Supplementary Far-Red Light for Photosynthetic Active Radiation Differentially Influences the Photochemical Efficiency and Biomass Accumulation in Greenhouse-Grown Lettuce

Adding far-red (FR, 700-800 nm) light to photosynthetic active radiation (400-700 nm) proved to be a possible approach to increasing plant biomass accumulation for lettuce production in indoor vertical farms with artificial lighting as a sole-source lighting. However, how FR light addition influences plant growth, development, and metabolic processes and the optimal value of FR photon flux density for greenhouse-grown lettuce under sunlight are still unclear. This work aims to quantify the value of supplementary FR light with different intensities on lettuce morphological and physiological characteristics in a greenhouse. Lettuce 'Dasusheng' (<i>Lactuca sativa</i> L.) was grown in a greenhouse under seven light treatments, including white plus red LEDs with FR photon flux density at 0, 10, 30, 50, 70, and 90 µmol m^-2 s^-1 (WR, WR + FR10, WR + FR30, WR + FR50, WR + FR70, and WR + FR90, respectively), and lettuce grown with sunlight only was marked as natural light (NL). FR light addition improved the electron transport flux per cross section and performance index (PI_abs, PI_total) and decreased the changes in relative variable fluorescence of lettuce leaves compared to plants under NL. Specifically, the PI_abs of lettuce leaves were 41%, 41%, 38%, 33%, 26%, and 25% lower under control than in plants under treatments WR + FR90, WR + FR70, WR + FR50, WR + FR30, WR + FR10, and WR, respectively. Leaf number, leaf area, and biomass accumulation of lettuce followed a quadratic function with increasing FR light intensity and were the highest under treatment WR + FR50. The shoot fresh weight and dry weight of lettuce were increased by 111% and 275%, respectively, under treatment WR + FR50 compared to NL. The contents of vitamin C, reducing sugar, total soluble sugar, and starch in lettuce showed a similar trend with biomass accumulation. In conclusion, with commonly used photosynthetic photon flux density (PPFD, 400-700 nm) around 200 μmol m^-2 s^-1, supplementary FR light intensity of 30~50 μmol m^-2 s^-1 was suggested to enhance the photochemistry efficiency, biomass accumulation, and carbohydrates' contents in greenhouse-grown lettuce.