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

Abstract

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 (Fe 3+ :HBED and Fe 3+ :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 Fe 3+ :G7 at various ratios. The effectiveness of the Fe fertilizers depend on the Fe 3+ :ligand ratio and the ligand type, the Fe 3+ :G7 (1:1 and 1:2) being the most effective. Fe 3+ :G7 (1:1) also presented a better response for the uptake of other micronutrients. CONCLUSION: Fe 3+ :G7 molar ratios have been shown to be critical for plant Fe uptake under hydroponic conditions and with calcareous soil. Thus, the Fe 3+ :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 plants.

Implications of the Mn:ligand ratio for Mn uptake by Glycine max L. plants fertilized with heptagluconate and gluconate complexes

“…These results are consistent with previous studies, 9,10 where G7 and G6 present higher stability than EDTA at maintaining the Mn soluble in alkaline solutions at pH 9; above pH 9, G7 has a greater capacity than G6. Moreover, in the present study, G7 also demonstrated a greater complexing capacity than G6 at alkaline pH, as has previously been obtained for Fe 16,18 …”

“…The ability of the Mn 2+ :G7 (1:1 and 1:2) complexes to provide Mn in hydroponics may be related to their metal:ligand affinity, which promotes an adequate equilibrium between the metallic micronutrients (Table 2), limiting Mn displacement. As shown in a previous experiment 16 in which Fe was applied as Fe 3+ :G7 in the same molar ratios (1:1 and 1:2) in Fe‐deficient plants under hydroponic conditions, a greater elongation of root and Mn uptake was promoted.…”

“…Moreover, the importance of the metal:ligand ratio in the preparation of the complexes has already been demonstrated for Fe complexes 16 . This study compared Mn–G6 and Mn–G7 complexes as biodegradable fertilizers and examined the importance of the Mn 2+ :ligand ratio in relation to their chemical characteristics.…”

“…In contrast, in nutrient solutions, the efficacy of Mn complexes has been related to the high metal:ligand affinity due to limited competition between the metals 5 . With regard to the metal:ligand affinity, several studies 14–16 have shown the effect of the metal:ligand ratio on complex stability and its ability to provide micronutrients to the plants. All these experiments have been conducted by testing the Fe:ligand molar ratio, but not the Mn:ligand ratio.…”

“…Furthermore, the use of biodegradable complexing agents such as G6 and G7 is another eco‐friendly, low‐cost alternative 17 . The strong ability of G6 and G7 to complex metals at alkaline pH, especially G7, 10,16,18 and the lower affinity of both ligands to complex Ca 2+ in comparison with traditional chelates 19,20 suggest their potential capacity for use as Mn fertilizers.…”