Nickel in higher plants: further evidence for an essential role

Soybeans (Glycine max [L.] Merr.) grown in Ni-deficient nutrient solutions accumulated toxic urea concentrations which resulted in necrosis of their leaflet tips, a characteristic of Ni deficiency. Estimates of the Ni requirement of a plant were made by using seeds produced with different initial Ni contents. When compared to soybeans grown from seeds containing 2.5 nanograms Ni, plants grown from seeds containing 13 nanograms Ni had a significantly reduced incidence of leaflet tip necrosis. Plants grown from seeds containing 160 nanograms Ni produced leaves with almost no leaflet tip necrosis symptoms. Neither Al, Cd, Sn, nor V were able to substitute for Ni.In other experiments, a small excess of EDTA was included in the nutrient solution in addition to that needed to chelate micronutrient metals. Under these conditions, nodulated nitrogen-fixing soybeans had a high incidence of leaflet tip necrosis, even when 1 micromolar NiEDTA was supplied. However, in nutrient solutions containing inorganic sources of N, 1 micromolar NiEDTA almost completely prevented leaflet tip necrosis, although no significant increase in leaf urease activity was observed. Cowpeas (Vigna unguiculata [L.] Walp) grown in Ni-deficient nutrient solutions containing NO(3) and NH(4) also developed leaflet tip necrosis, which was analogous to that produced in soybeans, and 1 micromolar NiEDTA additions prevented these symptoms.These findings further support our contention that Ni is an essential element for higher plants.