Zinc Absorption by Wheat Seedlings and the Nature of its Inhibition by Alkaline Earth Cations

Abstract

The effects and interactions of the alkaline earth cations on Zn²⁺ absorption were studied in. short-term experiments. At low concentrations of Zn²⁺ (≤ 2 μM), rates of Zn²⁺ absorption were linear even in the absence of Ca²⁺ or of other cations. At higher Zn²⁺ concentrations (5 and 10μm), rates were not linear in the absence of other cations but became linear on addition of 250 μM or more of Ca²⁺, Mg²⁺, Sr²⁺, or Ba²⁺. From 0.1 to 10μM Zn²⁺, all alkaline earth cations inhibited absorption in the order Mg²⁺ > Ba²⁺ ≥ Sr²⁺ = Ca²⁺. Increasing concentrations of Ca²⁺ or of Mg²⁺ from 0 to 40 mM progressively depressed absorption from 1μM ZnCl₂. Increasing Ca²⁺ or Mg²⁺ from 40 to 100 mM had no further effect on absorption. Over both high and low ranges of Ca²⁺ or Mg²⁺ concentrations, the affinity of plant roots for Zn²⁺ and the responses of Zn²⁺ absorption to temperature, H⁺, and Cu²⁺ were identical. At equal concentrations over the whole concentration range, Mg²⁺ was 30 per cent more effective than Ca²⁺ in inhibiting absorption. At concentrations below 40 mM, Ca²⁺ and Mg²⁺ competed with each other in their inhibiting effects. At concentrations above 40 mM, Ca²⁺ alleviated the extra inhibitory effects of Mg²⁺ in some unknown way. The alkaline earth cations inhibited Zn²⁺ absorption non-competitively. They depressed it to values which would limit vigorous plant growth. It is postulated that their effects are important in the zinc nutrition of plants in soil and in solution cultures.