Tissue NO3− concentrations in hydroponically-grown lettuce were reduced significantly with increasing NH4+: NO3− ratios in the nutrient solution. However, effects of mixed-Nitrogen nutrition on plant growth response varied considerably, depending on cultivar. With increasing solution NH4+: NO3− ratios up to 50:50 (in a 15.0 mM N solution), shoot NO3− concentrations in romaine lettuce at full head development were reduced by approximately 58% (52% on a fresh weight basis), while shoot dry weights increased by 46% (28% fresh weight). In butterhead cultivars, a solution NH4+: NO3− ratio of 50:50 decreased shoot NO3− concentrations by 58% (54% fresh weight basis), but shoot growth was also reduced by 9% (15% fresh weight basis).
