Influence of Microbial Inoculation on Growth, Fe and Zn Uptake and Biochemical Response of Hyoscyamus niger L. in Lead (Pb) Stress

Recently, due to enhancement of industrialization, urbanization and disposal of wastes, fertilizers and pesticides the concentration of heavy metals (HMs)in agricultural soil has increased. Heavy metals are serious threat for environment due to their hazardous effects. Lead (Pb) is one of the toxic heavy metal that threats the health of plants, living organisms and human. Excessive Pb concentrations in agricultural soils result in decreasing the soil fertility and health which affects the plant growth and leads to decrease in plant growth. Plants simultaneously exposed to Pb suffer morphological, biochemical and physiological injury. Pb adversely affect plant absorption of essential elements, chlorophyll biosynthesis and shoot and root growth. Arbuscular mycorrhizal fungi (AMF) and plant growth-promoting rhizobacteria (PGPR) are known to enhance nutrient uptake and improvement of plant growth and tolerance in heavy metal contaminated soils through different mechanisms including producing low molecular weight organic acids, siderophore, antibiotics and hormones. The objective of this study was to evaluate the effect of AMF and PGPR on yield, leaf relative water content (RWC), some biochemical properties and uptake of Pb, Fe and Zn by Hyoscyamus (Hyoscyamus niger L.) under soil Pb contamination.
This study was carried out in greenhouse condition as a factorial experiment based on a randomized complete block design with two factors including Pb concentration (in four levels) and microbial treatment (in three levels including arbuscular mycorrhizal fungi, plant growth-promoting rhizobacteria and control) and in three replications. Consequently, a soil was selected and spiked uniformly with concentrations of Pb (0, 250, 500 and 1000 mg Pb kg-1 soil). The contaminated soil was then sterilized and inoculated with the selected species of arbuscular mycorrhizal fungi (a mixture of Glomus species including G. intraradices, G. mosseae and G. fasciculatum) or plant growth-promoting rhizobacteria (a mixture of Pseudomonas species includeing P. putida, P. fluorescens, and P. aeruginosa). Seeds of Hyoscyamus niger L. plant were grown in pots containing the Pb spiked soil. At the end of growth period shoot length, dry weights of root and shoot, Fe, Zn and Pb concentration in shoot, and some biochemical and physiological properties of plant including relative water content (RWC) chlorophyll a, b and total chlorophyll, carotenoids, proline and soluble sugars, were measured.
Results indicated that with increasing soil Pb concentration, dry weights of root and shoot, shoot length, photosynthetic pigments contents (chlorophyll a, chlorophyll b, total chlorophyll and carotenoids), shoot Fe and Zn concentration decreased, while proline and soluble sugars contents and the shoot Pb concentration increased. With increasing of soil Pb concentration, relative water content decreased, however, this reduction in concentration of 1000 mg Pb kg-1 soil was not significant (P > 0.05) in compared with concentration of 1000 mg Pb kg-1 soil. Amounts of all measured properties in AMF and PGPR treatments were higher than that control treatment. The highest values of shoot weight and root weight, were observed in plants that inoculated with AMF. The lowest shoot weight was recorded in non-inoculated plants that were grown under 1000 mg Pb kg-1 soil concentration. In this study Arbuscular mycorrhizal fungi and plant growth-promoting rhizobacteria inoculation led to a significant increase (P≤0.05) in shoot length (12.9 -71.1%), shoot dry weight (11.5 – 81%), root dry weight (18.4 – 60.6%), chlorophyll (8.5 – 36.5%) and carotenoid (11.5 – 40.0%) pigments, proline (55 – 115.7%), soluble sugars (17.6 – 72.2%) and shoot Fe (9.5 – 57.2%) and Zn (25.0 – 165.5%) concentration in shoot at different levels of soil Pb. The highest and lowest amounts of shoot Fe, Zn and Pb concentration observed in AMF and control treatments respectively. Plant growth promoting rhizobacteria were more effective than arbuscular mycorrhizal fungi in shoot Fe, Zn and Pb concentration, while the mean of shoot length and shoot and root dry weight was higher in plants that inoculated with AMF compared to ones inoculated with PGPR. In general, there were not significant (P ≤ 0.05) differences in amounts of chlorophyll (chlorophyll a, b and chlorophyll a) and carotenoids pigments, proline and soluble sugars between AMF and PGPR treatments.
It could be concluded that microbial inoculation (mixture of AMF and PGPR species) with improvement of plant biochemical properties results in improved Hyoscyamus niger L. yield and increased tolerance to Pb toxicity. Thus, the use of microbial inoculation (mixture of AMF and PGPR species) inoculation might be suggested for enhancement of plant tolerance in Pb contaminated soils.