Nineteen isolates of rhizobacteria associated with sunflower (Helianthus annuus L.), collected from various locations in Pakistan, were screened for phosphate solubilization and indole-3-acetic acid (IAA) production. Two potential phosphate-solubilizing bacterial isolates with substantial IAA biosynthesis capacity, Ps-5 and Ss-2, were selected for further study. Based on 16S rRNA gene sequence analysis, isolate Ps-5 was identified as Bacillus sp. and Ss-2 as Alcaligenes faecalis. Both strains were found to be metabolically diverse in terms of the number and amount of different carbon substrates they utilized in the BIOLOG GN2/GP2 microplate assay. High-performance liquid chromatography analysis of the culture supernatant confirmed that Bacillus sp. Ps-5 produced considerable amounts of both lactic and tartaric acids, while A. faecalis Ss-2 secreted only lactic acid. There was a strong positive correlation between phosphate solubilization and organic acid production by both strains. Following inoculation, strain Ps-5 and Ss-2 were found to be good root colonizers and significantly (P ≤ 0.05) increased sunflower growth and phosphorus (P) uptake. However, inoculation had a non-significant (P ≤ 0.05) effect on sunflower yield parameters, including oil contents. Based on these results, we conclude that Ps-5 and Ss-2 are potent plant growth-promoting rhizobacteria strains with the ability to supplement the P requirements of sunflower crops. Further field inoculation studies are needed before these strains can be recommended as bio-inoculants. To the best of our knowledge, this is first report on the association and phytobeneficial potential of A. faecalis with sunflower.
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