Lettuce (Lactuca sativa) tipburn is a physiological disorder that leads to unappealing browning or necrosis of young leaf tips and stems, caused by localized calcium deficiency. It negatively impacts crop quality and yield, making proactive management essential for achieving optimal production. The objective was to evaluate the efficacy of a calcium-mobilizing chemical biostimulant, applied in the nutrient solution, on lettuce growth and tipburn. We conducted a greenhouse experiment on two lettuce cultivars (‘Dragoon’ and ‘Rex’) using a randomized complete block design. The seedlings were grown indoors under continuous white light from light-emitting diodes (LEDs) with a mean daily light integral (DLI) of 26 mol⋅m−2⋅d−1. We transferred 11-day-old seedlings to deep-water-culture hydroponic trays in a greenhouse. The two cultivars were subjected to three replications and five biostimulant concentrations (BC) of 0 (control), 0.125, 0.25, 0.5, and 1 mL⋅L−1 of the nutrient solution. Plants were grown under an 18-h photoperiod with a mean DLI of 16.6 ± 2.0 mol⋅m−2⋅d−1 from both sunlight and supplemental white LEDs, an air temperature of 24.6 ± 3.1 °C, and relative humidity of 33.2% ± 9.5%. Plant data were collected 14, 21, 28, and 35 days after transplant (DAT). There was no visible tipburn 14 DAT; however, plant diameter and shoot mass (fresh and dry) decreased with increasing BCs. We observed tipburn 21 DAT in both cultivars. The control had the highest severity on a 0–5 scale (0 = no tipburn; 5 = severe tipburn) for ‘Dragoon’ (0.6) and ‘Rex’ (1.3), whereas no tipburn occurred under higher BCs (i.e., 0.5 and 1 mL⋅L−1). Tipburn progressed 28 DAT, when increasing the BC from 0 to 1 mL⋅L−1 decreased the tipburn rating from 3.3 to 0 for ‘Dragoon’ and from 4.1 to 0 for ‘Rex’. Plant growth was stunted under the highest BC (i.e., 1 mL⋅L−1). At 35 DAT, both cultivars had severe tipburn under the control but had decreasing tipburn severity as the BC increased. Plant growth was unaffected under the control and low BCs (i.e., 0, 0.125, 0.25 mL⋅L−1). Under the highest BC, ‘Dragoon’ had the longest roots, but ‘Rex’ had the shortest. In contrast, plants experienced phytotoxicity (reduced biomass and chlorophyll concentration) under the highest BC, i.e., (1 mL⋅L−1) though no tipburn was recorded. In conclusion, the optimal calcium-mobilizing BC was 0.5 mL⋅L−1, which minimized tipburn of greenhouse hydroponic lettuce without affecting biomass accumulation or causing phytotoxicity during later development stages.
