A speciation model of essential trace metal ions in phloem

► 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