Pulsed LED Lighting effects on the cultivation of green and red lettuce cultivars in vertical farming

LED lighting has been increasingly used in vertical farming as a sustainable alternative to HPS (High-Pressure Sodium), offering benefits like enhanced plant growth, improved shelf-life, and a better nutritional quality of horticultural products at lower costs. Optimized light treatments can increase key metabolites, such as vitamins and bioactive compounds. Pulsed LED lighting, delivering high-intensity light with lower energy consumption, has shown promising effects. By adjusting the duty cycle, it is possible to achieve high production while saving energy. This study investigated the effects of pulsed LED light on two lettuce cultivars, green and red (Lactuca sativa L., var. “Multifoglia”), over a 48-day growing cycle. Two duty cycles were compared, 50% (D50) and 25% (D25), both at 1000 Hz frequency and a photoperiod of 16 hours, using a spectrum of 82% red, 11% green, and 6% blue lights. PPFD in the two chambers was measured: in D50, it was 122 µmol m^-2s^-1, while in D25, it was 71 µmol m^-2s^-1. A greenhouse with natural light during the autumn season served as the control. After harvest, the quality was evaluated over a period of 10 days at 4°C. Analyses of various physiological and biochemical parameters were conducted. The results showed that at harvest the red-leaf cultivar had the highest fresh weight compared with the green lettuce. Leaf pigments such as total chlorophyll and carotenoids were higher under the D25 treatment in both cultivars. Nitrate concentration was also higher in pulsed treatments compared to control plants in the greenhouse but remained under the EU limits (4000 mg kg^-1 FW). The maximum quantum efficiency of photosystem II (F_V/F_M) showed that plants under pulsed lighting were less stressed and showed better light use efficiency compared to the greenhouse conditions. The green lettuce showed better leaf performance. The effect of pulsed light was also observed during the postharvest period. Ethylene and CO₂ production, particularly after 7 days of cold storage, were significantly reduced under the D25 treatment. Considering energy saving, D25 reduced electric consumption by 34% compared with D50.

This study highlights the effectiveness of pulsed LED lighting, especially the D25 treatment, in improving growth, physiological traits, and energy efficiency in lettuce cultivars. Further analysis of secondary metabolites will be carried out to better understand plant responses and maximize the potential of these treatments.