Lettuce is one of the most popular vegetables, and the market niche of the baby leaf variety is expanding. The manner in which nitrogen (N) is supplied and the available concentration of the micronutrient molybdenum (Mo) affect N metabolism, with reflects on the nutritional quality of that vegetable. Here, two Mo concentrations (0.06 and 0.12 mg/L) and four proportions (%) of nitrate (NO₃ ^- )/ammonium (NH₄ ⁺ )(100/0, 75/25, 50/50, and 25/75) were supplied to hydroponic baby leaf lettuce floating type to evaluate their effects on plant growth and leaf mineral and NO₃ ^- compositions. Shoot dry mass did not differ among the different treatments, although fresh mass was lower in treatments with larger proportions of NH₄ ⁺ . Higher leaf concentrations of NO₃ ^- were observed in plants treated with 100% N-NO₃ ^- , but they were still below tolerable limits for human health. The enzyme nitrate reductase was not found to be sensitive to the nitrogen sources or to Mo concentrations. N proportions and Mo concentrations differently affected macro- (C, N, P, K, Ca, Mg, and S) and micronutrients (Cu, Fe, Mn, Mo, and Zn) leaf concentrations. Although treatment with 100% N-NO₃ ^- favored higher mineral concentrations in lettuce leaves, the addition of 25% N-NH₄ ⁺ allowed fresh mass production with the lowest NO₃ ^- concentrations. As such, and considering the healthy reduction of NO₃ ^- consumption by humans and the maintenance of plant productivity, the 75/25 NO₃ ^- /NH₄ ⁺ proportion is recommended for the hydroponic cultivation of baby leaf var. Mimosa lettuce. PRACTICAL APPLICATION: We demonstrate a direct link between the constitution of nutrient solution with nitrate accumulation by hydroponic lettuce and indicate the best source of N as well as the concentration of Mn to healthy reduction of NO₃ ^- consumption by humans and the maintenance of plant productivity.

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