Daytime Red and Nighttime Blue LED Lighting Allows Injury-Free Production in Greenhouse Tomatoes Grown with Continuous (24 h) Lighting.

The improvement in plant growth and yield by supplemental lighting in greenhouse vegetable production is mostly determined by the total amount of light supplemented during a day (daily light integral (DLI) – intensity × photoperiod). It is more economical to achieve target DLIs with long photoperiod of supplemental light at low light intensity because it uses less light fixtures, reducing initial capital costs. Furthermore, a long photoperiod of lighting such as continuous lighting (CL, 24h) can reduce overall energy costs because the heat released by the light fixtures reduces heating demand during the otherwise dark night period and electricity price is usually lower during the night in a lot of jurisdictions. However, lighting longer than 16-17 hours causes photo-injury such as leaf chlorosis and limits the yield increase in greenhouse tomatoes. In a previous study, we found that 12h red (at 200µmol m-2 s-1 during the daytime) and 12 blue (at 50µmol m-2 s-1 during the night) LED lighting can eliminate photo-injury. In this study, we further expanded the lighting treatments to longer daytime/red light photoperiods to reduce daytime light intensity. The study was conducted in 4 greenhouses during winter 2019-20. Each greenhouse was divided into 4 sections using white curtains which were impenetrable to light. Eight lighting strategies including 12h, 16h, 20h red (started in daytime) followed by 12h, 8h, and 4h of blue LED lighting during the night, conventional control (16h of red and blue together followed by 8h night darkness), 16h red and blue together followed by 8h blue or red, CL control (24h red and blue lighting together), and reversed CL lighting treatment (12h blue during the daytime followed by 12h red during the night). All 8 lighting treatments had the same daily light integral (12 mol m-2 d-1). Plants grown under CL control and reversed CL showed severe leaf chlorosis and yield reduction. The treatment with 20h red and 4h blue LED lighting also showed some leaf chlorosis and yield reduction but at much less degree. There was no leaf chlorosis in the conventional control and other CL treatments with 8h or 12h of blue light during the night. The CL treatments with 8h of blue light during the night used 12% lower light intensity in the daytime than the conventional control, and thus is a good CL lighting strategy for reducing energy costs and initial capital costs for greenhouse tomato production with supplemental lighting.