2019 ASHS Annual Conference

Supplemental lighting is essential for year-round greenhouse crop production in regions with low natural light conditions. Long photoperiods of supplemental lighting such as continuous (24h) lighting (CL) could significantly improve crop growth and yield at the same light fixture costs. Furthermore, CL can increase overall energy efficiency because the heat released by the light fixtures reduces the heating requirement during the otherwise dark night period. However, lighting longer than 17 or 18 hours causes photo-injury such as leaf chlorosis in greenhouse tomatoes, cucumbers and sweet peppers, and limits the yield increase with CL. Our previous study found that light spectrum can affect the response of plants to long photoperiods of lighting. Therefore, we investigated the response of greenhouse tomato to CL with alternating red and blue LED light in this study. The study was conducted in 2 greenhouses, each with 50 m² of growing area during winter 2018-19. Each greenhouse was divided into 2 sections using white curtains which were impenetrable to light. Two lighting strategies (12 h lighting of red and blue together followed by 12 h darkness (control), and 12 h of red followed by 12 h of blue lighting (CL with alternating red and blue LED)) were applied to the 2 sections. The two lighting strategies had the same daily light integral. Plants grown under CL with alternating red and blue LED lighting were taller and had larger leaf area than the plants grown with 12 h of lighting (red and blue together, control) after 3 weeks of lighting treatments. The difference became insignificant after 50 days of lighting. There was no leaf chlorosis on plants grown under CL with alternating red and blue LED light and no difference on leaf chlorophyll, chlorophyll fluorescence and quantum yield with the 12 h (control) lighting treatment. Therefore, the CL lighting strategy with alternating red and blue LED light eliminated the photo-injury with CL.