NO₃^-/NH₄⁺ proportions affect cadmium bioaccumulation and tolerance of tomato

With the growth of the world population, cadmium (Cd) concentration in the environment has increased considerably as a result of human activities such as foundry, battery disposal, mining, application of fertilizers containing toxic elements as impurities, and disposal of metal-containing waste. Higher plants uptake N as ammonium (NH₄⁺), nitrate (NO₃^- ), and many other water-soluble compounds such as urea and amino acids, and nourishing plants with N, providing part of it as NH₄⁺, is an interesting alternative to the supply of this nutrient in the exclusive form of NO₃^- under Cd toxicity. The objective was to evaluate the influence of NO₃^- /NH₄⁺ proportions on the development and tolerance of tomato plants grown under the presence of Cd in the culture medium. The experiment was conducted in a completely randomized block design in a 3 × 3 factorial arrangement consisting of three Cd rates (0, 50, and 100 μmol L^-1) and three NO₃^-/NH₄⁺ proportions (100/0, 70/30, and 50/50) in the nutrient solution. To this end, we quantified the responses of the antioxidant enzymatic system and productive and functional changes in Solanum lycopersicum var. esculentum (Calabash Rouge). Shoot biomass production decreased with the maximum Cd rate (100 μmol L^-1) tested in the growth medium, whereas the NO₃^- /NH₄⁺ proportions and other Cd rates did not significantly influence this variable. The lowest SPAD values were observed at the 100/0 NO₃^- /NH₄⁺ proportion and in plants exposed to Cd. The largest accumulation of the metal occurred in the shoots at the NO₃^- /NH₄⁺ proportion of 70/30 and at 100 μmol L^-1 Cd and in the roots at 100/0 NO₃^-/NH₄⁺ and with 50 and 100 μmol L^-1 Cd. The concentration and accumulation of NO₃^- were highest at the NO₃^-/NH₄⁺ proportion of 100/0 in the shoots and at 50/50 NO₃^-/NH₄⁺ in the roots, whereas for NH₄⁺, values were higher as the proportion of N supplied in the form of NH₄⁺ was increased. The nitrate reductase enzyme activity decreased with the Cd supply in the nutrient solution. The antioxidant system enzymes were activated as we increased the NO₃^-/NH₄⁺ proportion and/or Cd rates added to the nutrient solution in both shoots and roots of the tomato plant, except for ascorbate peroxidase. Based on the results obtained, if the plant is to be used as a food source as is the case of tomato, the 100/0 NO₃^-/NH₄⁺ proportion is the better alternative because it resulted in higher Cd accumulation in the root system over the translocation to the shoots and consequently to the fruit.