Role of Mycorrhizae in Heavy Metal Speciation

Roles of mycorrhizae in metal speciation in rhizosphere showed increased heavy metal tolerance in host plants and a significant change in the Cu, Zn and Pb spe-ciation was observed in rhizosphere of AM infected and uninfected maize. The greatest change was exchangeable Cu, which increased by 26% and 43% in the uninfected and AM-infected rhizospheres, respectively, compared to that in bulk soil. With the exception of organically bound Cu in AM, other metal species were stable in the rhizospheres of the AM and non-AM treatments. It is understandable that Cu was activated by inducing rhizobacteria (Huang et al. 2005). The organically bound Zn and Pb increased significantly in the rhizosphere in comparison to those in the bulked soil. In contrast, carbonate and Fe-Mn oxides of Zn and Pb did not exhibit significant changes. The results may indicate that the mycorrhiza can protect its host plants from the phytotoxicity of excessive copper, zinc, and lead by changing their speciations from bioavailable to nonbioavailable forms. The fact that copper and zinc accumulations in the roots and shoots of mycorrhiza-infected plants were significantly lower than those in the uninfected plants may also suggest that the mycorrhiza efficiently restricted excessive copper and zinc absorption by the host plants (Huang et al. 2005).

It is now well known that heavy metals cannot be chemically degraded. Therefore, remediation of metal-polluted soils is limited mainly to immobilization, for example by phytostabilization, which involves promoting plant growth in order to reduce or eliminate the bioavailability of metals. In this context, AMF constitute an important functional component of the soil-plant system that is crucial to sustainable productivity in stressed soils. A better understanding of the mechanisms behind these changes in AMF diversity - and particularly of those upon which AMF adaptation to and tolerance of metals are based - is important, since such an understanding could facilitate the management of these soil microorganisms for a restoration and/or bioremediation program.

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