The Effects of FR and UVA Irradiation Timing on Multi-Omics of Purple Lettuce in Plant Factories

The synergistic application of far-red (FR) and ultraviolet A (UVA) irradiation presents a promising approach for enhancing growth and the enrichment of secondary metabolites in plants. However, prolonged exposure to these combined light qualities imposes significant stress on plants, hindering their development. Therefore, an initial period of FR irradiation to promote plant growth, followed by a subsequent period of UVA irradiation to enhance the accumulation of plant quality, constitutes a viable strategy. Our study, focusing on purple lettuce, aims to elucidate the response mechanisms of the lettuce leaf under standard white light in commercial production, with the addition of different durations of FR and UVA irradiation, and to explore the complex dynamic changes at the multi-omics level. The results indicate that the duration of FR exposure is crucial in determining biomass-related phenotypes such as fresh weight, while the duration of UVA exposure significantly influences the accumulation of phenotypic markers like anthocyanins. At the transcriptional level, the most extensive transcriptional regulation was observed when FR was applied throughout the entire growth period, and UVA was applied eight days before harvest, significantly impacting pathways such as MAPK signaling cascades, plant hormone signal transduction, photosynthetic processes, and the biosynthesis of secondary metabolites. Metabolomic analysis corroborated the transcriptomic findings, with particular emphasis on antioxidant activity, photoprotection, and defense mechanisms. Our comprehensive analysis suggests that short-term UVA irradiation prior to harvest, based on full growth period FR irradiation, is feasible. The combined application of FR and UVA irradiation fine-tunes plant growth, developmental trajectories, and stress responses by modulating light signals, hormonal signals, and secondary metabolic pathways. These findings not only reveal the adaptive mechanisms of plants to fluctuating light environments but also provide a scientific basis for optimizing light management strategies in controlled plant production systems and precision agriculture.