The significance of biodegradable methylglycinediacetic acid (MGDA) for iron and arsenic bioavailability and uptake in rice plant

Methylglycinediacetic acid (MGDA) is a readily biodegradable complexing agent in compliance with Organization for Economic Cooperation and Development standards. In the present study, the use of MGDA for iron (Fe) and arsenic (As) bioavailability and uptake by rice plants (Oryza sativa L.) was investigated. The highest plant biomass was observed at pH 7, and the growth of rice seedlings decreased significantly (p < 0.05) with increasing pH of the nutrient solution. This might be due to Fe deficiency to the plant at alkaline pH. When rice seedlings were grown with different concentrations of MGDA (0.1, 0.25, 0.5, 1.0, 2.5, and 5 mM), the highest plant biomass was observed at 0.25 mM MGDA, while further increases of the ligand concentration decreased the plant growth. Fe concentrations on rice root surfaces decreased gradually with increasing MGDA concentrations in the growing medium, while Fe concentrations in rice roots and shoots increased with increasing MGDA concentrations up to 0.25 mM and then decreased gradually. This indicates that the concentration of the chelating ligand influences Fe uptake in the plant. Arsenic concentrations on rice root surfaces decreased, while As concentrations in roots and shoots increased with the addition of MGDA in the growing medium, indicating that the ligand enhanced As bioavailability and uptake in rice. The mechanism behind the MGDA effect on Fe and As uptake in plant is likely to be due to that Fe exists mostly in insoluble particulate forms [e.g., ferric oxide (Fe2O3), ferric hydroxide (Fe(OH)3) and ferric oxyhydroxide (FeOOH)] at neutral or alkaline pH, and the soluble [e.g., ferric ion (Fe3+), iron hydroxide ion (Fe(OH)2+) and iron dihydroxide ion ()] and apparently soluble (colloidal) fractions of Fe are increased at moderate concentrations of the ligand that increases Fe bioavailability. Since arsenate [As(V)] binds to the insoluble Fe-oxides/hydroxides, the binding sites for As(V) decreases with the increase of the soluble fractions of Fe by the ligand, which slightly increased As uptake in rice plants.