Abstract
<p class="1Body">Free-living diazotrophs play an important role as plant growth promoters by supplementing nitrogen to deficient soils. In this study, six diazotrophic isolates were selected from 36 previously screened diazotrophic isolates for their mode of action to promote plant growth. A nitrogen free medium was used to screen nitrogen fixing diazotrophs. All strains have exhibited some activity of indole-3-acetic acid production. Strain MAF7-1 shown the highest (0.448 mg ml<sup>-1</sup>), while strain NUL3-2 exhibited the least (0.032 mg ml<sup>-1</sup>) production of IAA. Though all isolates were unable to solubilize inorganic phosphate, were able to inhibit the growth of <em>Fusarium oxysporum</em> that causes wilting to plants. There was no significant difference in maize seed germination vigour and rate. However, MAF7-1 had better germination rate and seed vigour (27.5%) when compared to the control (16%). Except strain NUL3-2, all showed significant increase of root length and dry weight of maize seedlings. Therefore, the choice of strain to be used for production of biofertilizer should not only be based on the nitrogenase activity, but also on ability to produce phytohormones. Strain NUL3-2, MAF7-1 and MAF7-3 are potential strains that can be used in the production of biofertilizer. Further study of these strains in depth as plant growth agents and biofertilizer is commendable for future application.</p>
Highlights
Nitrogen (N), being a constituent of amino acids and nucleic acids, it remain as growth limiting nutrient to all living things
Six strains of diazotrophic isolates, three of which: PEK1-1, NUL3-2, and MAF7-3 belong to the genus Azospirillum; two strains: NUL18-1 and MAF7-1 that belong to genus Azotobacter and strain PEK5-1 that belongs to the genus Bacillus were evaluated for their maize seed vigour and plant growth activity
All the isolated strains have shown some ability of producing Indole Acetic Acid; of which, the Azotobacter strain, MAF7-1 found producing the largest amount followed by the Azospirillum strain, MAF7-3
Summary
Nitrogen (N), being a constituent of amino acids and nucleic acids, it remain as growth limiting nutrient to all living things. Nitrogen reserves of agricultural soils have to be replenished periodically in order to maintain an adequate or non-growth limiting level for crop production (Hubbell & Kidder, 2003). In order to restore the soil nitrogen, addition of nitrogen in the form of commercial inorganic fertilizers or the use of biological nitrogen fixation (BNF) is important. Rhizosphere microbial communities besides maintaining soil fertility, they serve as Plant Growth Promoting Rhizobacteria (PGPR) to prevent the deleterious effects of pathogenic organisms by the production of phytohormones, through phosphate solubilization, nitrogen fixation (Glick et al, 1999; Lugtenberg & Kamilova, 2007; Beneduzi et al, 2012; Ochoa-Velasco et al, 2016)
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