Role of arbuscular mycorrhizal fungi and /or Streptomyces sp. in alleviation of heavy metal toxicity on faba bean plants grown in soil amended with iron or sewage sludge

Abstract

1- Effect of the interaction between arbuscular mycorrhizal (AM) fungi and Streptomyces sp. isolated from heavy metals contaminated soil on faba bean (Vicia faba) plants grown on soil spiked with or without iron (250 ppm) was studied. Iron addition resulted in a decrease in the growth parameters of mycorrhizal and non-mycorrhizal plants. However, AM inoculation significantly increased all growth parameters of bean plants, compared to non-mycorrhizal ones especially when the Streptomyces sp. was added. Mycorrhizal symbiosis also significantly increased the photosynthetic pigments, total soluble sugars, proline content, free amino acids, total soluble proteins and crude proteins, in addition the activity of antioxidant enzymes {peroxidase(POX),catalase (CAT),and superoxide dismutase (SOD)} was increased, while iron concentrations in bean plants was decreased compared to untreated ones. The application of Streptomyces sp. in combined with AM fungi had positive effects on all physiological indices of bean plants. Also, the combination of the two types of microorganisms increased the metal tolerance index (93.5%) of bean plants compared to that of untreated ones (80%). Addition of iron into the soil had a negative effect on the rate of mycorrhizal colonization of plant roots as well as the spore density in the soil; however Streptomyces sp. was stimulated the mycorrhizal infection.

2- A pot experiment was conducted to study the effect of different concentrations of sewage sludge in the soil (1٫ 2 and 4% ٫w/w) on the growth and physiological indices of faba bean plants inoculated with a mixture of AM fungi, Streptomyces sp. and Rhizobium leguminosarum biovar viciae. The results showed that the application of sewage sludge at a rate of 1 and 2 % increased the growth parameters of bean plants as compared to those grown in unamended soil; however 4% sewage sludge amendment hadn’t any significant effect. Amendment of sewage sludge to the soil led to increase in the concentrations of N, P٫ K and some heavy metals in both the leaves and seeds of faba bean plants compared to those grown in unamended soil. Photosynthetic pigments, total soluble sugars, proline content, free amino acids, total soluble proteins, crude proteins and antioxidant enzymes activity were also enhanced by increasing sewage sludge concentrations in the soil. The rates of mycorrhizal colonization of bean plant roots as well as spores density in the soil were increased at 1 % and 2% sewage sludge; while they decreased at level of 4% sewage sludge.