Adsorption Of Heavy Metals By Ectomycorrhizal Fungi And Influence Of Suillus Granulatus On Heavy Metal Tolerance In Chinese Pine

Vegetation restoration on heavy metal contaminated soils is an important issue in the reclamation of degraded land. Ectomycorrhizal fungi are known to enhance growth of their host plants and increase the tolerance of woody plants growing under adverse soil conditions. It has therefore been suggested that ectomycorrhizal fungi can perhaps be used in the restoration of heavy metal contaminated soil in which woody plants are grown. The present study investigated the effect of inoculation of ectomycorrhizal fungi on plant tolerance to heavy metals and some of the mechanisms that may be involved, with the main focus on fungal adsorption of heavy metals. The study consisted of two major parts: the tolerance of two ectomycorrhizal fungi (Paxillus involutus Batsch Fr. and Suillus granulatus (L.ex Fr.) O.Kuntze) to Zn, Cd and Pb, and the adsorption of heavy metals by three fungi (Paxillus involutus, Suillus granulatus and Suillus bovinus (L.ex Fr.) O.Kuntze). The experiments were conducted in vitro, and a pot experiment was also carried out to investigate the effect of inoculation of one of the ectomycorrhizal fungi (Suillus granulatus (L.ex Fr.) O.Kuntze) on the tolerance of Chinese pine (Pinus tabulaeformis Carr.) to Zn and Cd.Yields of mycorrhizal plants were much higher than those of non-mycorrhizal controls in soil amended with either Zn or Cd. Concentrations of Zn and Cd were significantly lower in mycorrhizal plants, but P concentrations did not differ substantially between mycorrhizal and non-mycorrhizal treatments. The results indicate that the higher metal tolerance of mycorrhizal plants was due to the direct involvement of the mycorrhizal fungi in plant tolerance to heavy metals rather than a P- mediated nutritional effect.In order to investigate the tolerance of ectomycorrhizal fungi to heavy metals in vitro, three culture methods, namely liquid culture without agitation, liquid culture with agitation and solid agar culture, were investigated to determine which method would give the best combination of fungal biomass and EC50. The results indicated that liquid medium without agitation was the best culture method. The tolerance of Suillus granulatus and Paxillus involutus to Zn, Cd, and Pb was then compared using liquid culture medium without agitation. Compared to Suillus granulatus, Paxillus involutus was more tolerant to Zn and Pb, but more sensitive to Cd than Suillus granulatus.The adsorption of heavy metals in ectomycorrhizal fungi might be related to the base exchange capacity of the fungal mycelium. The base exchange capacity values of Paxillus involutus, Suillus granulatus and Suillus bovinus were measured in vitro. The results showed that the base exchange capacity of the three ectomycorrhizal fungi was much higher than cation exchange capacity of plant roots, indicating that the fungi may have great potential to adsorb heavy metals. The three fungi also differed in base exchange capacity, with Paxillus involutus having the highest base exchange capacity and Suillus bovinus the lowest. Adsorption of Pb, Zn and Cd among the three fungi differed appreciably and was largely influenced by the form of available N in the substrate. Adsorption of Zn, Pb and Cd was significantly higher by fungi previously grown in substrate supplied with NO3^--N compared with those grown in substrate supplied with NH₄⁺-N. When the fungi were grown in substrate supplied with-N, Suillus granulatus subsequently showed the highest adsorption of Pb and Paxillus involutus had the highest adsorption of Zn, but when the fungi were grown in substrate supplied with NH₄⁺-N, Suillus granulatus then showed the highest adsorption of Cd.