Low-Molecular-Weight Organic Acids Exuded by Millet (Setaria italica (L.) Beauv.) Roots and Their Effect on the Remediation of Cadmium-Contaminated Soil

Cadmium (Cd)-contaminated soils are of concern because of their possible effects on ecosystems and human health. Millet (Setaria italica (L.) Beauv.) is moderately tolerant to salinity and poor soil fertility as well as pristine soils contaminated with trace elements. It has been hypothesized that low-molecular-weight organic acids (LMWOA) exuded into the rhizosphere may play important roles in Cd uptake by millet. Thus the objectives of this study were to assess the LMWOA exudations of millet in Cd-contaminated soils and to evaluate the capacity of millet to remove Cd for phytoremediation. Millet was grown for 3 weeks in soils containing between 0 and 1,000 mg Cd kg(-1). These soils and plant tissues were analyzed for Cd content after harvest. The Cd accumulated in millet tissues was significantly increased with increasing Cd-amended rate. The total LMWOA concentrations in root exudates were significantly correlated with the amount of Cd accumulated in millet shoots and roots. High levels of Cd amendment stimulated secretion of LMWOA, a process capable of reducing Cd toxicity via the formation of Cd-organic complexes (Cd-LMWOA), thereby detoxifying the contaminated soil.