Evaluation of microbiological management strategy of herbicide toxicity to greengram plants

In order to circumvent the problems of herbicidal toxicity, tolerant N2 fixing and phosphate solubilizing bacterial strains were isolated and identified. Among 20 bacterial isolates, Azotobacter sp. strain AZ1 survived 2400, 3200, and 1600 μg/mL while Bacillus sp. strain PSB2 tolerated up to 1600, 2400 and 1600 μg/mL glyphosate, quizalofop and metribuzin, respectively. Under in vitro conditions, these bacterial strains secreted IAA, siderophores, exopolysaccharides, ammonia and transformed inorganic P into organic P even under herbicides stress. SEM and CLSM images revealed a clear toxic impact of herbicides on bacterial cells above tolerance limit. Phytotoxicity to greengram plants increased with increasing concentrations of herbicides. Herbicide tolerant Azotobacter strain AZ1 and Bacillus sp. strain PSB2 when used as inoculant, substantially reduced the herbicidal toxicity to greengram. For instance, strain AZ1 increased the length of roots (10%) and shoot (6%), dry biomass of root (28%) and shoot (6%), different symbiotic parameters like nodule number (6%), nodule dry biomass (8%), LHb (15%), photosynthetic pigments and seed yield (39%), whereas, Bacillus sp. strain PSB2 enhanced the measured parameters by 12%, 13%, 23%, 21%, 4%, 6%, 22%, 5% and 27%, respectively relative to positive control (1444 μg/kg glyphosate). Additionally, proline in shoot tissues declined rapidly in bio-inoculated plants. Conclusively, the microbial cultures resulted in better management of herbicidal toxicity to greengram plants. And hence, Azotobacter sp. strain AZ1 and Bacillus sp. strain PSB2 could be recommended for use as an effective and inexpensive microbial inoculant/biofertilizer to augment the production of greengram in herbicide contaminated soils.