Biosynthesis of auxins in the rhizosphere of different crops may vary because of quantitative and qualitative variations in microbial population and root exudation. A laboratory study was conducted to assess in vitro auxin biosynthesis, and biosynthesis in rhizosphere and non-rhizosphere soils of different crops (maize, sorghum, mungbean, cotton). Soils were inoculated with selected rhizobacteria with and without the auxin precursor L-tryptophan (L-TRP). Auxins were detected by colourimetry as indole acetic acid equivalents and confirmed by high performance liquid chromatography. Results revealed that 83% of the 60 rhizobacteria were capable of producing auxins in the absence of L-TRP. Auxin biosynthesis by the 8 most efficient rhizobacteria ranged from 5.0 to 12.1 mg/L broth medium. A comparison of rhizosphere v. non-rhizosphere soils indicated a greater accumulation of auxins in the rhizosphere soils than non-rhizosphere soils. Overall, inoculation of rhizosphere soils with selected rhizobacteria resulted in greater production of auxin (up to 10.4 mg/kg soil) than in inoculated non-rhizosphere soils (up to 5.76 mg/kg). Moreover, efficiency of these rhizobacteria for auxin biosynthesis in both rhizosphere and non-rhizosphere soils differed with crop and bacterial strain. Some rhizobacterial strains exhibited superiority over the indigenous microflora for auxin biosynthesis in soil. Application of L-TRP promoted auxin biosynthesis in both rhizosphere and non-rhizosphere soils. These findings imply that inoculation with suitable strains and/or amendment with L-TRP could promote auxin synthesis in the rhizosphere soil of a given crop, which may have consequences for better plant/crop growth.

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