2018 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 don’t 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. Light spectrum could affect the response of plants to long photoperiods of lighting. Therefore, this study was conducted to investigate the influence of light spectrum on the response of greenhouse cucumber to long photoperiods of intra-canopy LED lighting. The study was conducted in a large greenhouse (200 m²) during the winter 2017-18. The greenhouse was divided into 4 sections (50m²/section) so that 2 top LED light spectral compositions (100% red or mixed - red:blue:white =76:16:8, at 18 h photoperiod and 140µmol m^-2 s^-1) could be applied with 2 replications. Four intra-canopy light photoperiods (18, 20, 22, and 24 h at 40µmol m^-2 s^-1 of LED lighting, 87% red and 13% blue) were applied to the 4 plots inside each section. The long photoperiods (even if at 24 hours) did not cause any significant difference in leaf chlorosis but did result in mis-shaping (cupping) in the middle leaves. The plants grown at 100% red top light had less mis-shaped leaves than at the mixed top light. Therefore, leaf chlorosis might not be photoperiodic in nature or might be more related to the physiological age of the leaf at the time of light exposure, and light spectrum did impact the response to long photoperiods. The perspectives of using different light spectra and intra-canopy lighting to extend photoperiods and reduce initial light fixture costs in supplemental lighting will be discussed.