Variable survival ability of rhizobacteria in cumin (Cuminum cyminum L.) rhizosphere

Abstract

A study was undertaken to compare the survival efficacy of two native, previously characterized bacterial biovars viz. Bacillus subtilis BCU5 and Pseudomonas fluorescens PCU17 with Bacillus subtilis strain MTCC1789 and Pseudomonas fluorescens strain MTCC4828, procured from Institute of Microbial Technology, Chandigarh,India in cumin rhizosphere and bulk soil. All the four bacterial types were made rifampicin resistant and the mutants were applied as inoculants at the dosage of 6 log, 7 log and 8 log colony forming units (cfu) g-1 dry soil weight in pots containing cumin seedlings. The cfu of rhizosphere and bulk soil of pots was observed per week for four weeks. The results show that the initial population decline is a common feature of bioinoculants. In rhizosphere and bulk soil, the native bacterial biovars survived better than their procured counterparts. The population of P. fluorescens strain MTCC4828r in rhizosphere soil declined faster and reached below detection limit whereas the P. fluorescens biovar PCUr rhizosphere final population dropped to 3.1 log, 2.9 log and 2.13 log cfu g-1 soil dry weight with 8 log, 7 log and 6 log cfu g-1 soil dry weight inoculum treatment, respectively. In contrast to P. fluorescens strain MTCC4828r, the population of B. subtilis strain MTCC1789r stabilized after some decline and was comparable with B. subtilis biovar BCU5 population. Study concludes that the inoculant population decline in soil was the result of lower microbial load carrying capacity of soil than the provided inoculum densities. Also, the native bacteria survived better than procured ones in rhizosphere soil.