Development of a composite microbial agent beneficial to improve drought and salt tolerance of Glycyrrhiza uralensis Fisch.

Plant growth is enhanced by microbiological agents. In order to develop environment friendly composite microbial agent that can also improve the salt and drought tolerance of plants, the orthogonal experiment and response surface methodology were used to screen nutrients and fermentation conditions of composite microbial agent, and pot experiment was conducted to validate the application effect of optimized and complex microbial agent. Compatibility test showed that G2 (Bacillus cereus) and G5 (Bacillus pumilus) did not show any inhibitory effect on each other's growth in nutrient agar, moreover, in both the growth promotion characteristics and the seed germination tests, G2+G5 showed a synergistic effect. The process optimization results showed that the optimal carbon and nitrogen sources for the medium nutrients was maltose and yeast extract, and the optimal carbon-nitrogen ratio was 5:1; the optimal fermentation conditions included initial pH at 5.17, inoculation amount of 2.28%, liquid volume of 105.96 mL(250 mL)−1, and incubation temperature at 30.06℃. Pot experiment showed that application with composite microbial agent was able to increase antioxidant enzyme activities, ascorbat, proline content, and reduce reactive oxygen species accumulation and membrane peroxidation of G. uralensis seedlings under salt-drought stress. In conclusion, the composite G2+G5 agent possessed remarkable synergistic effect for improve plant tolerance to drought combined with salt stress, suggesting a noticeble practical potential in agricultural practice, and it can be used to develop newtype environment friendly microbial fertilizer.