Energetic efficiency of biomass production is affected by photoperiod in indoor lettuce cultivation

Some studies have shown that long photoperiods result in high biomass accumulation, others have reported damage caused by the lack of darkness. Since indoor production depends on a favorable cost-benefit ratio, it is essential to evaluate the biomass produced per energy consumed by LEDs. We expected that photosynthesis and energetic efficiency of biomass production would decrease under long photoperiods due to the circadian cycle. Lettuce (Lactuca sativa L.) plants were grown in nutrient solution under different photoperiod regimes. In each photoperiod (12, 14, 16, 18, 20, 22, and 24 h), plants were germinated and harvested after 21 days. Leaf gas exchange was monitored over 24 h, whereas plant growth and energy consumed by LEDs were evaluated after 20 days of growth in each regime. Although the maximum photosynthetic rates have not changed when varying photoperiod, the daily-integrated CO₂ assimilation on leaf area basis was higher at 20, 22, and 24 h. Biomass accumulation, leaf number, and leaf area increased linearly between 12 and 18 h of photoperiod. In regimes longer than 18 h, extending the photoperiod did not increase the number of leaves and dry mass accumulation and decreased leaf area and fresh mass. The highest energetic efficiency of biomass production was noticed between 12 and 18 h, while the lowest one was found at 22 and 24 h of photoperiod. The best photoperiod for growing lettuce was 18 h, as it was the shortest photoperiod that led to the highest plant growth while maintaining high energetic efficiency of biomass production.