Zinc Absorption by Wheat Seedlings: I. Inhibition by Macronutrient Ions in Short‐Term Experiments and its Relevance to Long‐Term Zinc Nutrition

Abstract

Short‐term Zn²⁺ absorption by intact wheat seedlings (Triticum aestivum L.) was studied to resolve large discrepancies between Zn²⁺ absorption in short‐term experiments with excised roots and in long‐term experiments with whole plants.Excision, pretreatment, and age of seedlings had no effect on short‐term Zn²⁺ absorption.Zn²⁺ absorption responded to changes in temperatures with Q₁₀ values of 6.1, 1.8, and 1.9 for the temperature ranges 2 to 10, 10 to 20, and 20 to 29C, respectively. Addition of macronutrient salts to 250µM Ca(NO₃)₂ in the absorbing solution decreased Zn²⁺ absorption by about 80%. Micronutrient salts had a small additional effect on Zn²⁺ absorption.Cations rather than anions inhibited Zn²⁺ absorption. Nitrate salts of alkali and alkaline earth cations strongly depressed Zn²⁺ absorption. In the presence of 250µM Ca(NO₃)₂, nitrates of alkali cations at 750µM and of additional Ca²⁺ at 250 and 375µM depressed Zn²⁺ absorption from 1µM ZnCl₂ in the order NH₄⁺ > Rb⁺ > K⁺ > Cs⁺ > Ca²⁺ > Na⁺ > Li⁺. Nitrates of alkaline earth cations at concentrations of 250µM depressed Zn²⁺ absorption in the order Mg²⁺ > Ba²⁺ ≥ Sr²⁺ = Ca²⁺. Replacing NO₃^‐ with Cl^‐, SO₄^2‐, or H₂PO₄^‐, had no effect on Zn²⁺ absorption.The inhibitory effects of alkali and alkaline earth cations appeared to be mutually competitive. Large effects of K⁺ at low concentrations of Ca²⁺ decreased and finally disappeared with increasing concentrations of Ca²⁺.The rates of Zn²⁺ absorption by 6‐day‐old, intact wheat seedlings closely approximated those reported for long‐term experiments over a wide range of solution Zn²⁺ concentrations from 0.05 to 5.0µM when the temperature and the composition of the absorbing solutions were identical. These results suggest that it may be possible to relate short‐term studies of Zn²⁺ absorption to long‐term studies of Zn nutrition.