2019 ASHS Annual Conference

Supplemental lighting is essential for year-round greenhouse crop production in regions with low natural light conditions. The supplemental lighting for increasing daily light integral can be added via lengthening photoperiods or increasing light intensity or both. Light addition via long photoperiods is more economical because less light fixtures are required, assuming the long photoperiods do not affect the response of plants to lighting. However, photoperiods longer than 17 or 18 hours cause photo-injury such as leaf chlorosis in greenhouse tomatoes, cucumbers and sweet peppers, limiting the yield increase at long photoperiods of lighting. In our previous study, we found that light spectrum affected the response of greenhouse cucumbers to long photoperiods of lighting. This study was conducted to investigate the influence of light spectrum on the response of greenhouse tomatoes to long photoperiods of LED lighting. The study was conducted in a large greenhouse (200 m²) during winter 2018-19. The greenhouse was divided into 8 sections with white curtain and 4 combinations of LED lighting treatments (2 top light spectral compositions and 2 photoperiods) were applied in the 8 sections. The 2 light spectral compositions were 100% red or mixed light (red:blue:white =76:16:8). The supplemental light was provided at 150µmol m^-2 s^-1 for 17 hours (23:00 to 16:00) or 111µmol m^-2 s^-1 for 23 hours (17:00 to 16:00) at the same daily supplemental light integral (9.2mol m^-2 d^-1). Because it was still daytime between 16:00 to 17:00. The actual photoperiod was 18 hours and 24 hours (continuous light) for the 2 photoperiod treatments, respectively. The 24-hours photoperiod resulted in severe leaf chlorosis and yield decrease in comparison to the 18-hours photoperiod. There was significant interaction between photoperiods and light spectral compositions. The leaf chlorosis and yield decrease at 100% red light was significantly less than at the mixed light compositions, which clearly demonstrated that response of tomatoes to long photoperiod (continuous light) was affected by light spectral compositions.