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University of Zululand (2015)

Phenotypic diversity of indigenous rhizobia nodulating different morphotypes of Vachellia karroo

Ngwenya, Nkazimulo

Titre : Phenotypic diversity of indigenous rhizobia nodulating different morphotypes of Vachellia karroo

Auteur : Ngwenya, Nkazimulo

Université de soutenance : University of Zululand

Grade : Master of Science (MS) in Agriculture (Agronomy) 2015

Résumé partiel
Vachellia karroo (sweet thorn) is one of the important pasture legumes in agriculture due to its ability to fix nitrogen through its symbiotic relationship with rhizobia. The nitrogen fixed through this biological way is more beneficial than chemical fertilizers as it does not have any harmful effects on the environment, and it is free. There is limited information on the rhizobia nodulating V. karroo growing in South African soils. Therefore, the isolation and determination of phenotypic characteristics of indigenous rhizobia nodulating V. karroo is of paramount in improving symbiotic effectiveness and thus enhance nitrogen fixation. Four different morphotypes of V. karroo obtained from four areas of South Africa with different agro-ecological conditions namely Kei Mouth, Richards Bay, Leeu Gamka and Tshwane were each grown in four soils, also obtained from these areas. Before planting the morphotypes, the chemical properties of the soil were first assessed and an experiment was conducted to isolate rhizobia in the soil to determine their presence. The morphology of the nodules of V. karroo morphotypes was observed after three months of growth. Thirty two rhizobia strains were isolated from the V. karroo morphotypes, two from each morphotype and soil combination. Fresh rhizobia cultures of each strain were used for the Gram test, bromomythyl blue test, determination of generation time and for colony morphology. Notable results from the soil chemical analysis was that soils from Richards Bay and Leeu Gamka were alkaline and those from Kei Mouth and Tshwane were acidic. Results of nodule morphology revealed that nodule morphology was influenced by morphotype and not by soil of origin. All the 32 isolates were Gram negative, 87.5% were fast growing rhizobia with below four hours of generation time and they turned bromomythyl blue indicator yellow while 12.5% were slow growing with a generation time of more than six hours and turned bromomythyl blue indicator blue. All the isolates which were slow growing were expectedly from acidic Tshwane soil as slow growing rhizobia prefer acidic conditions. The colonies for all isolates were generally mucoid, shiny and round. The phenotypic characteristics of the 32 rhizobia isolates were then studied. Variation was noted in the isolates’ response to temperature, pH, salinity, antibiotic resistance and heavy metal resistance. However, the isolates’ response to carbon source utilization was almost the same as they all utilized more than three quarters of the tested carbon sources

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