الفهرس 
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Abstract
All forms of life require nitrogen to synthesize proteins and other important cellular compounds. In order for nitrogen to be used for growth it must be ”fIxed” (combined) in the form of ammonium (~) or nitrate (NO) ions. It is now well accepted that nitrogen-fIxing bacteria colonizing graminaceous plants can be grouped into three categories: 1) rhizosphere diazotrophs, 2) facultative endophytic diazotrophs and 3) obligate endophytic diazoh·ophs. Azospirillum species are gram negative, aerobic heterotrophs and facultative endophytes that fIx N2 under micro aerobic conditions and found almost everywhere.
In the present study, forty three samples were collected from rhizosphere and free soil of different host plants that grown in different localities in Minia Governorate. The physicai and chemical characteristics of the soil samples were studied. All tested soil samples were successfully able to form subsurface pellicle of nitrogen fIxing bHcteria in N-free semisolid malate media. No clear relation was observed betwcen the counts of malate utilizer, nitrogen fIxing bacteria and any of the chemical or physical properties of the tested free soil samples except for soil texture since the highest numbers were recorded for clay soils specially those with the highest percentages of clay particles. Ammonium content of the free soil samples did not show any relation to the numbers of the nitrogen-fIxing bacteria.
Wheat seedling spermosphere model was used to study the colonization ability of Azospirilla from both the rhizosphere soil of different host plants and the bulk soil at different locations to wheat roots. Data revealed that, all rhizosphere soil inocula could successfully colonize wheat root surface (rhizoplane) where wheat root exudates are the only available carbon source for the growing bacterial population. A total of 46 isolates of pellicle forming nitrogen-fixing bacteria in nitrogen free Dobereiner’s N-free (DN) medium were obtained trom rhizoplane and endoplane of wheat seedlings inoculated
with various soil samples using the spermosphere model. Thirty Azospirillum isolates out of the 46 had the characteristic colony mOl]> of Azospirillum on RC medium (Colonies that have a Light-pink coloraftcr h that became scarlet after 72-96 h depending on the preferential absorptill Congo red dye by Azospirillum. The nitrogen-fixing ability of each isolate tested by using the Acetylene-reduction assay (ARA assay) in nitrogen· semi-solid DN medium; all Azospirillum isolates were able to r acetylene to ethylene within a range of 6.43 to 39.32 nmole/CzHJml/hat3 without any addition of yeast extract. The 32 strains of Azospirillum that were isolated from the rhizoplane endoplane of wheat seedling were examined for susceptibility to antibacterial agents (tetracycline, kanamycin, ampicillin, penicillin. chloramphenicol and streptomycin). Comparison of the antibiotype Azospirillum isolates indicated that 50% of them were resistant to all the tested antibiotics. Such antibiotype could be useful for identification of the strains in field experiments. Azospirillum isolates were grouped according to nitrogen-fixing activity and antibiotic sensitivity. Isolates that shared the same antibiotype and had a close nitrogenase activity roughly considered as the same and one of them was chosen for the following study tests, this rough screening reduced the number of isolates to 15 considerably different ones. Physiological differences between the twelve well characterized Azospirillum species were used to develop a flow chart for the biochemical identification of our locally isolated Azospirillum strains to their closest species. Results of the biochemical tests indicated that the isolates had similar characteristics to the type strain of Azospirillum melinis. The capacity of different Azospirillum isolates to hydrolyze starch, gelatin, cellulose, chitin and pectin was also studied. In the present study, most of Azospirillum isolates were unable to hydrolyze starch, on contrast the majority of them were able to hydrolyze gelatin. All isolates investigated were motile and showed catalase activity. Data of the present study regarding indole production indicated that, all
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