Active aeration enhances tissue hydration and fresh mass in hydroponic lettuce and modulates calcium-mobilizing biostimulant efficacy

Tipburn is a physiological disorder in lettuce (Lactuca sativa) that causes leaf margin necrosis, reducing marketability, and is exacerbated in hydroponic systems where calcium demand exceeds supply. Although a calcium-mobilizing biostimulant can mitigate tipburn, it is unclear whether its efficacy depends on nutrient solution dissolved oxygen (DO), which is critical for water and nutrient uptake. We evaluated the effects of this calcium-mobilizing biostimulant (0, 0.25, and 0.5 mL⋅L–1) and root-zone DO (achieved without and with varying degrees of moderate active aeration: 2.9, 3.5, and 4.0 mg⋅L–1) on growth and tipburn of hydroponic butterhead lettuce ‘Nancy’ in deep-water culture. At 21 days after transplanting (DAT), increasing DO from 2.9 to 3.5–4.0 mg⋅L–1 increased shoot fresh mass by 98–139%, moisture content by 5–7%, and plant diameter by 31–48%, without affecting dry mass, whereas the biostimulant did not affect growth. At medium DO, the biostimulant reduced percent burnt leaves from 33 to 12%. At 28 DAT, similar DO-dependent growth trends persisted, while high biostimulant concentration reduced tipburn, especially at low DO. These results indicate that active aeration enhances tissue hydration and fresh mass, and that the biostimulant mitigates tipburn primarily under root hypoxia.