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 Fe3+, Fe2+, Cu2+, Zn2+, Mn2+, Ca2+, and Mg2+ 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 Fe3+, 3.4% of the total Mn2+, and 2.3% of the total Mg2+ but has almost no effect on the speciation of Ca2+, Cu2+, Zn2+, and Fe2+. 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 Fe2+ and 2% for Zn2+. 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.