Influence of Ni Supply on Growth and Nitrogen Metabolism of Brassica napus L. Grown with NH₄NO₃ or Urea as N Source

The significance of nickel (Ni), which is essential for urease activity, for growth and nitrogen (N) metabolism of Brassica napus grown in nutrient solution with either NH₄NO₃ or urea as sole N source was investigated. Although Ni contents were below 25 µg kg⁻¹ d. wt, growth of plants relying on NH₄NO₃ was not affected by the Ni status. However, supplementing the growth medium with 0·04 µm Ni enhanced dry matter production of urea-grown plants significantly. Urease activity was significantly reduced in leaves and roots of plants grown without supplementary Ni irrespective of N source. Plants grown with urea without additional Ni accumulated large amounts of urea and had lower amino acid contents indicating impaired usage of the N supplied, while those grown with NH₄NO₃ under Nideprived conditions accumulated endogenous urea in their older leaves. It is suggested that Ni may not be strictly essential for plants receiving mineral N, or that the critical level is well below 25 µg kg⁻¹ d. wt. These results confirm that Ni is required for urease activity and thus for growth of plants on urea-based media, as well as for recycling endogenous urea.