Studies on rhizosphere microbial diversity of some commercially important medicinal plants

A preliminary study was made on four medicinal plants viz., Ocimum sanctum L., Coleus forskholii Briq, Catharanthus roseus (L.) G. Don. and Aloe vera in order to identify and enumerate the rhizosphere, non-rhizosphere and diazotrophic microorganisms in soil. The diazotrophic bacterial population studied includes Azospirillum, Azotobacter and Pseudomonas. The rhizosphere bacterial populations were 23.33 × 10⁶ g^-1 in O. sanctum followed by C. roseus (20.46 × 10⁶ g^-1), A. vera (18.44 × 10⁶ g^-1) and C. forskholii (16.64 × 10⁶ g^-1). The fungi populations were 19.44 × 10⁴ g^-1 in C. roseus, 18.66 × 10⁴ g^-1 in O. sanctum, 16.44 × 10⁴ g^-1 in A. vera and 14.22 × 10⁴ g^-1 in C. forskholii. The actinomycetes population was 12.22 × 10⁵ g^-1 in O. sanctum, 10.44 × 10⁵ g^-1 in C. roseus, 8.44 × 10⁵ g^-1 in A. vera and 6.22 × 10⁵ g^-1 in C. forskholii. The diazotrophic bacterial population of Azospirillum, Azotobacter and Pseudomonas is 8.2 × 10⁴ g^-1, 12 × 10⁴ g^-1, 6 × 10⁴ g^-1 in the rhizosphere soil. In all the four medicinal plants the microbial population is more in the rhizosphere soil, when compared to non-rhizosphere soil. These results are helpful in developing a biofertilizer consortium for these commercially grown medicinal plants.