Obligatory reduction of ferric chelates in iron uptake by soybeans

The contrasting Fe²⁺ and Fe³⁺ chelating properties of the synthetic chelators ethylenediaminedi (o-hydroxyphenylacetate) (EDDHA) and 4,7-di(4-phenylsulfonate)-1, 10-phenanthroline (bathophenanthrolinedisulfonate) (BPDS) were used to determine the valence form of Fe absorbed by soybean roots supplied with Fe³⁺-chelates. EDDHA binds Fe³⁺ strongly, but Fe²⁺ weakly; BPDS binds Fe²⁺ strongly but Fe³⁺ weakly. Addition of an excess of BPDS to nutrient solutions containing Fe³⁺-chelates inhibited soybean Fe uptake-translocation by 99+%; Fe(II) (BPDS)₃]⁴⁻ accumulated in the nutrient solution. The addition of EDDHA caused little or no inhibition. These results were observed with topped and intact soybeans. Thus, separation and absorption of Fe from Fe³⁺-chelates appear to require reduction of Fe³⁺-chelate to Fe²⁺-chelate at the root, with Fe²⁺ being the principal form of Fe absorbed by soybean.