Effect of sulfate anions on nitrates accumulation and total Kjeldahl nitrogen content in lettuce plants cultivated in peat-perlite mixture

Abstract

Intensive agriculture is characterized by increased use of synthetic nitrogen fertilizers. If the supplied nitrogen fertilizers are excessive, many vegetables, particularly leafy vegetables, accumulate high levels of nitrates. Increased vegetable nitrate content is related to possible implications for health, as nitrate metabolites can cause carcinogenesis, infantile methaemoglobinaemia and possibly even teratogenesis. The aim of the present study was to investigate the effect of sulfate ions on nitrate accumulation and total Kjeldahl nitrogen (TKN) content in lettuce (Lactuca sativa var. crispa ‘Lollo Bionda’), as an attempt to suggest possible ways to reduce nitrate levels. Plants were grown in peat-perlite mixture and fertilized by nutrient solutions under glasshouse conditions. Three levels of nitrogen concentrations (100, 200 and 400 mg L(‑1) N) have been applied in combination with three levels of sulfate anions (15, 150 and 300 mg L(‑1) S as SO(4)2-). At harvest stage, the fresh weight of leaves, nitrate accumulation, total sulfur concentration and TKN tissue content were determined. As expected, foliar biomass, nitrate accumulation and TKN content were enhanced when nitrogen concentration was increased. However, the increased levels of sulfates (150 and 300 mg L(‑1) S) had no effect on plant weight whereas a decline of nitrate accumulation was observed when sulfate fertilization increased. Moreover, a concentration of 150 mg L(‑1) S caused TKN increase while the reverse was the case for the concentration of 300 mg L(‑1) S. The results of the present study indicate that the enhanced concentrations of sulfates resulted in decreased nitrate accumulation in lettuce leaves. This may be due to interference in the uptake of nitrates by sulfates ions and may be proposed as a possible way to decrease nitrate content in lettuce, resulting in consumer safety.