Symbiotic and genomic diversity of ‘cowpea’ bradyrhizobia from soils in Botswana and South Africa |
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Authors: | Ian J Law Wilhelm F Botha Ugele C Majaule Francina L Phalane |
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Institution: | (1) ARC-Plant Protection Research Institute, Private Bag X134, Queenswood, 0121, South Africa;(2) Department of Agricultural Research, Private Bag 0033, Gaborone, Botswana |
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Abstract: | Strain CB756 is usually an effective competitor against indigenous bradyrhizobia for nodulation of peanut in South Africa.
Recently, inoculation of peanut and cowpea with CB756 in loamy sand soils of Botswana or a sandy clay loam in South Africa
proved unsuccessful, achieving <2% nodule occupancy. A survey of ‘cowpea’ bradyrhizobia from five soils in Botswana and one
in South Africa showed that many were effective in ability to fix N2 on peanut and cowpea. However, 15 isolates from Good Hope, Botswana were all effective on cowpea but ineffective on peanut,
three failing to nodulate the latter. Selected cowpea isolates were significantly more competitive than CB756 for nodulation
of cowpea in Leonard jars, but four were unsuccessful when inoculated at Roodeplaat, South Africa. When strain CB756 and two
isolates were inoculated in pots containing Roodeplaat soil, at a 4:1 inoculant to soil bradyrhizobia ratio, their average
nodule occupancy was 8% on cowpea compared to 40% on peanut. Significant differences in strain nodule occupancy were not detected
on either cowpea or peanut. In contrast, nodule occupancy in loamy sand from Good Hope, Botswana, inoculated at a 40:1 inoculant
to soil bradyrhizobia ratio, was 22.4% on cowpea and only 6.8% on peanut. In Good Hope soil, strain CB756 was the weakest
competitor on cowpea but strain differences were insignificant on peanut. Whereas the Good Hope soil population was effective
on cowpea, it was ineffective on peanut. DNA fingerprinting showed that isolates from Gaborone, Francistown and Roodeplaat
contained several different genotypes, whereas those from Good Hope, Rasesa and Maun were more homogeneous. The dominance
at Good Hope of genotypes effective on cowpea but ineffective on peanut emphasises the value of assessing the symbiotic capabilities
and structures of indigenous populations. |
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Keywords: | Cowpea Inoculation Soil bradyrhizobia Peanut |
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