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Persistence and Competition Aspects of Rhizobium japonicum Observed in Soil by Immunofluorescence Microscopy¹

ABSTRACT

Fluorescent antibody was used to study persistence and competition of Rhizobium japonicum in soils. Strain 110 (USDA) attached to microscope slides was studied by FA-staining after recovery of the slides from soil at various intervals. In the Clarion soil, reactive cells dropped from 32/microscope field, with 100% of the fields being positive, to 0.15/field, with only 10% positive fields in 120 days. In the Ulen soil, the drop was from 53/field (100% positive fields) to 2.5/field (40% positive fields) over the same period.

The question of competition was approached first by varying the numbers of strain 110 added to constant but low numbers of the resident rhizobia flora of Clarion soil. Each treatment was used to inoculate plants and each nodule was tested separately with the homologous FA. Relatively high inoculant/resident ratios were needed to overwhelm the resident population. When the log of the numbers of introduced 110 was plotted against the percentages of 110 nodules, a sigmoidal curve emerged which may reflect a region of resistance characteristic for the resident R. japonicum population of that soil. A second approach involved the competitive ability of a known number of the introduced strain in the soil over a period of time. An initial population of 2.2 x 10³ R. japonicum 110/g of Clarion soil, capable of initiating 85% of the nodules, formed only 55% and 45% of the nodules after 10 and 30 days, respectively, of incubation in the soil. This probably was a reflection of the inability of the initial added population to persist and maintain its numbers in the soil.

¹ Paper no. 8007, Scientific Journal Series, Minnesota Agr. Exp. Sta., St. Paul. The research was supported in part by Grant GB 29636 from the National Science Foundation.

² Research Fellow and Professor, respectively, Department of Soil Science, University of Minnesota, St. Paul, Minn. 55101.

Received for publication December 5, 1972. Accepted for publication March 7, 1973.