Nickel (Ni), the most recently discovered essential element (1), is unique among plant nutrients in that its metabolic function was determined well before it was determined that its deficiency could disrupt plant growth. Subsequent to the discovery of its essentiality in the laboratory, Ni deficiency has now been observed in field situations in several perennial species (2). The interest of plant scientists in the role of nickel was initiated following the discovery in 1975 (3) that it was a critical constituent of the plant enzyme, urease. The ultimate determination that nickel was essential for plant growth (1) depended heavily on the development of new techniques to purify growth media and to measure extremely low concentrations of nickel in plants. The establishment of nickel as an essential element, however, highlights the limitations of the current definition of essentiality of nutrients as applied to plants (4). It has been argued, for example, that even though nickel is clearly a normal and functional constituent of plants, it does not fulfill the definition of essentiality, since urease is not essential for plant growth and nickel deficiency apparently does not prevent the completion of the life cycle of all species, even though that criterion has not been explicitly satisfied for any element (5). Several authors (5,6) now suggest that the criteria for essentiality should be modified to include elements that are normal functional components of plants.

As our ability to determine the molecular structure, function, and regulation of biological systems improves, it is quite likely that additional elements will be shown to have irreplaceable functions in discrete biochemical processes that are important for plant life. This determination will be supplemented by advances in molecular and structural biology that will help predict the occurrence of similar processes across all organisms, allowing the relevance of discoveries made in bacterial systems to be immediately tested in plant and animal systems. The discovery of the essentiality of nickel is a good illustration of this principle and is likely to be repeated in the coming years. Nickel represents the first of several likely new essential elements that will be shown to be critical for certain metabolic processes normally active in plants, but not necessarily essential for the completion of the species’ life cycle under all conditions.

The current definition of essentiality is clearly inadequate and its acceptance likely stifles the search for new essential elements. It is proposed, therefore, that the definition of essentiality be modified to more closely resemble that utilized in animal biology (7).



An element shall be considered essential for plant life if a reduction in tissue concentrations of the element below a certain limit results consistently and reproducibly in an impairment of physiologically important functions and if restitution of the substance under otherwise identical conditions prevents the impairment; and, the severity of the signs of deficiency increases in proportion to the reduction of exposure to the substance. (Nielson (7))


By this criterion, nickel is an essential element as are silicon and cobalt, which are essential elements for nitrogen-fixing plants.