Effect of Fe:ligand ratios on hydroponic conditions and calcareous soil in Solanum lycopersicum L. and Glycine max L. fertilized with heptagluconate and gluconate

BACKGROUND: The environmental risk from the application of synthetic chelates has led to the use of biodegradable complexes to correct Fe deficiency in plants. In this article, the Fe oxidation state, the Fe:ligand ratio, and the molecular weight distribution for heptagluconate (G7) and gluconate (G6) are considered as key factors for the efficacy of complexes as fertilizers. Complexes with different Fe:ligand ratios were prepared and analyzed by gel filtration chromatography (GFC). The ability of Fe:ligand ratios to provide Fe to tomato in hydroponics and soybean in calcareous soil was tested and compared with synthetic chelates (Fe3+:HBED and Fe3+:EDTA). RESULTS: G7 presented greater capacity to complex both Fe(II) and Fe(III) than G6, but the Fe(II) complexes exhibited poor stability at pH 9 and oxidation in solution. Gel filtration chromatography demonstrated the polynuclear nature of the Fe3+:G7 at various ratios. The effectiveness of the Fe fertilizers depend on the Fe3+:ligand ratio and the ligand type, the Fe3+:G7 (1:1 and 1:2) being the most effective. Fe3+:G7 (1:1) also presented a better response for the uptake of other micronutrients. CONCLUSION: Fe3+:G7 molar ratios have been shown to be critical for plant Fe uptake under hydroponic conditions and with calcareous soil. Thus, the Fe3+:G7 at equimolar ratio and 1:2 molar ratio can be an environmentally friendly alternative to less degradable synthetic chelates to correct Fe chlorosis in strategy I plantsThis work was supported by the State Research Agency, Ministry of Science, Innovation and Universities of Spain (projects AGL2013-44474-R and RTI2018-096268-B-I00) and the Comunidad de Madrid (Spain) and Structural Funds 2014-2020 (ERDF and ESF) (project AGRISOST-CM S2018/BAA-4330). SIV would like to thank the Consejo Nacional de Ciencia y Tecnología (CONACYT, Mexico) for supporting her grant through PhD studentship number 278934, and the Consejo de Ciencia, Tecnología e Innovación de Hidalgo (CITNOVA, Mexico