Supramolecular Arrangement of Lignosulfonate-Based Iron Heteromolecular Complexes and Consequences of Their Interaction with Ca2+ at Alkaline pH and Fe Plant Root Uptake Mechanisms

Previous studies have shown that natural heteromolecular complexes might be an alternative to synthetic chelates to correct iron (Fe) deficiency. To investigate the mechanism of action of these complexes, we have studied their interaction with Ca²⁺ at alkaline pH, Fe-binding stability, Fe-root uptake in cucumber, and chemical structure using molecular modeling. The results show that a heteromolecular Fe complex including citric acid and lignosulfonate as binding ligands (Ls-Cit) forms a supramolecular system in solution with iron citrate interacting with the hydrophobic inner core of the lignosulfonate system. These structural features are associated with high stability against Ca²⁺ at basic pH. Likewise, unlike Fe-EDDHA, root Fe uptake from Ls-Cit implies the activation of the main root responses under Fe deficiency at the transcriptional level but not at the post-transcriptional level. These results are consistent with the involvement of some plant responses to Fe deficiency in the plant assimilation of complexed Fe in Ls-Cit under field conditions.