Enhancing Zinc Bioavailability in Rice Using the Novel Synthetic Siderophore Ligand Proline-2′-Deoxymugineic Acid (PDMA): Critical Insights from Metal Binding Studies and Geochemical Speciation Modeling

Bioavailable ligands that bind metals mediate their uptake in plants, leading to the study of artificial ligands as potential fertilizers. Proline-2′-deoxymugineic acid (PDMA) has shown a high affinity for Fe^III, enhancing iron uptake in rice and suggesting that it could be used for improving zinc uptake. This work studied chemical solution parameters, i.e., redox potential, ion strength, pH, and ligand/metal concentrations controlling Zn^II–PDMA complex formation in rice-producing soils using geochemical speciation modeling. We show that PDMA is generally selective for Zn^II in reducing, saline, and alkaline soil solutions. Comparison with a recent micronutrient uptake study in rice suggests that free PDMA should be added in reducing conditions to avoid competition with Cu^II and Fe^III or as the Zn^II–PDMA complex at pH below 9. The Zn/M ratios (M = Cu^II, Fe^III) needed to form stable Zn^II–PDMA complexes were also identified. This study shows the promise of PDMA as a fertilizer to overcome zinc deficiencies in alkaline and flooded soils.