The biological significance of nickel

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Abstract
The essentiality of Ni has not been established for higher plants, but many beneficial effects of Ni on plant growth have been reported. Evidence from several laboratories suggests that Ni is essential for animals, but its function has not been defined. High levels of Ni in plant growth media are phytotoxic, and possible beneficial effects of Ni on plant growth are poorly defined and not understood. Nickel reportedly is required for growth of a bacterium (Alcaligenes (Hydrogenomonas) eutrophus), a cyanobacterium (Oscillatoria) and a green alga (Chlorella vulgaris), but its function is unknown. In higher plants, Ni is required for optimum growth of certain pine tree species and some Ni accumulator species of Alyssum, but its function is not known. Slight increases in the growth of a number of plant species were attributed to low levels of Ni. Germination of seed of several species was stimulated by treatment with Ni salts. Nickel is an essential component of the enzyme urease isolated from lackbean seeds but the function of urease is not clear. Some higher plants require Ni when they are cultured with urea as the only source of N (soybean tissue cultures and Lemna, Spirodela and Wolffia species). Some researchers suggested that in higher plants urease might have functions other than the hydrolysis of urea, such as the utilization of carbamoyl, ureido, or amidino moiety of urea. Possibly urease and, therefore, Ni might be required for the mobilization of stored seed‐nitrogen through ureides or arginine during early stages of seedling growth. Several families of plants, including species of Leguminosae, accumulate large amounts of ureides. In soybeans, the ureides (allantoin and allantoic acid) are the major forms of nitrogen translocated from nodulated roots to tops of plants and are important in seed development. The only catabolic pathway known for allantoin and allantoic acid is through their enzymatic degradation to urea and glyoxylic acid. For use of urea‐N in anabolic processes plant cells hydrolize urea to NH3 and CO2 by the action of urease which requires Ni. Apparently Hi might be required by nodulated legumes that translocate large amounts of N from roots to tops via ureide compounds.