2018 ASHS Annual Conference

Quality of light plays an important role not only in photosynthesis and plant morphogenesis but also in the accumulation of many health-promoting phytochemicals. In this study, we investigate the effect of blue, red and far-red spectra on the growth characteristics and the phytochemical accumulation in lettuce (Lactuca sativa L. green-leaf variety ‘Two Star’ and red-leaf variety ‘New Red Leaf’) using light emitting diodes (LEDs). Seedlings were grown in Metro-Mix 360 in a growth chamber equipped with fluorescent lighting (PPFD 288 μmol.m^-2.s^-1) at 22^oC and 12 h photoperiod. Two-week old seedlings were treated with supplemental LEDs until harvest in a growth chamber at 22^oC and 12 h photoperiod. Control plants were grown with fluorescent lighting (274 μmol.m^-2.s^-1) as described above. Supplemental lighting generally produced better growth in both green and red-leaf varieties, however, the largest increase in the accumulation of biomass and plant height was in plants exposed to supplemental far-red LEDs. In red-leaf lettuce, red LEDs increased the accumulation of many phenolic acids and flavonoids while far-red light appeared to have a negative effect on the content of these phytochemicals. Significant increase in the accumulation of phenolic acids namely, caffeic acid and chicoric acid, and flavonoids such as luteolin, rutin and kaempferol occurred in red-leaf lettuce under supplemental red light while conversely, far-red light reduced the content of these phytochemicals compared to the control. For example, the caffeic acid content in ‘New Red Fire’ increased by 13-fold under supplemental red light while it decreased to less than 30% of the control value under far-red light. In the case of green-leaf lettuce, blue light had a positive impact on the accumulation of health-promoting phytochemicals. Levels of phenolic acids such as chlorogenic acid and chicoric acid, and flavonoids such as rutin, apigenin and kaempferol increased significantly under blue light. The largest increase was in the level of rutin which was approximately 20 times higher than that in the control. The results show that supplemental lighting and its spectral quality have a significant effect on the growth and the phytochemical content of lettuce and the response, however, varied depending on the variety.