Foliar uptake of zinc by vascular plants: radiometric study

The aim of this paper was to obtain quantitative data of foliar uptake kinetics and long distance transport of zinc in tobacco (Nicotiana tabacum L.) and hop (Humulus lupulus L.) plants. Zinc was used as a model of microelement and toxic metal, tobacco and hop as a representatives of agriculturally important plants. A tip of leaf blade was immersed in the solution spiked with ⁶⁵ZnCl₂ and foliar uptake and translocation to other parts of the plant grown in nutrient solution was measured by gamma-spectrometry and autoradiography. We found that foliar zinc uptake by both plants is dependent on the initial metal concentration within the range C ₀ = 10–100 μmol dm⁻³ ZnCl₂. Zinc is immobilized mainly in immersed part of the contact leaf and only <1% is transported to non-immersed parts of the leaf. At C ₀ = 0.1 mmol dm⁻³ ZnCl₂ concentrations >2.5 mg/g Zn and 4.8 mg/g Zn (dry wt.) in immersed part of tobacco and hop leaf plant, respectively were found after 5 days of exposure. Low mobility of zinc entering the plant via the leaf surface can be attributed to the immobilization of zinc into Zn–ligand complexes with high stability constants log K at pH 6.0–8.0, such as the reaction products of Zn²⁺ ions with citric acid, histidine or phosphates. Zinc can be extracted from dried leaves by the solutions of inorganic salts, carboxylic acids, amino acids and synthetic complexing ligands such as EDTA. Anionic (SDS) and non-ionic (Tween 40) surfactants causes the decrease of the Zn foliar uptake, but not translocation of Zn from the contact leaf area. Obtained data are discussed from the point of view of possible limited efficiency of liquid formulations designed for practical applications as Zn foliar fertilizers.