Studies On The Mechanisms Of Arbuscular Mycorrhizae(AM) In Heavy Metals Tolerance

The toxic metals in the soil pose severe problems to the environment and human health. As a functional link between soil and roots, mycorrhizae are supposed to play an important role in bioremediation of soil contaminated by heavy metals . Systematic studies in this field may lead to new bioremediation strategies.Although some metal-tolerant strains of arbuscular mycorrhizal fungi (AMF) have been isolated in recent decade, bioremediation requires not only tolerance to the heavy metals but also translocation of them from the soil to the plants. The mechanisms of alleviation of metal phytotoxicity in the mycorrhizal plants, as well as the interactions between mycorrhizae and toxic metals, are keys in this issue.Maize (Zea mays L., cv. nongda 108) was grown in calcareous soil pot cultures in the presence or absence of zinc (Zn), cadmium (Cd) and copper (Cu) ( applied separately or in combination), and inoculated or not with different strains of AMF (G. mosseae G. intraradices G. Caledonium G. clarum G. spp.). Mycorrhizal effects on plant growth, toxic elements uptake and distribution in host maize are evaluated in order to screening heavy metals tolerant mycorrhizal strain. Systematic studies on the mechanisms and interactions between mycorrhizae~host plants and heavy metals attempted to provide a theoretical basis for the possible application of arbuscular mycorrhizal symbiosis in bioremediation of heavy-metal-contaminated soil.The experiments showed that at different levels of Zn or Cd G. Caledonium is the best in promoting plant growth and mycorrhiza formation - the percentage of colonization was 41.6% and 33.6% respectively when the highest level of Zn (900 mg kg-1) or Cd (150 mg kg-1) was applied. So, it was selected as the inoculating strain in the following experiments.When exposed to Zn treatment alone, the forming of mycorrhizae promoted growth of maize and presented Zn-tolerance as well. At the highest Zn level (600 mg kg-1), total P uptake by mycorrhizal maize was 61.4%, while the dry matter of leaves and roots were 5.4% and 16.2%, more than that ofnon-mycorrhiza plant. At the Zn level of 300 mg kg-1, both the concentration and accumulation of Zn in roots of mycorrhizal maize enhanced. At the 600 mg ?kg'1 Zn level, not only Zn accumulation in roots increased, but also Zn translocation to shoots was promoted in mycorrhizal maize. The experiments indicated that the bio-dilution effect played an important role in alleviation of Zn phytotoxicity after inoculated by AMF. Since more Zn was removed from soil, thus, it suggested that mycorrhizal plants might help for the bioremediation in Zn contaminated soil. By analyzing the effect of mycorrhizae on mineral nutrition of maize, the experiments revealed that mycorrhizae significantly stabilized concentration of the elements such as Fe, Cu, Mn and Ca in the plants. Therefore, growth of maize under stress of Zn was much better than that of non- mycorrhizal plants. These results demonstrated how the establishment of mycorrhizae improved heavy-metal tolerance of host plants.In the experiments that Cu supplied alone, the infection rate of mycorrhizae was 54.7%, which suggested that G. caledonium was possessed of Cu-tolerance. The Cu-tolerance of Mycorrhizal maize was attributed to improving P nutrition, preventing acidification and decreasing Cu content in soil. Even at Cu level of 150 mg kg-1, biomass and length of the roots of mycorrhizal maize was enhanced 109.9% and 36.3% respectively. Cu concentration in shoots and roots of mycorrhizal maize was 25.7% and 24.1% less than in non-mycorrhizal plant, respectively. Moreover, total Cu accumulation in mycorrhizal maize increased 58.7%, but the most was immobilized in root. The experiments strongly indicated that mycorrhizal effect could promote Cu uptake and accumulation in roots of host plants by decreasing Cu concentration relatively, therefore this kind of bio-dilution might help to remove Cu from soil.At the Cd level of 25 mg kg-1, total P uptake by mycorrhizal maize was about 4 times greater than non- my...