Evaluation of the effects of mycorrhizal inoculation on Pb uptake and growth of alfalfa in Pb-contaminated soil

Establishment of arbuscular mycorrhizal symbiosis in plant roots can affect plant physiological and morphological characteristics and may induce tolerance to heavy metals in plants grown in polluted soils; therefore, it can play an important role in phytoremediation. In present study, to investigate the effect of mycorrhizae on alfalfa growth and Pb uptake, a factorial experiment was designed with two factor: (1) plants non-inoculated (NM) or inoculated with Rhizophagus intraradices (Ri) or Funeliformis mosseae (Fm) and (2) soil non-contaminated (Pb0) and contaminated with 200 (Pb1), 400 (Pb2) and 600 (Pb3) mg kg-1 Pb2+. All plants were evenly inoculated with Sinorhizobium meliloti. The results showed that at high levels of Pb2+, both fungi compared to the NM controls enhanced root nodulation and phosphorus nutrition. Moreover, the dry weight of shoots, leaf area and chlorophyll index of the leaves were significantly increased in mycorrhizal plants compared to the NM plants. Mycorrhizal dependency increased by four- and three-folds in Ri and Fm plants, respectively, at Pb3 level in comparison with the non-polluted condition (p < 0.05).  Moreover, Pb translocation from root to the shoot was significantly declined at Pb3 level in Ri plants by 1.75-folds as compared with NM plants (p < 0.05). The comparison of the results obtained by principal component analysis demonstrated that R. intraradices symbiosis was more efficient for host plant protection against the phytotoxic effect of Pb. These results highlight the assisting role of AM fungi in protecting plants from metal toxicity and in plant establishment in Pb polluted soils.