Discovery of the Essentiality of Nickel

The discovery in 1975 that nickel is a component of plant urease (3) prompted the first detailed studies on the essentiality of nickel for plant life. In 1977, Polacco (8) determined that tissuecultured soybean (Glycine max Merr.) cells could not grow in the absence of nickel when provided with urea as the sole nitrogen source. Subsequently, many researchers demonstrated that plant growth is severely impacted by nickel deficiency when urea is the sole nitrogen source (9-14).

These results, though compelling, demonstrated a role for nickel only in certain species when grown with urea as the sole nitrogen source and as such did not satisfy the established criteria for essentiality, which state that an element is essential if without the element, the plant cannot complete its life cycle and the element is a constituent of an essential plant metabolite or molecule (4). Essentiality of nickel was subsequently established in 1987, when Brown et al. (1) demonstrated that barley (Hordeum vulgare L. cv. ‘Onda’) could not complete its life cycle in the absence of added nickel, even when plants were supplied with a nonurea source of nitrogen. In addition, it was shown that growth of oats (Avena sativa L. cv. ‘Astro’) and wheat (Triticum aestivum L. cv. ‘Era’) were significantly depressed under nickel-deficient conditions (15). The laboratory-based observations that Ni deficiency impacts a diversity of plant species has recently been verified in a diverse number of perennial species (Carya, Betula, Pyracantha) growing in the acidic low-nutrient soils of southeastern United States (2).

Nickel is now generally accepted as an essential ultra-micronutrient (16); however, the only defined role of nickel is in the metabolism of urea, a process that is not thought to be essential for plants supplied with a nitrogen source other than urea. The possibility that additional roles for nickel in plants exist was suggested by the results of Brown et al. (1,15), who demonstrated an effect of nickel deprivation in plants grown in the absence of urea and is implied in the work of Wood et al. (2), who demonstrated field responses to Ni supplementation in many ureide-transporting hydrophiles. A broader biological significance of nickel is also implied in the demonstration that nickel is essential for animal life and for a range of bacterial enzymes, including key enzymes in the nitrogen-fixing symbiont, Bradyrhizobium japonicum (17).

Our knowledge of the complete biological significance of nickel for plant productivity is still quite limited; however, with the demonstration of the essentiality of nickel in diverse species (1,2) and the increased use of urea as a nitrogen source, the importance of understanding the chemistry and biology of nickel and its potential impact on agricultural production has never been greater. Evidence that nickel plays an important function in animal and bacterial systems also suggests that nickel plays a larger role in plant productivity than is currently recognized. To obtain a full understanding of the potential role and management of nickel in agricultural systems, it is necessary to review the roles of nickel in other biological systems and to understand the plant and soil conditions under which nickel deficiency is likely to occur.