Role Of Arbuscular Mycorrhizae In Alleviation Of Zinc And Cadmium Phytotoxicity

The possible use of arbuscular mycorrhizal symbiosis for bioremediation of heavy metal contaminated soils was suggested considering of the limitation of currently practicable soil remediation strategies and the complexity of heavy metal contaminations. Systematic studies on the critical role of arbuscular mycorrhiza in metal translocation from soil to plants, with focus on the mechanisms of mycorrhizae mediated alleviation of metal phytotoxicity, were conducted on a calcareous soil experimentally contaminated with zinc or cadmium. Mycorrhizal effects on plant growth and metal uptake by maize plants (Zea mays L.) inoculated with each of five AM fungi (G. mosseae, G. intraradices, G. caledonium, G. clarum and G. spp.) were investigated in pot experiments. Addition of Zn or Cd had marked inhibitory effects on mycorrhizal colonization, but plants colonized by G. caledonium or G. intraradices had generally higher root infection rates. The largest plant yields occurred with the moderate Zn addition level of 300 mg kg-1, under which mycorrhizal colonization had no effect on plant growth. When no metal added or the highest rate of Zn (900 mg kg-1) was added, mycorrhizal colonization, especially by G. caledonium or G. mosseae, increased plant yield. Under the moderate Zn addition level, shoot Zn concentration and uptake were generally lower in mycorrhizal plants and root Zn concentrations unaffected by AM inoculation, while under the highest Zn addition level both shoot and root Zn concentrations decreased with mycorrhizal colonization. On the other hand, plant yields decreased markedly following Cd addition, and colonization by G. caledonium or G. intraradices led to plant yield increases as a result of lowered shoot Cd concentrations. Among the five fungal strains investigated, G. mosseae and G. caledonium showed the most pronounced effects on plant P nutrition. The experiments thus demonstrated some general influences of arbuscular mycorrhizae in the balance of plant mineral nutrition and alleviation of heavy metal toxicity, also the clear variation in fungal adaptation to soil contamination and affinity to host plants, and indicated the possibility of screening of fungal strains for specific application purposes. Effects of metal chelate (EDTA) application and mycorrhizal colonization on Zn uptake by maize plants were investigated using soil experimentally contaminated with Zn in pot culture. Plant growth was generally inhibited by EDTA application but promoted by mycorrhizal colonization by G. caledonium, especially under a Zn addition level of 600 mg kg-1. Application of EDTA increased plant Zn concentrations, and Zn accumulation in the roots increased with increasing EDTA addition levels. On the other hand, the effects of inoculation treatments on plant Zn uptake varied with different Zn addition levels. When no Zn was added, Zn translocation from roots to shoots was enhanced by mycorrhizal colonization, while under Zn addtions, arbuscular mycorrhizae showed protective effects on the host plants, resulting in lower shoot Zn concentrations for mycorrhizal plants. The P nutrition of maize plants was greatly affected by AM inoculation, with mycorrhizal plants containing more P and higher P concentrations. The results indicate that application of EDTA mobilized soil metals and enhanced Zn accumulation by the roots, with consequent plant metal toxicity and growth inhibition. On the other hand, mycorrhizal colonization alleviated both Zn deficiency and contamination, and also increased host plant growth by influencing mineral nutrition. However, both EDTA application and arbuscular mycorrhizae showed no stimulation of metal translocation from roots to shoots, or metal phytoextraction under the experimental conditions. For understanding the direct and indirect involvement of arbuscular mycorrhiza in alleviation of Cd phytotoxicity, investigations were conducted on mycorrhiza-mediated improvement of plant P nutrition and alleviation of Cd toxicity, together with studies on the mycor-rhizosphere changes, and Cd uptake and transfer by the extraradical hyphae in compartmented pot cultures under two P addition levels. Experimental results indicated that mycorrhizal colonization by G. mosseae of maize plants was unaffected by Cd or P additions, while plant yields markedly decreased with increasing Cd addition levels. Cd partitioning to the shoots tended to decrease due to mycorrhizal colonization or P addition. Under all Cd addition levels, P uptake was markedly enhanced by mycorrhizal colonization, and residual extractable soil P concentrations were significantly lower and soil pH were higher in mycorrhizal treatments. The results showed that binding of Cd by specific mycorrhizal structures may be the principal mechanism of the protective effects of arbuscular mycorrhizae on host plants under conditions of Cd contamination, and physiochemical changes in the rhizosphere could help to immobilize metals in the soil. Furthermore, improved P nutrition of host plants by mycorrhizal colonization played a critical role in Cd binding by the roots and accelerated plant growth with consequent enhanced plant tolerance to Cd toxicity. In a modified glass bead compartment cultivation system for producing AM fungal materials, mycorrhizal associations were established using the two host plants maize and red clover (Trifolium pratense L.) and the two AM fungi G. mosseae and G. versiforme. Multi-element analysis showed that concentrations of P, Cu and Zn were much higher in the fungal biomass than in host plant material. There were also significant differences in nutrient concentrations between the two AM fungi. The metal binding capacity of the mycorrhizal fungi using fungal mycelium recovered from the modified cultivation system. Fungal mycelium showed different binding capacities for different metal ions, and on a dry matter basis the excised mycelium could bind Zn and Cd by up to 2.8 and 13.3% respectively under the experimental conditions. Root adsorption characteristics were also altered by fungal colonization: mycorrhizae had higher CEC and showed stronger Cd binding capacity compared with non-mycorrhizal roots. The results provided direct support for the occurrence of mycorrhizal protection of host plants against heavy metal contamination.