canopy, especially during the ES. The total chlorophyll (Chl) content of leaves was higher under all irradiated treatments compared to the untreated control. However, Chl a/b ratios decreased for all treatments except in leaves under continuous LED irradiation. The maximum photosynthetic rate was higher in leaves closer to the supplemental interlighting exposure, but was lower in the 17th and 13th leaves at 6 mu mol center dot m2 center dot s-1 and 4 mu mol center dot m2 center dot s-1, respectively. Fruit dry-matter weight increased significantly to 143.2-156.5 g in all supplemental interlighting treatments compared with 119.6 g for the control. Interlighting treatment during VES achieved the highest yield and the greatest increase in fruit and total dry-matter weight. Therefore, VES-irradiation is most efficient to increase dry-matter weight.