Summary
Azotobacter vinelandii strains caused the formation of increased numbers of root nodules onGlycine max, Vigna unguiculata andTrifolium repens by their respective rhizobial symbionts. Increased nodulation due to inoculation withA. vinelandii also occurred in field grownG. max. Mutant strains ofA. vinelandii unable to fix nitrogen caused nodulation increases comparable to those caused by nitrogen-fixing strains. This indicates that nitrogen fixation byA. vinelandii was not responsible for the enhanced nodulation. The effect ofA. vinelandii on nodulation was greatest when cells from the mid-exponential phase of growth were applied as inoculants. Non viable cell preparations ofAzotobacter vinelandii were also found to cause an increase in the number of root nodules formed onGlycine max Rhizobium japonicum under greenhouse conditions. The nodulation enhancement activity was influenced by the method chosen to kill theA. vinelandii cells. Heat treatment and treatment with lethal levels of streptomycin destroyed the activity, whereas the activity was unaffected by ultraviolet-light treatment of the cells. Cell-free extracts ofA. vinelandii were found to enhance nodulation. On the other hand, culture supernatants ofA. vinelandii had no effect on nodulation. A split-root experiment suggested that the agent(s) responsible for the increased nodulation was not translocatable throughout the plant. The results suggest a non-excretable protein, produced byA. vinelandii, as a possible mechanism for nodulation enhancement.
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Cooperative investigations of the United States Department of Agriculture, Science, and Education Administration, Agricultural Research and the North Carolina Agricultural Research Service, Raleigh, North Carolina. Paper No.6754 of the Journal Series of the North Carolina Agricultural Research Service at Raleigh.
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Burns, T.A., Bishop, P.E. & Israel, D.W. Enhanced nodulation of leguminous plant roots by mixed cultures ofAzotobacter vinelandii and rhizobium. Plant Soil 62, 399–412 (1981). https://doi.org/10.1007/BF02374137
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DOI: https://doi.org/10.1007/BF02374137