Because indoor vertical farms require sole-source lighting, light spectra that maximize desired crop attributes are pivotal. Although previous research has revealed growth responses of leafy greens to blue (B), red (R), and far-red (FR) light, the combined effects of the FR fraction [FR:(R FR)] and the B photon flux density (PFD) on biomass accumulation and nutrient uptake in lettuce (Lactuca sativa) remain unclear. We conducted an indoor hydroponic experiment on red-leaf lettuce ‘Rouxai’ using a randomized complete block design (with four blocks) to investigate how the FR fraction and the B PFD influenced growth, morphology, coloration, and nutrient uptake. The first two blocks and the second two blocks were performed over time. From day 0 to 11, we grew plants from seed in rockwool indoors under continuous warm-white light-emitting diodes (LEDs), with a mean photosynthetic PFD of 180 μmol∙m−2∙s−1. From day 11 to 28, we grew plants in hydroponic units under nine continuous LED lighting treatments, which were combinations of three FR fractions (0%, 25%, and 40%) and three B PFDs (30, 60, and 90 μmol∙m−2∙s−1). The mean air temperature and humidity were 23.6 °C and 62.5%, respectively, in the first two blocks, and 25.0 °C and 20.0%, respectively, for the second two blocks. Foliage coloration was measured on day 26, while growth and morphology data were collected on day 28. Increasing the FR fraction from 0% to 40% increased shoot biomass (by 24%–78%), leaf length (by 21%–41%), and plant diameter (by 17%–32%) across the three B PFDs. Decreasing the B PFD from 90 to 30 μmol∙m−2∙s−1 also increased shoot biomass (by 24%–50%) across the three FR fractions and increased plant diameter (by 9%–11%) at FR fractions of 0% and 25%, but not 40%. Despite some interactions, the FR fraction and the B PFD mostly influenced plant growth and morphology independently. Interestingly, chlorophyll index and coloration were largely unaffected by light spectra, indicating that the lowest B PFD might have saturated pigmentation. Increasing the FR fraction or decreasing the B PFD generally increased total nitrogen uptake to support growth promotion, but decreased tissue nutrient concentrations of nitrogen and potassium, possibly due to the dilution effect. We conclude that increasing the FR fraction (from 0% to 40%) or decreasing the B PFD (from 90 to 30 μmol∙m−2∙s−1) generally increased shoot biomass, leaf expansion, and nitrogen demand in indoor hydroponic lettuce ‘Rouxai’, without influencing foliage coloration.
