Effects of CO2 enrichment and light spectrum on lettuce growth, morphology, and metabolites in controlled environment agriculture

Controlled environment agriculture offers opportunities to regulate environmental factors that enhance crop productivity and quality. This study aimed to investigate the effects of CO2 enrichment (400, 800, and 1200 ppm) and light spectrum (broad-spectrum white, combined narrow amber + blue, and combined white + far-red + ultraviolet-A) on the growth and development of lettuce (Lactuca sativa). Environmental parameters were controlled at 21 °C during the 18 h light period and 19 °C during the 6 h dark period, with 65% relative humidity. After 28 days, fresh mass, dry mass, plant height, head diameter, chlorophyll content, leaf area, specific leaf area, and anthocyanin content were measured. CO2 enrichment significantly (p < 0.05) increased fresh mass, dry mass, leaf area, and specific leaf area, with the largest increases observed at 800 ppm CO2. Light treatments had a significant (p < 0.05) effect on fresh mass and plant height. The combined narrow amber and blue spectrum yielded the highest fresh and dry mass, the white, far-red and UV-A combination promoted taller plants, and broad-spectrum white light produced the largest head diameters. Chlorophyll content did not show statistically significant differences across CO2 or light treatments, whereas anthocyanin content differed significantly among light treatments (p = 0.04), with the narrow amber and blue treatment producing the highest anthocyanin content. Specific leaf area decreased significantly with rising CO2 concentrations. Leaf area increased significantly at 800 ppm but plateaued at 1200 ppm (p < 0.05). These findings suggest that CO2 enrichment at 800 ppm, combined with a narrow amber and blue light spectrum, enhances indoor lettuce production by improving both biomass and quality.