2017 ASHS Annual Conference

The objective of this experiment was to determine impact of different light spectral treatments on lettuce (Lactuca sativa L.) under sole source lighting (i.e. indoors only, no sunlight). Seedlings of lettuce ‘Black Seeded Simpson’ (BSS), ‘Parris Island’ (PI), and ‘New Red Fire’ (NRF) were germinated and established in rockwool. Fourteen days after seeding, transplants were established in 20 L hydroponic tubs with a complete nutrient solution at pH of 5.5 ± 0.3 and EC of 2.3 ± 0.3 dS⋅m^-1 at a spacing of 5 plants per square foot. Eight replicate plants per cultivar were randomly placed under one of four LED lighting treatments 1) R:B, 61% red (peak 665 nm) and 39% blue (peak 460 nm); 2) R:B:FR, trt #1 plus an additional 38 µmol⋅m^-2⋅s^-1 far-red light (peak 740 nm); 3) R:G:B, 61% red, 14% green (peak 530 nm), and 25% blue; and 4) W, a phosphor coated blue white LEDs (peaks at 455 and 550 nm). In all treatments, lights were adjusted to 180 µmol⋅m^-2⋅s^-1 at canopy level for 24 h continuous lighting resulting in a daily light integral (DLI) of 15.5 mol⋅m^-2⋅d^-1. Air temperature was maintained at 21 °C. Plants were destructively harvested 18 d after sowing, and the experiment was replicated three times. For BSS and NFF, shoot fresh weight (FW) and dry weight (DW) was greatest for the R:B:FR treatment and least for the B:R and W treatments. For example, FW of BSS was 147, 129, 114, and 110 g, respectively for R:B:FR, R:G:B, R:B, and W. FW of PI was 94, 78, 70, and 80 g, respectively for R:B:FR, R:G:B, R:B, and W. Cultivar NRF was less responsive to light treatment, there was a 10% reduction in FW under W and all other lighting treatments had similar biomass. Leaf surface area followed similar trends to FW/DW. While far-red led to increases in plant biomass it also led to earlier bolting of lettuce. This suggests the far-red light treatment might be useful in early plant development, but not for the entire crop cycle.