Increasing DLI through photoperiod: a strategy for optimizing functional quality and yield in hydroponic lettuce

In vertical farming, photoperiod management is a decisive tool for modulating plant growth and phytochemical composition to optimize energy use. This study evaluated the impact of four Daily Light Integrals (DLI: 7.2, 10.8, 14.4, and 21.6 mol m⁻² d⁻¹), delivered through photoperiod extension (8, 12, 16, and 24 h, respectively) at a constant photosynthetic photon flux density (PPFD), on lettuce cultivars ‘Carmolí’ (CM: red) and ‘Levistro’ (LV: green) across three phenological stages: seedling (SD-0), juvenile (JV-15), and adult (AD-30). Measured parameters included biomass, photosynthetic efficiency (Y(II); Fv/Fm), bioactive compounds, and enzymatic activity. Independent stage analyses revealed that light availability significantly modulated crop performance across different developmental windows. A medium-high DLI (14.4 mol m -2 d -1 ) maximized fresh weight, leaf area, and texture in both cultivars, with the most pronounced differences observed at the mature adult stage. At the initial SD-0 stage, the restrictive 7.2 mol m -2 d -1 DLI treatment maximized total flavonoid content (1251.58 mg RE 100 g -1 FW) and DPPH antioxidant capacity (11.59 µmol TE g -1 FW) in CM, suggesting options for early-stage operational energy savings. Conversely, LV reached its highest initial antioxidant capacity under continuous light (21.6 mol m -2 d -1 ) and exhibited peak peroxidase activity (89.38 µmol g -1 min -1 FW) at the intermediate JV-15 stage. By the final harvest phase (AD-30), the continuous 21.6 mol m -2 d -1 treatment proved most effective at retaining bioactive compounds against the natural ontogenetic decline observed during late vegetative development. This research demonstrates that metabolic requirements vary by stage and genotype, providing the physiological basis to transition from static photoperiods to adaptive, stage-specific lighting strategies in controlled-environment agriculture.