Abstract

Light conditions affect lettuce plant growth, development and secondary compounds, and the response may depend on the stage of development. This study evaluated the effect of ambient light enrichment with different LED spectra (red, blue, white, and red-blue) on biomass accumulation, photosynthesis, and polyphenol biosynthesis in green lettuce (Lactuca sativa cv. 'Levistro') grown hydroponically in a greenhouse. Both young and mature lettuce showed similar fresh weight, dry weight, and leaf number across treatments, with values ranging from 17.1 to 19.4 g plant(-1), 6.9 % to 7.2 %, and 10.6 to 11 leaf plant(-1) in young plants, and from 38.5 to 44.8 g plant(-1), 9.2 % to 10.4 %, and 12.3 to 13.8 leaf plant(-1) in mature plants, respectively, indicating that spectral enrichment maintained yield even though they received 35-45 % less daily light integral (DLI) than plants grown in ambient light. Light enrichment differentially affected gas exchange depending on plant developmental stage. In young lettuce, control increased CO2 assimilation relative to blue and white light enrichment by 59.2 % and 45.5 %, respectively, with no significant differences observed in stomatal conductance, transpiration, and intercellular COQ. Mature plants showed comparable responses, although only red-blue light enrichment increased COQ assimilation by 34.5 % compared to the blue treatment. Meanwhile, the control enhanced stomatal conductance and transpiration compared to red, white, and blue light enrichment. On the other hand, red-blue light significantly increased the accumulation of quercetin and chlorogenic acid in young lettuce by 113.5 % and 36.8 %, respectively, coinciding with the upregulation of the FLS and C3H gene expressions. In mature lettuce, blue light promoted quercetin accumulation (39.1 % and 34.8 % more than the red and white light enrichment, respectively) without changing gene expression, while red-blue enrichment enhanced chlorogenic and chicoric acid levels relative to the another treatments, within ranges of 7.7-47.4 % and 14.5-29.0 %, respectively. The young and mature 'Levistro' lettuce exhibited normal growth under light spectrum modulation with a lower DLI compared to ambient light. This study demonstrated that spectral optimization can significantly reduce energy usage (via a lower DLI) without compromising yield, while enhancing bioactive quality, offering a tangible strategy to support sustainable practices in greenhouse agriculture. This response was mediated by a growth stage-specific regulation of key biosynthetic genes, highlighting the importance of adapting light recipes according to the plant's development stage. This study demonstrates that dynamic spectral adjustment is an effective tool for enhancing the sustainability and nutritional quality of greenhouse lettuce production.