S08 - Session P5 - Optimizing red, green and blue LEDs ratio of butterhead lettuce at seedling stage through mixture design based on growth parameters and its verification

Authors: EunJeong Lim *, Myung-Min Oh

Raising high-quality seedlings is one of the significant issues for intensive production of leafy vegetables to get a high yield at harvest. Light quality is a crucial factor affecting seedling quality which is associated with mature crop yield in a plant factory with artificial lighting. In this study, we aimed to determine the optimal ratio of red (R), green (G), and blue (B) light-emitting diodes (LEDs) for making high-quality lettuce seedlings using a mixture design method. Seedlings of butterhead lettuce 'Fairly' were cultivated under 10 different light spectra (R, G, B, RG 1:1, RB 1:1, GB 1:1, RGB 4:1:1, RGB 1:4:1, RGB 1:1:4 and RGB 1:1:1) based on simplex axial design after 3 days of sowing. After 2 weeks of treatment, all the seedlings were transplanted into white LED-based growth conditions and cultivated for 4 weeks to validate the effect of light quality at the seedling stage. We measured shoot fresh and dry weights, root dry weight, leaf area, ground area, and SPAD value at the end of the seedling stage and harvest. In addition, another cultivation was conducted to verify the new optimal RGB ratio from response surface. At first cultivation, R showed the best result in shoot fresh weight of seedlings, but the final result was not identical. RGB 1:4:1 at the seedling stage had the highest value at harvest. Among response surfaces of each growth parameter at the end of the seedling stage, the most identical parameter for final shoot fresh weight was leaf area×SPAD value. Based on this parameter, the optimal ratio of RGB LEDs during the seedling stage was 31:44:25. Moreover, seedlings under the optimal ratio had the highest leaf area×SPAD value, yet not in shoot fresh weight in the verification study. After 4 weeks of cultivation under white LEDs, what treated the optimal ratio at seedling stage recorded the best final production yield. Our results suggested that RGB mixed light with a large proportion of green light may positively affect making high-quality lettuce seedlings and leaf area×SPAD value, which is closely associated with photosynthesis ability, could be a suitable indicator for final production yield among various growth parameters at seedling stage. This research is supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIT) (No.2020R1A4A4079705). *Corresponding author: moh@cbnu.ac.kr