ASHS 2025 Conference

Tipburn is a physiological disorder that compromises the appearance and marketability of lettuce (Lactuca sativa) by causing necrosis and curling at leaf margins. It is problematic in controlled-environment hydroponic systems, where calcium demand outpaces calcium supply. Previous research demonstrated that nutrient solution application of a calcium-mobilizing biostimulant curbed lettuce tipburn. However, it remained unclear whether its efficacy depended on nutrient solution dissolved oxygen (DO), which is crucial for water and nutrient uptake. We evaluated effects of this calcium-mobilizing biostimulant and the DO level on growth and tipburn severity of hydroponic butterhead lettuce ‘Nancy’. Seeds were planted in a temperature-controlled greenhouse under a 24-hour photoperiod with supplemental white red light-emitting diodes providing a mean photosynthetic photon flux density of 300 μmol⋅m–2⋅s–1. After 11 days, seedlings were transplanted into deep-water-culture trays in the same greenhouse at 22.7 ± 4.1 °C air temperature and 23.6% ± 5.1% relative humidity, with the same continuous supplemental lighting. In a randomized complete block design with three blocks, plants were grown in nine nutrient solution treatments per block, which were combinations of three biostimulant concentrations (0, 0.25, and 0.5 mL⋅L–1) and three DO levels (2.9, 3.5, and 4.0 mg⋅L–1). The low DO level was achieved without active aeration, whereas the medium and high DO levels were achieved with active aeration by adjusting airflow rates. Plants were evaluated on 21 and 28 days after transplant (DAT). On 21 DAT, plant growth benefited from active aeration but was unaffected by the biostimulant. Compared with no active aeration, active aeration increased shoot fresh mass by 127% to 139% and moisture content by 5% to 7%, but did not affect shoot dry mass. This indicates that growth promotion was attributed to increases in both relative and absolute water uptake, but not whole-plant photosynthesis, despite increased canopy size. The biostimulant decreased the percent of tipburn-affected leaves from 33% to 12% under the medium DO level. On 28 DAT, shoot fresh mass, shoot dry mass, moisture content, and canopy size had similar trends to those on 21 DAT. The high biostimulant concentration decreased the tipburn rating by 36% and the number of tipburn-affected leaves by 50% without active aeration, but not with it, indicating that the biostimulant was more efficacious under root hypoxia than under sufficient DO levels. We conclude that active aeration enhances lettuce water uptake and shoot fresh mass, whereas the biostimulant moderately controls tipburn at lower DO levels.