Wang, B. and Shen, Q. R. 2011. NH₄ -N/NO₃^- -N ratios on growth and NO₃^--N remobilization in root vacuoles and cytoplasm of lettuce genotypes. Can. J. Plant Sci. 91: 411-417. Aerobically growing crops generally prefer nitrate (NO₃^-) to ammonium (NH₄ ), but partial replacement of NO₃^- by NH₄ usually makes crops grow better. Five cultivars of lettuce (Lactuca sativa L.) frequently cultivated in southeast China, including Ny, Sx1, Rw, Nct and Nrnct, were hydroponically grown in four ratios of NH₄-N:NO₃-N: 0:100, 10:90, 25:75 and 50:50. Based on biomass of roots and shoots of plants, Sx1 was a mostly sensitive genotype, while Nrnct was a mostly insensitive genotype to partial NO₃^- replacement by NH₄ among the five cultivars. Total root surface area, root length, SPAD readings and net photosynthesis rate of Sx1 and Nrnct were found to be highest at 25:75 of NH₄-N:NO₃-N. Sx1 had a greater improvement than Nrnct under moderate NH₄ nutrition (NH₄-N:NO₃-N=25:75). The capacities of NO₃^- release and remobilization from vacuoles to cytoplasm of Sx1 were stronger than those of Nrnct. The results suggest that we can improve the utilization of nitrogen and decrease the NO₃^- content in lettuce through planting the appropriate lettuce cultivar in a proper NH₄-N/NO₃-N ratio.