Application of flow field visualization technology in analysing the influence of nutrient solution flow on hydroponic lettuce growth

• Flow field visualization is used in horticultural research. • Visualized the flow field of hydroponic lettuce at different growth stages and flow rates. • Nutrient solution flow significantly effected lettuce growth and root morphology. • This effect varied with the growth stage of the lettuce. Nutrient solution flow is important for the growth and root morphology of lettuce in hydroponics, requiring precise regulation to optimise yield and quality. However, the mechanisms involved remain poorly understood. We examined the influence of varying nutrient solution flow rates on lettuce growth, root morphology, and nitrogen uptake. We assessed lettuce performance at five growth stages, measuring shoot and root dry and fresh weights, root morphology, and nitrogen uptake. Particle image velocimetry was employed to visualise the flow field, providing a deeper understanding of how flow patterns impact the root environment. In the early growth stage, lettuce under no flow conditions exhibited higher shoot and root biomass. However, moderate flow consistently outperformed other conditions as growth progressed, demonstrating significantly higher fresh and dry weights. High flow initially suppressed growth, highlighting the detrimental effects of excessively fast flow rates. No flow initially promoted root development, while moderate flow enhanced root growth later in the lifecycle. Nitrogen uptake analysis showed that moderate flow achieved the highest efficiency, while high flow increased nitrogen uptake flux in later stages. PIV visualisation revealed that moderate flow delivered uniform flow vectors and moderate velocity, enhancing nutrient ion contact with roots and uptake efficiency. In contrast, high flow resulted in chaotic flow vectors, high vorticity, and potential root damage, reducing uptake efficiency. Under no flow conditions, nutrient ion transport relied solely on diffusion, limiting nutrient availability during rapid growth and maturation. In conclusion, moderate flow was optimal for promoting lettuce growth and root development.