Competition studies with non-nodulating mutants of cowpea rhizobia
1992
Budall, S.E.M.
Competitive interaction between two non-nodulating mutants and four nodulating cowpea rhizobia strains varied from no suppression in some instances in vermiculite at the 1:1 inoculum ratio to 27.8 % and between 31 and 72 % suppression at the 10:1 ratio on cowpeas. In non-sterile soils, suppression of nodulation was more pronounced than that in vermiculite. Shoot dry weight generally showed no particular trend, while nodule weight related quite well to nodule numbers. Showed that the proportions of competing strain as well as total rhizobial numbers influenced competitiveness. Competition in broth and supernatant studies revealed that suppression was not being effected by production of antagonistic compounds. Attachment studies however, showed more firm attachment of Nod** (-) than Nod+ and indicated this as the possible means by which Nod** (-) exerts its suppressive action. Further, the competitive interaction between Nod+ and Nod** (-) strains was significantly altered by pre-exposure of cowpea seedlings. Pre-exposure of cowpea seedlings to Nod** (-) for 6, 24, 48, and 72 hours even at a 1:1 ratio resulted in nodule suppression of 25 - 42 % with increased suppression of 31 - 63 % from 0 -72 hours at the 10:1 ratio (Nod** (-) and Nod+). Reversal of inoculum order with Nod+ applied first resulted in normal nodulation of Nod+ from the 24 hour treatment. Suggests that the critical period for nodulation success was 24 hours when Nod** (-) was supplied ten times greater than Nod+ in the inoculum.
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