Detection of inoculated plant growth-promoting rhizobacteria in the rhizosphere of rice

A mixture of five plant growth-promoting rhizobacterial strains belonging to the genera Azospirillum, Azoarcus, Pseudomonas and Zoogloea was used to inoculate rice seedlings growing in microplots. The population of these bacteria, as well as the indigenous bacterial population in the rice rhizosphere, was determined at 4-week intervals throughout the growth season. In rhizosphere soil a continuous increase in total bacterial population (CCM-plate counts) in both inoculated and noninoculated plots was noted until 3 months after transplantation, while the maximum population of diazotrophic bacteria (ARA-based MPN counts) was observed 1 month after transplantation. A larger number of bacteria was found associated with plant roots (10(7)/g) as compared to those present in rhizosphere soil (10(6)/g). The total bacterial population (plate counts) in plant roots showed a continuous increase until harvest, while the population of diazotrophs (MPN counts) started declining 1 month after transplantation. Considerably higher levels of nitrogenase activity (ARA) were noted in the roots of inoculated plants as compared to those of noninoculated plants, where only a small number of diazotrophic bacteria were detected. The population of one Azospirillum strain (N-4) used as inoculum was determined by using selective media containing antibiotics, morphological characteristics, reaction with fluorescent antibodies and ARA of re-isolates. The maximum number of cells (1.4x10(5)/g) of this strain were detected in the roots 1-month after seedling transplantation from the nursery, constituting about 0.4% of the total bacterial population and 2% of the diazotrophic population.