Limited information is available on biological effects of various levels of nickel (Ni) (deficiency to toxicity levels) on growth and yield of certain crops, particularly vegetables. In this sand-culture study, we investigated the effects of four levels of Ni (0, 50, 100, and 200 lM) on growth, yield, and fruit-quality attributes of two cucumber cultivars (Cucumis sativus L. cvs. Super Dominus and Negin) supplied with urea or NH 4 NO 3 as nitrogen source. Addition of 50 lM Ni to the nutrient solution resulted in a significant increase of shoot and root dry-matter yield of cv. Negin although this increase was greater in the urea-fed plants than those fed with NH 4 NO 3 . In both cultivars, addition of 50 lM Ni increased urease activity and thereby decreased the urea concentration in the urea treatment. Addition of 100 and 200 lM Ni caused a significant decrease in root and shoot growth of cucumber although this decrease was insignificant for cv. Super Dominus in the 100 lM treatment. The highest fruit yield, total soluble solids (TSS), and fruit firmness were achieved at the 50 lM Ni treatment. Regardless of nitrogen source, Ni addition proportional to the concentration used increased leaf Ni concentration and fruit acid ascorbic concentration. The concentration of Ni required for optimum growth and yield of cucumber varied with cultivars. The level of 50 lM was sufficient for optimum growth of cv. Negin in nutrient-solution culture while lower concentration of Ni was required for cv. Super Dominus. While the beneficial effects of sufficient levels of Ni on growth and yield of urea-fed plants was greater than with NH 4 NO 3 -fed plants, the toxic effects of Ni in these plants were also greater.
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.
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