A hundred of bacterial strains, belonging to the genera Azotobacter, Azospirillum, Bacillus, Enterobacter and Pseudomonas, were isolated from the rhizosphere of various crops. They were characterized as to the ability of directly enhancing plant growth, root colonizing, and growth using root exudates as the only energy and carbon source. In vitro bacterial inoculation on maize seedlings resulted in changes of average yield ranging from a 50% decrease to a 70% increase over controls. Most isolates were found to grow on nutrients exuded by roots of both maize and wheat. On the contrary, colonization ability varied greatly with crop.\ud The occurrence and distribution of some bacterial properties, which could be involved in plant-growth stimulation, was investigated in fifty-one non-deleterious strains. Nitrogenase activity, and production of siderophore, indole-3-acetic acid and pectinolytic enzymes were measured under pure culture conditions. A significant nitrogen fixation rate was detectable only with Azotobacter and Azospirillum isolates. Siderophore and pectinase production was found to be restricted to Enterobacter and Bacillus strains, respectively. More widespread resulted the synthesis of the phytohormone indoleacetic acid. A total or partial replacement of the carbon source in the culture medium with root exudates failed to stimulate these bacterial activities
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