Serdar Bilen

Effect of plant growth-promoting bacteria and soil compaction on barley seedling growth, nutrient uptake, soil properties and rhizosphere microflora

Inoculants are of great importance in sustainable and/or organic agriculture. In the present study, plant growth of barley (Hordeum vulgare) has been studied in sterile soil inoculated with four plant growth-promoting bacteria and mineral fertilizers at three different soil bulk densities and in three harvests of plants. Three bacterial species were isolated from the rhizosphere of barley and wheat. These bacteria fixed N2, dissolved P and significantly increased growth of barley seedlings. Available phosphate in soil was significantly increased by seed inoculation of Bacillus M-13 and Bacillus RC01. Total culturable bacteria, fungi and P-solubilizing bacteria count increased with time. Data suggest that seed inoculation of barley with Bacillus RC01, Bacillus RC02, Bacillus RC03 and Bacillus M-13 increased root weight by 16.7, 12.5, 8.9 and 12.5% as compared to the control (without bacteria inoculation and mineral fertilizers) and shoot weight by 34.7, 34.7, 28.6 and 32.7%, respectively. Bacterial inoculation gave increases of 20.3–25.7% over the control as compared with 18.9 and 35.1% total biomass weight increases by P and NP application. The concentration of N and P in soil was decreased by increasing soil compaction. In contrast to macronutrients, the concentration of Fe, Cu and Mn was lower in plants grown in the loosest soil. Soil compaction induced a limitation in root and shoot growth that was reflected by a decrease in the microbial population and activity. Our results show that bacterial population was stimulated by the decrease in soil bulk density. The results suggest that the N2-fixing and P-solubilizing bacterial strains tested have a potential on plant growth activity of barley.

The Role of Plant Growth-Promoting Rhizosphere Bacteria in Toxic Metal Extraction by Brassica spp.

Brassicaceae are scattered all over the world, where they exclusively grow on serpentine rocks in Western Australia, New Zealand, South Africa, Japan, Philippines, Brazil, Portugal, Italy, Turkey, Cuba, eastern Canada, and western north America. Although serpentine rocks cover only less than 1% of the earth’s surface their worldwide distribution has recently attracted many researchers in exploring their distinctive potential for phytoremediation plant communities, mainly members of Brassicaceae plant family inhabiting on serpentine rocks of these countries. On the other hand, the majority of Brassicaceae plant family are slow-growing plants producing little biomass and their use for phytoextraction purposes may not be practical, especially when bioavailable metal concentration is high in the contaminated conditions. Therefore, recently emerging practices in the field of phytoremediation have pointed out various focuses such as the utility of high-biomass crops such as maize, peas, oats and Indian mustard and associated soil practices including application of synthetic chelators such as ethylenediaminetetraacetic acid and nitrilotriacetate and elemental sulphur to enhance metal uptake by these plants. These approaches may meet the conditions required for the phytoremediation. However, one of the most critical components of phytoextraction process is the bioavailability of heavy metals meaning the portion of the metals that is available for absorption into living organisms such as plants. It has been known that various plant growth-promoting rhizobacteria (PGPR) associated with plant roots may provide some beneficial effects on plant growth and nutrition through a series of well known mechanisms, namely, nitrogen fixation, production of phytohormones and siderophores, and transformation of nutrients once they are either applied to seeds or incorporated into the soil. Similarly, heavy metal mobility and availability can substantially be driven by PGPR populations through their release of chelating agents, acidification, and phosphate solubilization in rhizosphere. Miscellaneous PGPR were also shown to tolerate heavy metals in different ways including the mechanisms of exclusion, active removal, biosorption, precipitation, and extra- or intracellular bioaccumulation. Since these processes may affect the solubility and the bioavailability of heavy metals to the plant and hence modifying their toxic effects, interactions between hyperaccumulator plants such as Brassicaceae spp., and metal tolerant or resistant PGPR are considered to have an increasing biotechnological potential in the remediation of anthropogenically polluted soils. Present chapter/review considers the role of PGPR on soil-heavy metal-plant interactions and more specifically bioaccumulation of toxic metals by Brassicaceae plant family.