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Articles

Metal Complexation in Xylem Fluid ¹

III. ELECTROPHORETIC EVIDENCE

Rufus L. Chaney

A. Morris Decker

Maryland Environmental Service, Annapolis, Maryland 21401, Biological Waste Management and Organic Resources Laboratory, Agricultural Environmental Quality Institute, Beltsville, Maryland 20705, United States Department of Agriculture, Science and Education Administration-Agricultural Research, Beltsville, Maryland 20705, Department of Agronomy, University of Maryland, College Park, Maryland 20742

The capacity of ligands in xylem fluid to form metal complexes was tested with a series of in vitro experiments using paper electrophoresis and radiographs. The xylem fluid was collected hourly for 8 hours from soybean (Glycine max L. Merr.) and tomato (Lycopersicon esculentum Mill.) plants grown in normal and Zn-phytotoxic nutrient solutions. Metal complexation was assayed by anodic or reduced cathodic movement of radionuclides (⁶³Ni, ⁶⁵Zn, ¹⁰⁹Cd, ⁵⁴Mn) that were presumed to have formed negatively charged complexes.

Electrophoretic migration of Ni, Zn, Cd, and Mn added to xylem exudate and spotted on KCl- or KNO₃-wetted paper showed that stable Ni, Zn, and Cd metal complexes were formed by exudate ligands. No anodic Mn complexes were observed in this test system. Solution pH, plant species, exudate collection time, and Zn phytotoxicity all affected the amount of metal complex formed in exudate. As the pH increased, there was increased anodic metal movement. Soybean exudate generally bound more of each metal than did tomato exudate. Metal binding usually decreased with increasing exudate collection time, and less metal was bound by the high-Zn exudate.

Ni, Zn, Cd, and Mn in exudate added to exudate-wetted paper demonstrated the effect of ligand concentration on stable metal complex formation. Complexes for each metal were demonstratable with this method. Cathodic metal movement increased with time of exudate collection, and it was greater in the high-Zn exudate than in the normal-Zn exudate. A model study illustrated the effect of ligand concentration on metal complex stability in the electrophoretic field. Higher ligand (citric acid) concentrations increased the stability for all metals tested.

¹ This work represents cooperative research from the Department of Agronomy (University of Maryland, College Park, MD), the Biological Waste Management and Organic Resources Laboratory (Agricultural Environmental Quality Institute, United States Department of Agriculture, Science and Education Administration-Agricultural Research, Beltsville, MD), and the Maryland Environmental Service (Annapolis, MD). It is also part of a dissertation submitted by M. C. W. in partial fulfillment of Ph.D. degree requirements.

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