Effects of LED spectrum and daily light integral on growth and energy use efficiency of tomato seedlings

Low temperature and weak light environment in winter and spring have become major bottlenecks restricting the production of high-quality seedlings in a greenhouse. Compared with conventional seeding production systems, the plant factory with artificial lighting (PFAL) can effectively shorten breeding cycle, while improve seedling quality. However, the high investment cost of infrastructure and power consumption have seriously effects on the commercial production of PFAL. Most previous studies focused on the monochromatic red (R)/blue (B) light or different R: B ratio created by mixture of red and blue lights in the seedling growth, not suitable for the commercial production. In this study, the effects of light quality and daily light integral (DLI) on the morphogenesis and biomass accumulation of tomato (Lycopersicon esculentum Mill.) seedling were investigated using the LED lighting environmental control in a PFAL, while, the electric energy use efficiency (EUE) and light energy use efficiency (LUE) of a system were evaluated, in order to reduce the electricity cost of a PFAL. Tomato seeds (cv. Fengshou) were sown in the 128-hole tray for 31 days. Four kinds of light sources were set, with the R: B ratio of 0.9 provided by a white LED lamp (L0.9), 1.2 and 2.2 provided by a white plus a red LED lamps (L1.2 and L2.2), and 1.8 provided by a white fluorescent lamp (F1.8) (control) along the three levels of DLIs (10.1, 12.6, and 15.1 mol/(m2•d), marked as D10.1, D12.6, and D15.1, respectively). The results indicated that the tomato seedlings grown under LEDs had the higher health index, biomass, and net photosynthetic rate than those under fluorescent lamps. The morphology of tomato seedlings showed remarkable differences among all treatments with the R: B ratio of 1.2. In the L1.2-D12.6 treatment, the seedling height of tomato, stem diameter, total leaf area, average daily growth of dry weight and health index were 16.6 cm, 3.7 mm, 86.1 cm2, 16.2 mg/d and 12.04, respectively, indicating much higher than those in other treatments. There was no influence of DLI on the photosynthetic capacity of tomato seedlings under the similar light quality, indicating the higher or lower DLI cannot contribute to the plant photosynthesis. The dry weight was 473.3 mg, when the seedling was grown in a DLI of 12.6 mol/(m2•d) under the white plus red LED with an R: B ratio of 1.2. The light quality significantly dominated the chlorophyll contents and net photosynthetic rate of tomato seedling leaves. The net photosynthetic rate reached the maximum of 15.4 µmol/(m2•s), when the tomato seedlings were grown in the white plus red LED with the R: B ratio of 1.2 under the DLI of 12.6 mol/(m2•d). The LUE and EUE were much higher in the white plus red LED with the R: B ratio of 1.2 than those in other treatments. Compared with other treatments, the total leaf area of tomato seedlings increased by at least 43% in the treatment of L1.2-D12.6, while, the ability of utilization and interception of light has also improved, indicating a great contribute to the highest LUE and EUE. The LUE and EUE reached the maximums of 0.078 and 0.026 in the treatment of L1.2-D12.6, indicating 79% and 321% higher than those in the F1.8-D12.6 treatment, respectively. An optimal combination to produce tomato seedlings was achieved, including the white plus red LED with the R: B ratio of 1.2 in the DLI of 12.6 mol/(m2•d). The finding can provide a promising LED lamps to replace the conventional fluorescent lamps, thereby to improve the seedling quality in tomato production. © 2020, Editorial Department of the Transactions of the Chinese Society of Agricultural Engineering. All right reserved.