Phenotypic diversity of indigenous rhizobia nodulating different morphotypes of Vachellia karroo
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Date
2015
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University of Zululand
Abstract
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. When grown under a temperature range of 5 to 450C, all the isolates grew in 15 to 300C. 18.75%, 87.5%, and 28.13% grew at 5, 40, 450C respectively. The temperature tolerance of the isolates was not related to the climatic conditions of soil of origin nor to those of the host morphotypes. None of the tested isolates grew at pH 3 and all grew at pH 6-7. At pH 4, 5, 8 and 9 the percentages of isolates which grew were 22, 38, 91 and 51%, respectively. Isolates extracted from the alkaline soil from Richards bay were unable to grow at low pH of 3-5. However, for the other soils no such relationship was found and also no relationship was found between morphotypes and growth behaviour with respect to pH. The growth performance of the 32 Vachellia karroo rhizobia isolates in relation to salinity varied greatly in all isolates and was not related to the salinity characteristics of the soils from which they were extracted. Fast growing rhizobia were more tolerant to high salt concentrations compared to the slow growing rhizobia. In this research most rhizobia isolates exhibited resistance to antibiotics which was not correlated to conditions of geographic origin and host morphotype and the same pattern was noted in their resistance to heavy metals and utilisation of carbon sources. The dendrogram obtained from the computer numeric analysis of the isolates’ phenotypic characteristics produced six clusters at 83% level of relative similarity. This revealed the high variation of indigenous rhizobia. The clustering of isolates showed some degree of relatedness to the soil of origin and host morphotype contrary to their behaviour under each single treatment. The last study focused on the ability of V. karroo rhizobia from each morphotype to cross nodulate with other morphotypes or with other Vachellia species (V. nilotica and V. tortilis). All the rhizobia strains from the four morphotypes of V. karroo, two from V. nilotica and V. tortilis were able to form symbiosis nodules with the other morphotypes and with other Vachellia species. These results confirm that V. karroo rhizobia are able to form symbiosis with other morphotypes of the host species and also with other species of Vachellia. The same was true for the host plants, they were not specific in terms of the rhizobia nodulating them.
From this study, it was discovered that there the four V. karroo morphotypes are generally not distinct in terms of rhizobia nodulating them. It was also noted that behavior of rhizobia under certain conditions is not co-related to soil of origin. Furthermore a wide diversity in characteristic was noted in the rhizobia nodulating V. karroo which is advantageous when used in the selection of rhizobia which are tolerant to certain adverse condition, symbiotic effectiveness and competitive ability.
Description
A dissertation submitted to the Faculty of Science and Agriculture in fulfilment of the requirements for the degree of Masters of Science in Agriculture (Agronomy) in the Department of Agriculture at the University Of Zululand, South Africa, 2015
Keywords
indigenous rhizobia --morphotypes --Vachellia karroo