Responses of cucumber to deficient and toxic amounts of nickel in nutrient solution containing urea as nitrogen source

Nickel (Ni) is an irreplaceable component of urease which reduces urea toxicity, but excess of Ni has detrimental effects on plant growth. The responses of cucumber (Cucumis sativus L. cvs. Negin and Dominus) plants supplied with urea as sole N source to four Ni concentrations (0, 50, 100 and 200 μM) were investigated. Nickel at a 50 μM concentration stimulated growth and reduced urea accumulation and lipid peroxidation in the leaves. However, the application of 100 and 200 μM Ni reduced a shoot dry mass and increased a malondialdehyde (MDA) content. An activity of catalase (CAT) was not affected by 50 μM Ni, whereas it was significantly increased by 200 μM Ni. The application of Ni resulted in an enhancement of a guaiacol peroxidase (GPX) activity in the leaves. An ascorbate peroxidase (APX) activity was reduced by 200 μM Ni in cv. Negin and by 100 μM Ni in cv. Dominus.