الفهرس 
Introduction
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Abstract
The concept ”environment select” for diazotrophic cyanobacteria was monitored in a series of short-term laboratory and a field experiments. The laboratory experiments were first started by the isolation and identification of different cyanobacterial isolates (Nostoc maculiforme, Nostoc humifusum and Wollea sp.) from wheat rhizosphere soils. Besides the isolated cyanobacterial strains, i.e., Nostoc calcicola, Anabaena flos aquae, Nostoc muscorum, Anabaena laxa and Microchate tenra, five local cyanobacterial strains were all being initially evaluated for growth and atmospheric N2-fixation through determination of the nitrogenase activity (N-ase). The most six efficient nitrogen fixing cyanobacterial strains were examined when being exposed to various environmental stimulators or constraints. Cyanobacterial strains were cultivated in synthetic culture medium supplemented with increased concentrations of NaCl, phosphorus and glucose. Biomass and nitrogenase activity (N-ase) of the chosen cyanobacterial strains were estimated at 1, 2, and 3 week incubation intervals. The soil based cyanobacterial inoculum was prepared including those chosen cyanobacteria strains. As well as, the cyanobacterial soil based inoculum along with Azotobacter inoculum was evaluated for its effect on wheat yield and yield components of wheat plants cultivated in either clay or sandy soil and on soil available NPK, the soil biological activity in terms of dehydrogenase activity, CO2 evolution, total bacterial count, total cyanobacterial count and Azotobacter count at three different growth stages of wheat, i.e., vegetation, panicle initiation and harvest stages as affected by cyanobacteria and/or inoculation under different nitrogen levels. The obtained results could be summarized in the following:Part I: Laboratory experiments:1. Biomass production and nitrogen fixation of local and isolated cyanobacterial strains:Both biomass and N-ase activity increased with increasing the incubation period. The highest biomass which was 0.097 g/100 mL medium was recorded with Anabaena flos aquae after 4 weeks while Nostoc calcicola gave the highest N-ase activity which reached to 515.00 mmole C2H4 g-1 dry weight cyanobacteria h-1 2. Factors affecting the growth and nitrogen fixation capacity of cyanobacteria: 2.1. Salinity: The inclusion of NaCl into cyanobacterial culture media up to 8000 mgl-1 supported higher biomass yields for cyanobacterial strains over the control whereas increasing NaCl level up to 12000 mgL-1 in the growth medium decreased the threshold of cyanobacterial growth of all the tested strains. Increasing salt concentration up to 8000 mgL-1 NaCl enhanced the N-ase activity whereas the cyanobacterial culture exposed to 12000 mgL-1 NaCl exhibited considerable decreases in N-ase activity. 2.2. Phosphorus: Cyanobacterial cultures supplemented with P up to 60 mg PL-1 were characterized by high biomass yield and fixed more N2 compared with the others which were supplemented with limited level of phosphorus. Raising phosphorus level in growth medium suppressed, to some extent, the cyanobacterial development of all cyanobacterial strains at all incubation periods. 2.3. Glucose: Increasing both glucose concentration and incubation period up to 3 weeks elevated linearly both cyanobacterial biomass and N-ase activity. Identification of the isolated cyanobacteria isolates: Nostoc maculiforme, Nostoc humifusum and Wollea sp.) from wheat rhizosphere soils and identified. Nostoc maculiforme, Nostoc humifusum were only used in the preparation of cyanobacteria soil based inoculum. Part II: Greenhouse experiment: Wheat experiment: A pot experiment was conducted using two soil types of clayey and sandy soils to study the effect of single and/or dual inoculation of both cyanobacteria and Azotobacter on wheat yield and its components as well as to study its effect on soil available NPK and the soil biological activity in terms of dehydrogenase activity, CO2 evolution, the total count of bacteria Azotobacter and cyanobacteria. Both Soil available NPK and the microorganisms count were performed at three wheat growth stages, i.e., vegetation stage, panicle initiation stage and harvest stage. The important results can be summarized as in the following:1- Clayey soil: The inoculation with a mixture of Azotobacter and cyanobacteria combined with ½ N dose attained the superior effect on grain and straw yields compared to that achieved due to single inoculation. Dual inoculation increased NPK contents in grains and straw over those recorded by the single inoculation treatments. Single inoculation with cyanobacteria plus ½ N dose was proceeding only with nitrogen and potassium uptake by grains and straw and followed by Azotobacter plus ½ N dose but not with phosphorus. Soil dehydrogenase activity and CO2 evolution, total bacterial count, total cyanobacterial count and Azotobacter count increased significantly due all the tested treatments over the control treatments at all tested wheat growth stages. Sandy Soil: The Same trend was observed for the measured parameters in clayey soil in response to the tested treatments due to the single or dual inoculation with both cyanobacteria and Azotobacter in presence or absence of different levels of nitrogen sandy soil. In such soil, recorded values were lower than those recorded in the clayey soil. The priority was also for the treatment of a mixture of Azotobacter and cyanobacteria combined with ½ N dose that gave the splendid values due to wheat grain and straw yields, better NPK of wheat grains and straw uptake, favorable soil available NPK and characterized soil biological activity in terms of better dehydrogenase activity, higher CO2 evolution and wealthy total count of any of bacteria, cyanobacteria and Azotobacter. Generally, it could that dual inoculation with a mixture of Azotobacter and cyanobacteria combined with ½ N dose is more beneficial for wheat plants than the single inoculation with either Azotobacter or cyanobacteria.