الفهرس 
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Abstract
The beginning of 21st century is marked by global scarcity of water resources and increased salinization of soil and water. Increasing human population and reduction in land available for cultivation are two threats for agricultural sustainability. There is a need to develop simple and low cost biological methods for salinity stress management, which can be used on short term basis. Microorganisms play a significant role in this respect.
The present study aimed to isolate PGPR from the rhizosphere of salt affected soils. These isolates were screened in vitro for different plant growth promoting traits like N2-fixation, Gibberellins production, IAA production, siderophores production, HCN production and phosphate solubilization. Different methods undertaken by bacteria for alleviating salt stress were studied. Further studies were done in pot experiments to evaluate the potential use of these isolates as biofertilizers for growth performance of sorghum and barley.
Obtained results could be summarized as follows:
5.1. Isolation of halotolerant bacteria
One hundred and thirty five bacterial isolates were obtained from different geographic regions in Egypt (Raas Sedr, Tour Sinai and El-Malahat in Alexandaria).
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-5.2. Screening of bacterial isolates in vitro for different functional properties could be summarized as follow:
5.2.1. Screening for nitrogen fixation
Fifty eight bacterial isolates gave nitrogenase activity using ARA assay with different variation between them, the isolates A19, A22 recorded the highest nitrogenase activity (631.53, 98.40 nmole C2H4/ml/h respectively) at 1% NaCl concentration, while the isolate D26 recorded the least activity (0.29 nmole C2H4/ml/hr)
5.2.2. Production of auxin
Ninety five bacterial isolates grown at different NaCl concentrations (0.6, 0.8, 1, 3 and 5%) produced IAA using the colorimetric salkowski assay. The isolates D50 and D52 exhibited the highest IAA production (236.5 and 214.75 μg/ml respectively) at 3% of salt concentration while the isolate A15 gave the least production (3.00 μg/ml) at 0.8% of salt concentration.
5.2.3. Production of gibberellins
The isolates K61, A8, A19, K80 and D53 produced the highest amounts of gibberellins at (0.6, 0.8, 1, 3 and 5 % NaCl respectively). The highest gibberellins production was recorded for the isolate K80 (318.72 μg/ml) at 3% of salt concentration, while K83 gave the least production (1.12 μg/ml) at the same concentration.
5.2.4. Detection of siderophores production
Out of 37 bacterial isoltes, 14 isolates only were able to grow on tryptic soya agar medium (TSA) amended with 8-hydroxyquinoline. For verifying the nature of the produced siderophores, 83.7 % of the tested isolates showed catechol type siderophores, while 45.9% produced citric acid type. Data showed also 16 bacterial isolates were able to produce both types of siderophores while 5 isolates didn’t produce any type.
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5.2.5. HCN Production
Out of 37 bacterial isolates, the results revealed that 21 isolates produced HCN while 16 isolates didn’t show any production.
5.2.6. Phosphate-solubilizing activity
Data showed that 21 bacterial isolates could solubilize inorganic phosphate on solid medium. The solubilization of tricalcium phosphate (TCP) in NBRIP broth medium by different isolates was accompanied by a DROP in pH from an initial pH of 7.0 after 6 days of incubation. The highest amount of solubilization was recorded for the bacterial isolate B116 (229.53 μg/ml) with a maximum DROP in pH to 4.8 followed by B100 (130.46 μg/ml), and then B128 (106.09 μg/ml). The minimum amount of soluble P (0.625 μg/ml) was observed in the cultures of B96. There was a strong negative correlation between P release and decrease of pH.
5.3. Identification of the most effective isolates
The isolates A19, D48, K80 and B128 were chosen for their efficient PGP activities, Table 18.
Table (18). Different PGP activities produced by the highest efficient isolates.
Bacteial isolate
Specific media
Salt concentration (%)
PGP activities
A19
Ashby’s medium
1
nitrogen fixation , IAA and gibbrellins production
D48
seim-solid malate medium
3
nitrogen fixation , IAA and gibbrellins production
K80
King’s medium
3
IAA, gibbrellins , siderophores and HCN production
B128
modified Bunt and Rovira agar medium
3
Phosphate solubilization on both solid and liquid media
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Molecular analysis based on 16S rRNA sequencing identified the isolates A19, D48, K80 and B128 as Azotobacter salinestris, Azospirillum canadense, Pseudomonas aeruginosa and Bacillus subtilis respectively.
5.4. Detection of nif H gene in Azotobacter salinestris A19
The nifH gene appeared as a single band on Agarose gel electrophoresis of approx. 293 bp comparing to the gene marker used in this study. In addition, the phylogenetic tree revealed similarity to other nitrogen fixing bacteria such as different strains of Azotobacter vinelandii, Azotobacter tropicalis and Azotobacter salinestris.