A speciation model of essential trace metal ions in phloem

Phloem sap is the nutrient rich fluid that transports sugars, amino acids and metal ions from leaves to other parts of the plant. A computer model for the speciation of the trace elements Fe(3+), Fe(2+), Cu(2+), Zn(2+), and Mn(2+) in phloem sap has been developed. The literature on phloem has been reviewed to determine the appropriate concentrations for the metal ions, amino acids and carboxylic acids in phloem sap. Stability constants for the metal complexes with these ligands have been selected from the literature. These data have been used with the program ECCLES to calculate the speciation of the trace elements in the phloem sap. The macronutrients Ca(2+) and Mg(2+) were also included in the model. The model indicates that the hexadentate chelating agent nicotianamine is the most important chelating agent in phloem. This ligand binds almost all the Fe(2+), but it binds only 20 to 50% of the other trace elements. The remainder of each trace element is bound primarily by a combination of several amino acids, most importantly glutamic acid (Fe(3+)), cysteine (Zn(2+)) and histidine (Cu(2+)). The carboxylic acids, including citrate, play a minor role in binding Mn(2+) and Fe(3+), but have almost no impact on the speciation of the other trace elements.