Chelation of Trace Metals in Nutrient Solutions.

Plant nutrition studies frequently require nearly complete elimination of trace metals from the nutrient media in which the plants are to be grown. Conventional methods of purification by recrystallization and precipitation have been used for this purpose in some studies. These have proved more difficult and less effective than the adsorption technique developed by STEINBERG (11) and improved by ARNON and STOUT (4), in which heavy metals are coprecipitated with calcium phosphate. In recent years attention has been called to the reaction of heavy metals with certain organic molecules to form chelate complexes which can be removed from nutrient solutions by extraction (7, 8, 12). This means of eliminating trace metals from nutrient solutions suggested the possibility that such chelating agents might be used in nutrient solutions in direct contact with the roots of growing plants to provide a continuing control of trace metal contamination in the nutrient media. The present study was undertaken to determine the effect of chelating agents added to nutrient solutions on plants growing in these solutions. Tests reported describe the responses of seedlings of several species and of older tomato plants to six such chelating compounds present in nutrient solutions. A chelating agent (from the Greek Chele, meaning claw) usually is a complex organic molecule containing two electron-donating groups which, acting similar to the two jaws of a claw, may remove simple metal ions from solution. These chelating reactions are slightly reversible, the degree of reversibility, characterized as the "stability constant," depending on the agent used and the metal ion involved. The mechanism of chelation is illustrated with the following example. Oxine (fig. 1, A), 8-hydroxyquinoline (HOC6H3N: CHCH: CH), is a double ring compound formed by the fusion (in the ortho position) of a pyridine ring to a benzene ring with the hydroxyl group on the eighth position of the double ring compound. Oxine ionizes to form oxinate and hydrogen ions, the hydrogen coming from the hydroxyl group. The chelating effect of oxine results when the oxygen and nitrogen, each acting as a claw, join to a metal ion (Zn++) thus forming a ring, called a chelate ring. The oxygen, with a free negative valence, forms an electrovalent linkage with the zinc ion while nitrogen forms a co-ordinate covalent bond. Thus the zinc ion is bound securely to the oxine molecule. However, the zinc ion has two free valences and for this reason two molecules of the oxine are required to 1 Contribution from the Botanical Laboratory, The IUniversity of Tennessee, New Ser. no. 119. 573