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

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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.
Funding Information
  • West Australian Wheat Research Committee
  • Colombo Plan Studentship

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