Rapid photosynthetic induction after short dark periods mitigates growth reduction in lettuce exposed to frequent light-dark cycles

In indoor crop production, light intensity is typically maintained constant throughout the photoperiod. Dynamic lighting strategies, such as dimming or turning off lights during periods of high electricity prices, can help reduce energy costs. However, fluctuating light conditions may negatively affect plant growth. In this study, we quantified the effects of frequent light-dark cycles on lettuce growth and physiological responses. Two lettuce cultivars, ‘Rex’ and ‘Rouxai’, were grown under seven treatments that differed in the number of light-dark cycles per 24-h period (1–96 cycles): 16 h light/8 h dark (1 cycle; control), 8 h/4 h (2 cycles), 4 h/2 h (4 cycles), 1 h/30 min (16 cycles), 30 min/15 min (32 cycles), 20 min/10 min (48 cycles), and 10 min/5 min (96 cycles). Across all treatments, light intensity during the light periods was maintained at 300 µmol m -2 s -1 . The ‘16 h/8 h’ and ‘8 h/4 h’ treatments resulted in the greatest plant growth in both cultivars. In contrast, the ‘1 h/30 min’ and ‘20 min/10 min’ treatments resulted in the lowest leaf area and shoot biomass in ‘Rouxai’. Plants under these treatments exhibited a slow increase in photosynthetic rate (P net ) following transitions from dark to light. This slow photosynthetic induction, combined with the frequent light-dark cycles, likely attributed to the reduced growth. Similarly, the ‘20 min/10 min’ treatment resulted in the lowest steady-state P net and shoot mass in ‘Rex’. However, both cultivars grown under the most frequent light-dark cycle (‘10 min/5 min’) showed rapid photosynthetic induction and comparable growth to those grown under the control treatment. Pigment levels, including total chlorophylls, carotenoids, and anthocyanins, decreased under frequent light-dark cycles (32–96 cycles per 24-h). Overall, frequent light-dark cycles reduced lettuce growth and pigmentation. However, when the dark period was brief (5 min), the negative effects on plant growth were mitigated, likely due to rapid photosynthetic induction. The duration of dark periods and cycle frequency should be carefully considered to minimize growth reductions when implementing non-static lighting strategies in indoor production.