FANKEM Henri, NGO NKOT Laurette, DEUBEL Annette, QUINN John, MERBACH Wolfgang, ETOA FranÃ§ois-Xavier and NWAGADieudonnÃ©*
A trial of a screening and selection strategy for phosphate-solubilizing bacteria based on phosphate solubilization ability, and the subsequent effect of these strains on plant growth promotion under in situ conditions was conducted. Of all the bacteria tested, three Pseudomonas fluorescens strains (CB501, CD511 and CE509) were selected. On agar plates, two strains (CB501 and CE509) showed an ability to solubilize the three phosphate types (Ca3 (PO4)2, AlPO 4·H2O or FePO4·2H2O), while strain CD511 showed a halo zone only on an agar plate supplemented with iron phosphate (Fe-P). However, in liquid media, all the strains were able to mobilize significant amounts of phosphorus (P) depending on the phosphate type. Calcium phosphate (Ca-P) solubilization resulted from the combined effects of pH decrease and carboxylic acids synthesis. At pH 4, it was solubilized by most of the organic acids. However, the synthesis of carboxylic acids was the main mechanism involved in the process of aluminium phosphate (Al-P) and Fe-P solubilization. Both were mobilized at pH 4 by citrate, malate, tartrate, and on a much lower level by gluconate and trans-aconitate. Subsequently, a greenhouse trial was conducted using Zea mays, the results of which obtained using 5 parameters including grain yield and P uptake, revealed that strain CB501 was the best plant growth promotor with a global effect of +37%, followed by strain CE509 (+21.2%) and then by strain CD511 (+16.7%). However, the selection of phosphate-solubilizing Pseudomonas strains as possible inoculation tools for phosphate-deficient soils should focus on the integral interpretation of laboratory assays, greenhouse experiments and field trials.
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