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.

Computer simulation of the interactions of glyphosate with metal ions in phloem.

Essential nutrients such as trace metal ions, amino acids, and sugars are transported in the phloem from leaves to other parts of the plant. The major chelating agents in phloem include nicotianamine, histidine, cysteine, glutamic acid, and citrate. A computer model for the speciation of metal ions in phloem has been used to assess the degree to which the widely used herbicide glyphosate binds to Fe(3+), Fe(2+), Cu(2+), Zn(2+), Mn(2+), Ca(2+), and Mg(2+) in this fluid over the pH range of 8 to 6.5. The calculations show that glyphosate is largely unable to compete effectively with the biological chelating agents in phloem. At a typical phloem pH of 8, 1.5 mM glyphosate binds 8.4% of the total Fe(3+), 3.4% of the total Mn(2+), and 2.3% of the total Mg(2+) but has almost no effect on the speciation of Ca(2+), Cu(2+), Zn(2+), and Fe(2+). As the pH decreases to 6.5, there are some major shifts of the metal ions among the biological chelators, but only modest increases in glyphosate binding to 6% for Fe(2+) and 2% for Zn(2+). The calculations also indicate that over 90% of the glyphosate in phloem is not bound to any metal ion and that none of the metal-glyphosate complexes exceed their solubility limits.