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 Fe3+, Fe2+, Cu2+, Zn2+, and Mn2+ 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 Ca2+ and Mg2+ 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 Fe2+, 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 (Fe3+), cysteine (Zn2+) and histidine (Cu2+). The carboxylic acids, including citrate, play a minor role in binding Mn2+ and Fe3+, but have almost no impact on the speciation of the other trace elements. A model for the metal and ligand components in phloem has been developed and used to calculate the speciation of Fe3+, Fe2+, Cu2+, Zn2+, and Mn2+. Nicotianamine is an important ligand for these metal ions, but there is significant metal binding by glutamate, histidine, and cysteine. [Display omitted] ► Phloem contains low concentrations of the aquo ions of Fe3+, Cu2+, Zn2+ or Fe2+ ► Nicotianamine binds over 99% of Fe2+ in phloem but only ~20 – 50% of Fe3+, Cu2+, Zn2+ and Mn2+ ► About 70% of low-molecular-weight Fe3+ is bound to glutamate ► Almost 60% of Cu2+ is bound as Cu(his)(L) mixed ligand complexes of histidine ► Over 95% of Zn2+ is bound to either nicotianamine or cysteine