Metals essentials for plants: the nickel case

The long period of research that preceded the discovery of nickel (Ni) essentiality for plants constitutes a paradigmatic case of doubts and uncertainties that often occur in experimental biology. The history of the essentiality of chemical elements that are present as traces in the plant ash (micronutrients) began in the mid-Nineteenth, but it had blurred outlines until Daniel Arnon, towards the mid-twentieth century, fixed the now historic 'criteria of essentiality'. During this rather long time, seven micronutrients were recognised, step by step, as essential for higher plants, (iron, manganese, boron, Zinc, copper, molybdenum, and chloride), at first thanks to meticulous observations of deficiency symptoms and then to the culture of plant on aqueous solutions. The last element to be recognised as essential for plant nutrition was Ni, which was considered a very toxic element for more than a century. Towards the Thirties, Ni became to be regarded as a useful element by some researchers, but the ultimate proof of its essentiality was obtained only in the Eighties, when the American group of Ross M. Welch demonstrated that Ni is a cofactor of the enzyme urease. More recent research shows that Ni improves the nitrogen (N) metabolism and appears to be important for the efficiency of N fixation.