Significance of root exudates in acquisition of heavy metals from a contaminated calcareous soil by graminaceous species

Abstract

Graminaceous species release phytosiderophores for mobilization of iron (Fe) under Fe deficiency stress. In this study the possible risk that these root exudates might also enhance the uptake of toxic heavy metals from contaminated soils, was investigated. For this purpose wheat (Triticum aestivum L. cv. Piko) and sorghum (Sorghum bicolor (L.) Moench cv. E‐610), differing in release of phytosiderophores were precultured with roots in nylon bags in a sand culture system for low and high Fe nutritional status (+/‐Fe supply) for two weeks. After this preculture the nylon bags were brought in contact with a sewage sludge or heavy metal salt‐contaminated calcareous soil in a climate chamber for an additional 8 days growth period. At harvest the concentration of Fe, Zn, Ni and Cd in roots and shoots were analysed. The results clearly show that both plant species have mobilized Fe and heavy metals from the contaminated soil. Plants precultured for low Fe nutritional status had a higher uptake of Fe, Zn, Ni and Cd (up to 200%) than control plants with adequate Fe nutritional status. This enhanced acquisition of heavy metals was particularly expressed in wheat as an Fe efficient graminaceous species with corresponding higher release of phytosiderophores under Fe deficiency stress than sorghum.