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Zinc Absorption by Wheat Seedlings: II. Inhibition by Hydrogen Ions and by Micronutrient Cations¹

ABSTRACT

At very low concentrations of Ca²⁺ in solution both H⁺ and Cu²⁺ retarded Zn²⁺ absorption through two distinct effects—toxicity and inhibition. Concentrations of 350µM Ca²⁺ overcame toxic effects of H⁺ at pH 3 but even 500µM Ca²⁺ did not prevent toxicity of Cu²⁺ at 10µM.

At Ca²⁺ concentrations commonly encountered in soil solutions no toxicities appeared. Copper inhibited Zn²⁺ absorption strongly and competitively, Co²⁺ competitively but weakly, and Mn²⁺ and Fe²⁺ had no effect. Also, Ca²⁺ had no effect on Cu²⁺ inhibition.

Increasing H⁺ concentration from pH 7 to 3 in the presence of 350µM, 10mM, or 50mM Ca²⁺ severely inhibited Zn²⁺ absorption. Hydrogen ions inhibited Zn²⁺ absorption by two mechanisms—one Ca²⁺-sensitive, the other Ca²⁺-insensitive. The Ca²⁺-sensitive mechanism only operated at very low Ca²⁺ or under very acid conditions. At the usual H⁺ and Ca²⁺ concentrations of soil solutions, H⁺ inhibited Zn²⁺ absorption only by the Ca²⁺ insensitive mechanism; in this mechanism H⁺ was non-competitive with Zn²⁺.

In soils the effects of Ca²⁺, Cu²⁺, and of H⁺ would be additive. However the effects of Cu²⁺ and especially of H⁺ on soil chemical reactions may operate in the opposite direction to their effects on Zn²⁺ absorption. More information is needed on the behavior of chelating compounds before the effects of Cu²⁺ on Zn²⁺ absorption from soils can be assessed.

¹ Contribution from the Department of Soil Science & Plant Nutrition, Institute of Agriculture, University of Western Australia, Nedlands 6009. This work was supported by the West Australian Wheat Research Committee and by the grant of a Colombo Plan Studentship to the senior author.

² Ph.D. student and Associate Professor in Plant Nutrition, respectively. Present address of the senior author, Radiation Genetics Institute, Atomic Energy Commission, Lyallpur, West Pakistan.

Received for publication June 18, 1971. Accepted for publication November 9, 1971.