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The role of Trichoderma species as phosphate solubilizing microorganism was studied in vitro, by means of the Modified Pikovskaya's Broth medium (MPB) and in vivo, under greenhouse conditions using Rock Phosphate (RP) as a source of phosphorus (P). In broth medium, Trichoderma strain FR-NST-009 gave the highest P-solubilization (80.25%), followed by FR-NST-353 (77.51%), CB-Pin-01 (66.91%), RB-NST-028 (46.05%), and RB-NST-003 (21.20%) as compared with a control (broth medium non-inoculated with Trichoderma strain) after incubation at room temperature (27 ± 2 o C) for 7 days. In addition, the treatments with Trichoderma species provided the final pH of broth culture lower than the control. Three kinds of organic acids including citric acid, lactic acid and succinic acid were detected by High Performance Liquid Chromatography (HPLC) in all treatments inoculated with Trichoderma strain. Trichoderma species produced citric acid in higher than lactic acid and succinic acid. Production of organic acids by Trichoderma species is one of the mechanisms for phosphate solubilization. In greenhouse conditions, at 60 days after planting, the treatments with Trichoderma strain FR-NST-009+RP provided the percentage of available phosphorus increasing more than the control 1 (with only RP) to 14.91%. However, after 60 days, the available phosphorus in the planting medium continually decreased in all treatments. At 180 days after planting, the treatments with FR-NST-009+RP increased plant height (22.19%), stem circumference (13.81%), leaf number (71.43%), total phosphorus in the rubber tree leaves (18.90%), shoot fresh weight (43.95%), root fresh weight (19.36%), shoot dry weight (39.96%), and root dry weight (21.13%), as compared with the control 1 (with only RP) Furthermore, the treatments with FR-NST-009+RP provided the population of Trichoderma species in the planting medium with 1.78 × 10 5 Colony-Forming Units (CFU) per gram planting medium and gave the root colonization percentages with 100.00%. The selected indigenous strain, FR-NST-009 was Trichoderma harzianum identified by using morphological characteristics.

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