Study on the Root–Nodule Bacteria and the Cross–Inoculation Groups of the Legumenous Plants in the Sudan.

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Sherif Mustafa Khairi, Sherif Mustafa Khairi
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A survey of nodulation of legumes commonly found in the Sudan was made in four regions of agricultural importance. Extensive field survey was carried out in Shambat while nodulation in the soils of Hudeiba, Gezira and Abu Na'ama was studied in pot experiments using ten legumes of economic importance. Nodules were found in all plants examined both in the field and in the pots. The number of nodules varied from region to region and generally the number was higher in the field than in the pot tests. Further observations were made on the nodulation and nitrogen fixation by a number of legumes in the field. Nodules were present in all cases and the nitrogen content of the plant was greater in the legume than in wheat. The nitrogen content of the soil under legume was not greater than that under wheat and this was attributed to the fact that estimation of soil nitrogen was made immediately after the plants were removed. It is suggested that higher nitrogen content of soil under legume would obtain after the plant remains had decomposed and their nitrogen released into the soil. Cross – inoculation tests were made on 21 species of legumes and it was found that depending upon both production of nodules and nitrogen fixation, those could be broadly grouped according to the accepted system of classifications into the medicago or Alfalfa group, the Bean group, the Lupin group, the Clover group, the Soybean group and the Cowpea group. Important inconsistencies were found in the Cowpea group. For instance Arachis hypogaea and Acacia spp., did not cross – inoculate with typical members of the Cowpea group such as Vigna sinesis and Dolichos lablab. The Arachis strain of Rhizobium was further culturally and physiologically different from the Cowpea strain. Considering the variation in the physiological characters, e.g. acid production reported by Norris (1965) among different isolates of groundnuts Rhizobium it is suggested that the isolate studied in the present work may be an additional example of that variation. This strain was isolated from Shambat soil and it is suggested successful cross-inoculation between groundnut and cowpea may occur under the conditions of the Gezira Scheme were groundnut and Dolichos lablab are grown in closer sequence. As to Acacia spp., the difference between them and the Cowpea group is apparently more basic since A. seyal, A. Senegal, A. mellifera which did not cross-inoculate with the members of the Cowpea group cross-inoculated among themselves. It should be added that such cross-inoculation in all directions does not occur in the Cowpea group. A tentative suggestion is made to place these Acacia spp., in a separate group and the need for additional data is emphasised. Cultural and physiological studies were made on eight selected strains of Rhizobium and it was found that their characters were generally similar to that known for other rhizobia. Similarly the correlation between some of these characters and the ability to nodulate legumes was established as for Rhizobium strain studied by other workers. Thus the Lupin, the soya bean and the Cowpea strains were slow-growing and alkali-producers whereas the Lucerne, the Pea, the Bean and the Groundnut strains were fast-growing and acidic. The optimum temperature for growth was between 27-30°. All strains resisted heating at 65° for ten minutes. This and microscopic examination of stained cells showed that the acidic strains were more tolerant of acidity than the alkaline strains. It is known that the acidic strains of rhizobia such as the Lucerne strain are more sensitive to acidity than alkaline strains. Additional tests are necessary on the present isolates in comparison with others of known tolerance to acidity in order to clarify this matter.
Study on the Root–Nodule Bacteria and the Cross–Inoculation Groups of the Legumenous Plants in the Sudan.