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Accueil du site → Doctorat → Royaume-Uni → 1993 → The ecology of nitrogen - fixing symbioses under arid conditions of Kenya

University of Dundee (1993)

The ecology of nitrogen - fixing symbioses under arid conditions of Kenya

Odee, David Warambo

Titre : The ecology of nitrogen - fixing symbioses under arid conditions of Kenya

Auteur : Odee, David Warambo

Université de soutenance : University of Dundee

Grade : Doctor of Philosophy (PhD) 1993

The rhizobial populations and nodulation status of both indigenous (mainly Acacia species) and some introduced woody legume species were assessed under glasshouse conditions in soils collected from 12 sites located in different ecological zones of Kenya. The soils were collected on 6 dates spanning a period of 3 years (1989-1992) and at least once every year, covering wet and dry seasons. The rhizobial populations among the sites, as estimated by the most-probable-number (MPN) technique, varied from <3.6 to >2.3 x 10 cells g-l soil. There were some intrasite variations in population estimates depending on the trap host species and the method used in sampling the soils. Nodulation also varied across the sites with test species showing frequently higher nodulation ability in native soils. (L.) Merr. was the most prolific nodulating species while (Forsk.) Hayne was very erratic in nodulation. Most species showed interplant and intraspecific variability within a single soil source. Nodule shapes were consistent within each species irrespective of soil source but nodulation pattern on the root systems differed when grown in axenic conditions. A comparison of nodule internal structure between Willd., and (Del.) A. Chev. showed differences in the distribution and sizes of infected cells vis-a-vis uninfected cells.Over 480 isolates were recovered from nodules of test plant species in the various soils. The bulk of the isolates (91 were watery, milky-translucent and curdled milk types with moderate to copious extracellular polysaccharide slime (EPS) production. The rest were creamy or white opaque with none or moderate EPS production. They showed a wide range of growth rates on yeast mannitol agar (YMA) plates : very fast growing (34.3%), fast growing (47%), intermediate between fast and slow growing (8.4%) and slow-growing (10%). Characterisation by intrinsic antibiotic resistance (IAR) and salt (NaCl) tolerance showed that was generally more sensitive to antibiotics (streptomycin, kanamycin and ampicillin) and NaCI than Numerical’ analyses of the IAR and NaCI sensitivity data showed in most cases, clustering of the isolates according to their cultural and morphological characteristics. There was no apparent relationship of isolates within phenons due to site and date of collection, but isolates from S. tended to cluster within the same phenon. Nitrogen fixation potential as determined by acetylene reduction assay (ARA) ranged from 0.2 0.01 to 19.1 1.2 umoles plane ! h- !. The woody legumes were vesicular-arbuscular mycorrhizal (VAM). An intraspecific comparison between two provenances of Marigat and Mwatate in a native soil (Marigat) showed that the native provenance was inferior in nitrogen fixation attributes (i.e. nodulation and acetylene reduction rates) but superior in phosphorus (P) uptake. The results in this work portray the major influences of site (habitat), host and micro symbiont in the nitrogen fixation symbioses under the Kenyan arid and semi-arid lands conditions. The ecological and practical implications are discussed in each chapter.

Mots clés : Acacia spp / Nitrogen / Rhizobia / Marigat, Kenya / Mwatate, Kenya


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