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Population Dynamics of Mung Bean Rhizobia and their Interaction with Mycorrhiza in an Acid Soil
Thesis Abstract:
Changes in rhizobia population in acid soil and in the mung bean rhizosphere were assessed in pot experiments. Acid tolerant M79-17-19 and acid sensitive M79-20-124 mung bean cultivars grown in unlimed (pH 4.6) and limed (pH 6.2) Luisiana clay were inoculated with antibiotic resistant mutants of Bradyrhizobium sp. (Vigna): BVr 9 (acid-tolerant) and TAL 209 (acid-sensitive).
In the unlimed soil, regardless of cultivars and strain, total population of rhizobia in the soiI and rhizosphere was depressed at two and five weeks after planting (WAP), but it was significantly increased by liming. The total population of rhizobia either in the soil or in the rhizosphere tended to decrease from 2 to 5 WAP.
In general, inoculum strain population was higher in limed than in unlimed soil. However, higher numbers were observed at 5 WAP than at 2 WAP.
Regardless of cultivar, the rhizosphere population of Bvr 9 was similar with or without lime. TAL 209 had a significantly lower number in the rhizosphere without lime application.
The interaction of lime application, strain, and cultivar had significantly influenced nodule occupancy by the inoculum strain in the primary and secondary roots. Without lime, more nodules were formed by the inoculum strains on the acid-tolerant cultivar than on the acid sensitive cultivar.
A separate pot experiment showed that in unlimed and limed Luisiana clay, inoculation with mycorrhiza resulted in higher population of Bvr 9 in the mung bean rhizosphere than phosphorus application. In the presence of mycorrhiza, Bvr 9 formed 100 percent of the nodules in both secondary and primary roots while the average nodule occupancy of TAL 209 was 80 percent at 6 WAP.
In the absence of the host plant, Bvr 9 showed better survival than TAL 209 both in unlimed and limed soils.