| Summary: | Vegetables can be treated with biofertilizers as an alternative to chemical fertilizers because of their low toxicity. We investigated the effects of foliar spraying of <i>Azotobacter</i> under different levels of nitrogen (100, 150 and 200 mg/L in nutrient solution) on the growth, nutritional value, nitrate accumulation and antioxidant enzyme activities of hydroponically grown lettuce. The experiment was laid out in a completely randomized design with four replicates in a factorial combination. Plants treated with <i>Azotobacter</i> and 200 mg/L nitrogen had greater leaf area and photosynthetic pigments than plants treated with 200 mg/L nitrogen without spraying with <i>Azotobacter</i>. Increasing nitrogen levels increased leaf number, fresh and dry weights, leaf area and nitrate accumulation in lettuce plants. Peroxidase (POD) activity increased by 95.4% at a nitrogen level of 200 mg/L compared to a nitrogen level of 100 mg/L. Ascorbate peroxidase (APX) activity and leaf phosphorous (P) and potassium (K) concentrations were the highest in plants treated with a nitrogen source of 100 mg/L without foliar application of <i>Azotobacter</i>. As nitrogen levels increased in all treatments, nitrate reductase (NR) activity decreased and reached a minimum at the 200 mg/L nitrogen level. In general, foliar application of <i>Azotobacter</i> sp. can be used to promote plant growth and reduce nitrate accumulation in lettuce.
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