Significance of N source (urea vs. NH4NO3) and Ni supply for growth, urease activity and nitrogen metabolism of zucchini (Cucurbita pepo convar. giromontiina)

The effect of Ni supply on growth and N metabolism of zucchini plants grown with either NH4NO3 or urea as sole N source was investigated. Dry matter production of plants grown with NH4NO3 was not affected by the Ni status, while urea-based nutrition led to reduced growth, particularly when plants were grown without Ni supplementation. The activity of urease, which requires Ni for activation, was hardly detectable in leaves and roots of plants grown without supplementary Ni irrespective of N source. Low-Ni urea-grown plants were chlorotic, accumulated large amounts of urea and had lower amino acid contents indicating impaired usage of the N supplied. The lack of urease activation made these plants metabolically N deficient. The amino acid pools of plants grown with NH4NO3 was not markedly affected by the Ni supply, although these plants accumulated endogenous urea in their leaves when grown without supplementary Ni. In urea-grown plants the glutamine content was considerably increased by Ni supply, indicating that the efficient use of urea N is Ni (urease) dependant. Based on growth and urease activity, a critical Ni concentration in the leaves of around 100 mu g kg(-1) can be deduced. These results confirm the necessity of Ni for urease activation and thus for growth of plants on urea-based media, as well as for recycling endogenous urea.