Growing food crops in space supports astronauts’ dietary needs in long-duration space missions and necessitates efficient use of light. In red-leaf lettuce (Lactuca sativa), sufficient red (R) and far-red (FR) light promote extension growth, whereas sufficient blue (B) light restricts extension growth but enhances secondary metabolite accumulation. Green (G) light also contributes to photosynthesis and improves visual quality. Compared to fixed light spectra, we evaluated dynamic light spectra to balance harvestable biomass and nutritional quality of red-leaf lettuce ‘Outredgeous’ under elevated CO2 concentration (≈2794 μmol⋅mol–1) and intermediate relative humidity (≈48%), typical on the International Space Station. This ground-based growth chamber experiment was performed twice following a randomized complete block design. We grew plants hydroponically at ≈22 °C under light-emitting diodes (LEDs) with four fixed light spectra and four dynamic light-spectrum alternations. The four fixed light spectra from seed to harvest were B60R140, B10R190, B10G50R140, and B10R140FR50 (the subscript following each waveband denotes its photon flux density in μmol·m−2·s−1). The four dynamic light-spectrum alternations switched among B10R190, B10G50R140, and B10R140FR50 in the lag (day 0–11) and exponential growth (day 11–25) phases, followed by B60R140 in the finish phase (day 25–28). Plant data were collected 11 and 28 days after sowing for seedlings and mature plants, respectively. Among the fixed light spectra, increasing the B photon flux density decreased shoot mass by 28% to 39% but increased total phenolic concentration by 27% to 45% in mature plants. Partial substitution of R light with G light decreased shoot mass by 31% to 42% in seedlings, but not mature plants. Partial substitution of R light with FR light did not influence shoot mass of seedlings or mature plants. Compared with fixed low B light treatments, dynamic light-spectrum alternations with high B light in the finish phase did not affect shoot mass, root mass, or leaf number while increasing total phenolic concentration by 8% to 25%. In addition, partial substitution of R light with G or FR light during the lag or exponential growth phase did not influence shoot or root mass. We conclude that low B light in the lag and exponential growth phases followed by short-term high B light in the finish phase improves lettuce nutritional quality without decreasing biomass as seen under long-term high B light. Spectrum selection in the earlier phases should prioritize the photosynthetic photon efficacy of LEDs to maximize light use efficiency.
