NOTE: End date changed to 08/31/2024 per D. Freeland/KSC. Original end date per NSSC information was 12/31/2023 (Ed., 10/5/23).

We conducted this experiment using four reach-in plant growth chambers with precise environmental control capabilities to maintain target levels. At the same light intensity (˜200 µmol·m-2·s-1), we tested four fixed light spectra (from seed to harvest) and four dynamic light-spectrum alternations, from combinations of blue, green, red, and/or far-red light. The dynamic light-spectrum alternations had light switching between the lag phase, the exponential growth phase, and the finish phase. We collected data on plant growth, morphology, coloration, and nutritional quality and analyzed data in statistical software, JMP Pro, using Tukey's honest significant difference test.

We found that low blue light in the lag and exponential growth phases, followed by short-term high blue light in the finish phase, improves lettuce nutritional quality without decreasing biomass as seen under long-term high blue light. Spectrum selection in the earlier phases should prioritize the photosynthetic photon efficacy of light-emitting diodes to maximize light use efficiency.

We are in the process of completing a second ground-based experiment, the objective of which is to characterize how alternating light quantity over time affects lettuce light use efficiency and final nutritional quality under elevated CO2 and moderate relative humidity. Several hardware-related issues from the plant growth chambers caused delays in completing the second experiment by now; however, they have been resolved through dedicated troubleshooting and testing. We plan to finish performing two replications of the second experiment by the end of 2024, and analyzing and visualizing all data by summer of 2025. A one-year no-cost extension is requested to ensure satisfactory completion of all aims in this project.