Light Spectrum Strategies to Optimize Yield and Physiological Performance of Korean Thistle (Cirsium setidens (Dunn) Nakai) in a Closed-type Plant Production System

Background and objective: In closed-type plant production systems, controlling the spectral distribution of light strongly influences plant morphophysiological traits. This study provides a comprehensive assessment of the growth, yield, plant quality, chlorophyll fluorescence, and remote sensing vegetation indices of Korean thistle (Cirsium setidens (Dunn) Nakai) exposed to different light qualities with the aim of identifying spectral conditions that enhance plant quality. Methods: Eight different light-emitting diode (LED) spectra were applied: monochromatic red (630 nm), green (520 nm), and blue (450 nm); a purple LED composed of red and blue wavelengths supplemented with far-red (approximately 17.6%); and white LEDs at 2100, 3000, 4100, and 6500 K. Measurements included plant growth, yield (biomass), plant quality indices (relative moisture content, S/R ratio, compactness, and Dickson quality index), chlorophyll fluorescence parameters, and remote sensing vegetation indices. Subsequently, principal component analysis, Pearson correlation analysis, and hierarchical clustering were performed. Results: According to the results, the red LED effectively increased shoot size and biomass; however, this was accompanied by elevated photochemical stress-related indicators. Across the various white LED treatments, SPAD units and normalized difference vegetation index (NDVI) were comparatively high, and photochemical efficiency indices such as PIABS and Fv/Fm were favorable, whereas plant size was relatively smaller than under red LED treatment. Under the purple LED, leaf length and width increased together with higher carotenoid reflectance index 2 (CRI2) and anthocyanin reflectance index 2 (ARI2) values, suggesting pigment accumulation in leaves, which we interpret as an effect of the relatively high proportion of far-red wavelength. Under the red and green LEDs, relative declines in Fv/Fm and NDVI, together with increases in ABS/RC and DIo/RC, indicated a typical photochemical stress pattern. However, these may represent downregulation signals associated with PSII-PSI excitation partitioning and spectral acclimation, rather than damage responses. Consequently, reliance on any single metric should be avoided, and an integrative approach that utilizes multiple parameters and indices is recommended. Conclusion: In conclusion, white LED treatments can be useful options for cultivating C. setidens by concurrently enhancing plant quality and maintaining favorable photochemical efficiency, whereas the monochromatic red LED treatment is effective at increasing plant size and biomass but is more likely to be accompanied by photochemical stress-associated signals. These findings offer practical insights for spectral optimization of C. setidens production in closed plant production systems. Future studies should further refine supplemental wavelengths, R:B:FR ratios, and irradiance settings to improve physiological vigor.