Supplements of Nickel Affect Yield, Quality, and Nitrogen Metabolism When Urea or Nitrate is the Sole Nitrogen Source for Cucumber

Influences of nickel (Ni) concentrations in the nutrient solution on yield, quality, and nitrogen (N) metabolism of cucumber plants (Cucumis sativus cv 'RS189' and 'Vikima') were evaluated when plants were grown either with urea or nitrate as the sole N source. The cucumber plants were treated with two N sources, urea and nitrate as sodium nitrate (NaNO3) at 200 mg L-1, and three concentrations of Ni as nickel sulfate (NiSO46H2O; 0, 0.5, and 1 mg L-1). Treatments were arranged in a randomized block design with six replicates. The highest concentration of Ni in the leaves (1.2 mg kg-1 Dwt) was observed in the urea-fed plants at 1 mg L-1 Ni concentration. Additions of Ni up to 0.5 mg L-1 had no effect on the fruit Ni concentration in the both urea and nitrate-fed plants. Yield significantly (p 0.05) increased with the Ni supplements from 0 to 0.5 mg L-1 (10 and 15% in 'RS189' and 'Vikima', respectively), but decreased when 1 mg L-1 Ni applied to the solutions in urea-fed plants. Nitrate-fed plants had a higher percentage of total soluble solids compare to those urea-fed plants. Nitrate concentrations of the fruits in urea-fed plants in both cultivars were reduced by approximately 50% compared to those nitrate-fed plants. The reduction of nitrate concentration in the fruits became more pronounced as the Ni concentration increased in the solution. The rate of photosynthesis (Pn) increased with the increase of the Ni concentration in the solution with urea-fed plants. Both N concentration and nitrate reductase (NR) activity of young leaves were higher in urea-fed plants at 0.5 mg l-1 Ni concentration. Ni supplements enhanced the growth and yield of urea-fed plants by increasing Pn, N concentration and NR activity. It can be concluded that Ni supplements (0.5 mg l-1) improve yield, quality, and NR activity in urea-fed cucumber plants.