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Improving the legume-rhizobium symbiosis in Zimbabwean agriculture: A study of rhizobia diversity & symbiotic potential focussed on soybean root nodule bacteria

Chiduwa, Mazvita Sheila (2021) Improving the legume-rhizobium symbiosis in Zimbabwean agriculture: A study of rhizobia diversity & symbiotic potential focussed on soybean root nodule bacteria. PhD thesis, Murdoch University.

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Legumes are important components for both smallholder and commercial agriculture in Zimbabwe in relation to food and income security and improvement of soil fertility through a symbiotic association with rhizobia. The efficiency of biological nitrogen fixation is largely unknown in most situations in Zimbabwe. While rhizobia inoculant is available for many legumes, only soybean is consistently inoculated. Native soybean rhizobia have not been genetically characterized or taxonomically identified.

The inoculation response of cowpea, groundnut, lablab, sunn hemp, pigeon pea and soybean was investigated under Zimbabwean field conditions, together with effects on a subsequent maize crop. Separately, soybean microsymbionts were obtained from soils with known inoculation histories from nine smallholder and three commercial farms to isolate naturalized inoculant strains and native rhizobia. Isolates were genetically characterized using partial recA gene sequences. Phylogenetic analysis of representative isolates was undertaken using recA, glnII, and 16S rDNA sequences. Symbiotic genes nifH and nodC were analysed. Isolates were screened for nitrogen fixation efficiency and the two best fixers per species were tested for compatibility with three soybean varieties under glasshouse conditions. The best isolate of each species was tested across different field sites in Zimbabwe.

Inoculation generally increased grain yield, shoot biomass and nitrogen accumulation. Maize biomass was higher when succeeding inoculated legumes than when succeeding uninoculated legumes. Partial recA gene sequencing grouped the isolates into four species: Bradyrhizobium diazoefficiens (13%), B. japonicum (21%), B. elkanii (61%) and B. ottawaense (5%). B. ottawaense had the widest host range across 13 legumes, followed by B. elkanii, B. diazoefficiens and B. japonicum. Phylogenetic analyses were consistent with vertical transmission of core genes and horizontal transfer of symbiotic genes. Based on symbiotic performance and edaphic competence, strains B. japonicum NAZ554 and NAZ710 and B. diazoefficiens NAZ629 were identified as potential elite inoculant strains for soybean in Zimbabwe.

Item Type: Thesis (PhD)
Murdoch Affiliation(s): Agricultural Sciences
United Nations SDGs: Goal 12: Responsible Consumption and Production
Supervisor(s): Ardley, Julie, Howieson, John, Mapfumo, P., O'Hara, Graham and Tiwari, Ravi
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