Toxicity of Pb and Cd to symbioses between faba bean plant and rhizobia as well as bioremediation using microsymbiont

Laboratory and green house experiments were conducted to investigate the effect of pollution with Pb and Cd on microsymbiont (Rhizobium leguminosarum biovar Viceae) and its macrosymbiont (faba bean plant) as well as the role of inoculation with heavy metals tolerant rhizobia on amelioration of heavy metals toxicity effects Many rhizobial isolates (R. leguminosarum bv. Viceae) were isolated from faba bean fields irrigated with polluted sewage water (EI-Gabal EI-Asfar and EI- Hamoul locations) which had different degrees of heavy metals in addition to control site irrigated with normal water (EI-Reyad location). The highest concentration of Pb and Cd in soil and plant were found in EI- Gabal EI-Asfar followed by EI-Hamoul then EI-Reyad. Rhizobial isolates which isolated from the above study areas subjected to different concentrations of Pb and Cd in YM medium, the rhizobial number of isolates differentially decreased by increasing heavy metals concentrations. Isolates of RH6 and RH8, which isolated from polluted sites were the most tolerant to heavy metals. On the other hand, isolates of RH1 and RH2 (isolates from normal site) showed a high sensitivity to heavy metals, The increases of Pb and Cd concentrations significantly decreased plant dry weight (g/plant), nodules dry weight (g/plant) and N%. Inoculation of faba bean plants with different rhizobial isolates increased plant parameters (as compared to) N- fertilized un-inoculated plants under normal or pollution conditions. Inoculated plant with isolates RHe and RH8 attained good tolerance to heavy metals toxicity and gave highest records of dry weight of plant and nodules as well as N% even at high concentrations of Pb or Cd. Consequently, it is to recommend that inoculate of faba bean plants grown in heavy metals cultures or in polluted sites using tolerant and efficient rhizobial strains to ameliorate and reduce toxicity effect of heavy metals.