Phosphate solubilization activity of rhizobia native to Iranian soils
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Agricultural soils in Iran are predominantly calcareous with very low plant available phosphorus (P) content. In addition to their beneficial N2-fixing activity with legumes, rhizobia can improve plant P nutrition by mobilizing inorganic and organic P. Isolates from different cross-inoculation groups of rhizobia, obtained from Iranian soils were tested for their ability to dissolve inorganic and organic phosphate. From a total of 446 rhizobial isolates tested for P solubilization by the formation of visible dissolution halos on agar plates, 198 (44%) and 341(76%) of the isolates, solubilized Ca3(PO4)2 (TCP) and inositol hexaphosphate (IHP), respectively. In the liquid Sperber TCP medium, phosphate-solubilizing bacteria (Bacillus sp. and Pseudomonas fluorescens) used as positive controls released an average of 268.6 mg L−1 of P after 360 h incubation. This amount was significantly (P < 0.05) higher than those observed with all rhizobia tested. The group of Rhizobium leguminosarum bv. viciae mobilized in liquid TCP Sperber medium significantly (P < 0.05) more P (197.1 mg L−1 in 360 h) than other rhizobia tested,. This group also showed the highest dissolution halo on the TCP solid Sperber medium. The release of soluble P was significantly correlated with a drop in the pH of the culture filtrates indicating the importance of acid production in the mobilization process. None of the 70 bradyrhizobial isolates tested was able to solubilize TCP. These results indicate that many rhizobia isolated from soils in Iran are able to mobilize P from organic and inorganic sources and this beneficial effect should be tested with crops grown in Iran.
Keywordscalcium phosphate legumes inositol hexaphosphate PGPR pH Rhizobium
The authors are grateful to professor Malakouti head of the Soil and Water Research Institute in Tehran and to the staff of the Department of Soil Biology of the same Institute for their help with the rhizobial collection and for their constructive discussions. Many thanks to professor Ghannadha for his help in the statistical analyses and the interpretation of the results, and to Miss Pashaky for her excellent technical assistance. Financial support for this investigation was provided by grants from the Iranian Government “Studies and Researches Between Universities” program for collaboration between the Universities of Tehran and Rafsanjan.
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