Impact of ambient light spectrum modifications on agronomic traits and phenolic profile of green lettuce plants

Modifying the light spectrum that plants are exposed to may be an optimal strategy to balance growth, yield, and the accumulation of antioxidant compounds, particularly in lettuce plants since it is one the most economically important vegetables worldwide due to its high consumption and export. The purpose of this study was to evaluate the impact of ambient light enrichment with different LED light spectra on biomass, the relative index of chlorophyll concentration (RICC), polyphenol concentration, and the relative gene expression of enzymes associated with their formation in young green 'Levistro' lettuce leaves grown in an NFT hydroponic system under greenhouse conditions. Ambient light was enriched with blue (B), white (W), red-blue (RB), and red (R) LED lights for 14 days. Each lamp was mounted on a 120 × 35 cm thin wooden panel which shaded the experimental unit, resulting in a mean daily light integral (DLI) of 14.0 mol m⁻² d⁻¹. The control was ambient greenhouse light (DLI = 21.8 mol m⁻² d⁻¹). Fresh weight was similar across the enrichment treatments and the control, while dry matter (%) significantly decreased under B compared to the control, by 5.7%. The number of leaves increased significantly under RB compared to the control, by 24.4%. RICC rose as the plants grew, with no significant difference between the enrichment treatments and the control. Chlorogenic acid (ChlAc) concentration was significantly higher under RB compared to R, B, and W, by 136.4%, 116.7%, and 62.5%, respectively. Meanwhile, total quercetin (TQ) concentration was significantly higher under W and RB compared to the control, by 81.0% and 71.4%, respectively. Interestingly, the expression of the coumarate 3-hydroxylase (C3H) gene, associated with ChlAc formation, was higher under RB compared to the control and the other enrichment treatments, while the expression of the flavonol synthase (FLS) gene, which is linked to quercetin formation, increased under W and RB compared to the control and the other light enrichment treatments. Therefore, the modulation of the environmental light spectrum with a lower DLI than the control allowed for the maintenance of normal and adequate morphology and growth of green lettuce 'Levistro,' while RB promoted the concentration of ChlAc and TQ by increasing the expression of the C3H and FLS genes, respectively.

Keywords: LED light; greenhouse; agronomic characteristics; polyphenol; gene expression