Abstract: To understand the roles of mycorrhiza in metal speciation in the
rhizosphere and the impact on increasing host plant tolerance against excessive
heavy metals in soil, maize (Zea mays L.) inoculated with arbuscular
mycorrhizal fungus (Glomus mosseae) was cultivated in heavy metal contaminated
soil. Speciations of copper, zinc and lead in the soil were analyzed with the
technique of sequential extraction. The results showed that, in comparison to
the bolked soil, the exchangeable copper increased from 26% to 43% in
non-infected and AM-infected rhizoshpere respectively; while other speciation
(organic, carbonate and Fe-Mn oxide copper) remained constant and the organic
bound zinc and lead also increased but the exchangeable zinc and lead were
undetectable. The organic bound copper, zinc and lead were higher by 15%, 40%
and 20%, respectively, in the rhizosphere of arbuscular mycorrhiza infected
maize in comparison to the non-infected maize. The results might indicate that
mycorrhiza could protect its host plants from the phytotoxicity of excessive
copper, zinc and lead by changing the speciation from bio-available to the
non-bio-available form. The fact that copper and zinc accumulation in the roots
and shoots of mycorrhia infected plants were significantly lower than those in
the non-infected plants might also suggest that mycorrhiza efficiently
restricted excessive copper and zinc absorptions into the host plants. Compared
to the non-infected seedlings, the lead content of infected seedlings was 60%
higher in shoots. This might illustrate that mycorrhiza have a different
mechanism for protecting its host from excessive lead phytotoxicity by
chelating lead in the shoots.
Keywords: heavy metal speciation, arbuscular mycorrhiza, Glomus mosseae, rhizosphere
