Growth analysis of lettuce seedlings adding green or far-red to an optimized red and blue LED light spectrum

In vertical farms with artificial lighting, light spectrum management represents an opportunity to regulate plant growth and morphology. Red and blue spectral regions are among the wavebands most used in horticulture since they perform well both in terms of photosynthetic performances and energy efficiency. On the other hand, also green and far-red lights were recently shown to play a role on overall crop photosynthesis. In this research, the effect of adding green or far-red light to an optimized red and blue spectrum has been explored. Plants of green lettuce (Lactuca sativa L.) were grown for 25 days in a growth chamber. Plants were supplied with four LED light treatments (each replicated three times in light insulated compartments) under a constant PPFD of 250 µmol m^‑2 s^‑1. Light treatments included a control treatment (RB3) composed of an optimized red and blue light spectrum (R:B ratio of 3), a green added spectrum (RB3G), where the red and blue spectrum was partially substituted by 50 µmol m^‑2 s^‑1 of green light, a far-red added spectrum (RB3Fr) where 50 µmol m^‑2 s^‑1 of far-red light was added and, finally, a white light with a high component of green light (W). From 13 to 25 days after sowing (DAS), five destructive measurements (fresh and dry weight, leaf area and leaf number) were performed. Dry weight, fresh weight and leaf area were significantly increased at final harvest when supplied with either RB3Fr or W spectrum, while the effect of additional green light was present only on dry weight and leaf area. Growth analysis was applied in order to assess if and how plants would differently react to the light spectrum according to the different developmental stage reached. At 25 DAS, the yield increase obtained in seedlings grown with additional far-red and W spectrum rich in green light was mainly caused by an increase in the specific leaf area (SLA) throughout the growth cycle. W spectrum was found to be also particularly efficient in the early stage of growth, by increasing net assimilation rate (NAR) and leaf weight ratio (LWR).