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Articles

Metal Complexation in Xylem Fluid ¹

II. THEORETICAL EQUILIBRIUM MODEL AND COMPUTATIONAL COMPUTER PROGRAM

Fred D. Baker

Rufus L. Chaney

A. Morris Decker

Maryland Environmental Service, Annapolis, Maryland 21401, Statistical Applications Branch, Communications and Data Services Division, United States Department of Agriculture, Science and Education Administration, Beltsville, Maryland 20705, 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

Theoretical considerations of metal complex formation in aqueous solutions were used to develop a computer program (CHELATE) to calculate all equilibrium species (free metal ions, metal complexes, etc.) in any user-defined system, such as xylem fluid. Mass-balance equations were established to describe each free metal ion and each free ligand concentration as a function of solution pH, total metal or total ligand, hydrogen-association constants, and the stability constants of known metal complexes. A default data base can be altered by the user to define any desired system covered by the stored equilibrium data. The program can currently handle nine metal ions, 35 ligands, and 500 complex species. The validity of the program was confirmed by using experimental test systems in which free-metal ion activity measurements were made with ion-selective electrodes.

Program CHELATE was used to calculate the distribution of six metals in 0- to 1-hour exudate from soybean (Glycine max L. Merr.) and tomato (Lycopersicon esculentum Mill.) plants grown in normal and Zn-phytotoxic nutrient solutions. The results indicated that Fe is bound by citric acid, and Cu is bound by several amino acids in the normal-Zn exudate. Most of the Cu in soybean exudate is bound to asparagine and histidine. In tomato, Cu is bound to histidine, glutamine, and asparagine. Zinc, Mn, Ca, and Mg are bound primarily by citric acid and malic acid in both species; the per cent bound for these metals is lower than that for Fe and Cu. Zinc phytotoxicity caused equilibrium concentration shifts and resulted in the formation of several additional metal complexes not found in the normal-Zn exudate.

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 the M. C. W. in partial fulfillment of Ph.D. degree requirements.

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