2019 ASHS Annual Conference

Previous research in growth chambers showed that spectral changes at the beginning and end of the photoperiod can impact plant growth and morphology. In addition, our previous research showed that lettuce can be grown in long photoperiods (20h+) without a growth penalty. The objective of this project was to improve plant growth through light signals (low light intensity) during the hours without solar radiation. Four-day-old lettuce seedlings of two cultivars, ‘Red Sails’ and ‘Cherokee’, were grown in a greenhouse for 16 and 34 days under a 24 h photoperiod. From 0400 to 2000 (16 h) plants were grown under sun light supplemented with (average ± SD) 162.0 ± 3.2 μmol m^-2 s^-1 of 15% blue, 3% green, and 82% red (15B:3G:82R) PF ratio. From 2000 to 0400 (8 h) plants were grown under lower PF (24.9 ± 0.3 μmol m^-2 s^-1) of the light signaling treatments; the three light signaling treatments consisted of: 1) 16B:3G:80R (control), 2) 100B, and 3) 100R during 2000-0000 (4 h) followed by 100B during 0000-0400 (4 h) (100R-100B). Photoperiod and spectrum were controlled using the SmartPAR^TM system from LumiGrow®. The greenhouse was split in six zones (two repetitions), photosynthetic photo flux (PPF) and temperature were recorded in each zone. After 16 days of treatment, leaf area of ‘Red Sails’ in 100B was 14% greater than in control and 100R-100B. No differences were detected on fresh mass and dry mass between the three light signaling treatments. After, 34 days of treatment, plant diameter of both cultivars was 6% and 5% greater in 100R-100B than in 100B and control, respectively. However, no differences were detected on leaf area, fresh mass, and dry mass.