Supplemental daily light integral by LED light to improve the growth of leafy vegetables in aquaponics system

Aquaponics is a highly efficient production system that relies on the fish food as the only energy system’s input for growing both fish and vegetables, without chemicals. In Mediterranean climates, during winter, due to the combined effects of low solar radiation and low-temperature levels, plant metabolic activity, growth rate, and nutrient uptake decrease, which may, in turn, cause harmful accumulation of nitrogen compounds for fish in the water. Growers renew the water periodically by discharging nutrients enriched water with severe environmental and economic costs. In winter, aquaponics cultivation cycles can be integrated with supplemental light to guarantee a constant daily light integral (DLI) that could promote plant growth and nutrient uptake. The aim of the study was to evaluate the morpho-physiological response of lettuce (Lactuca sativa L.), escarole endive (Cichorium endivia var. latifolia), and curly endive (Cichorium endivia var. crispum) grown in a floating raft, in combination with tilapia (Oreochromis niloticus L.) in a recirculating aquaponics system (RAS). Plant were grown under natural light (NL) or natural light integrated with 16 h of supplemental white LED lighting (IL, PPFD: 173 µmol m-2 s-1, DLI, 10 mol m-2 d-1). The results showed species-specific morpho-physiological responses, with higher productivity in the endives than lettuce, under both light regimes. Compared to NL, plant biomass and leaf area were promoted under IL, accounting for higher plant photosynthetic rates. IL boosted endive growth, while lettuce leaf expansion and biomass allocation into the canopy were increased. Both lettuce and endives performed well in the adopted RAS system. It appears recommendable to provide a fixed DLI during the winter months to shorten the crop cycle by improving plant growth and nutrient uptake. However, further studies are needed to optimize the lighting application protocols and reduce energy costs too.