Time-Course Evaluation of Water and Fertilizer Use Efficiencies and Growth Characteristics of Lettuce Cultivated Under Artificial Light Using a One-Way, Reduced-Discharge Hydroponic Cultivation Unit

The recirculating flow hydroponic cultivation unit (RFC) used in plant factories with artificial lighting requires nutrient solution replacement to prevent growth inhibition caused by nutrient imbalances and allelochemical accumulation. Although sustainable systems are needed to reduce environmental impacts, the effects of non-flowing conditions on plant growth and nutrient uptake dynamics remain unclear. We used a one-way, reduced-discharge hydroponic cultivation unit (ORC) to evaluate frill-type lettuce growth and quantify changes in water use efficiency (WUE), fertilizer use efficiency (FUE), and inorganic nutrient absorption under non-flowing conditions. The ORC reduced nutrient solution uptake by ~28% compared with the RFC, improving physiological and system-level WUE and FUE. Root dry weight in the ORC was ~1.5 times higher, likely owing to morphological adaptation—specifically, enhanced root system development—that expanded the nutrient acquisition area under reduced diffusion. Fluid movement in the RFC may also have physically inhibited root growth. Conversely, initial growth in the ORC was lower because of reduced nutrient supply to the rhizosphere. Overall, the ORC achieved high WUE and FUE while minimizing waste. Quantifying the time-course absorption characteristics of inorganic components in this unit is expected to enable precise adjustment of individual nutrients, further reduce discharge, and facilitate high-quality production.