The effect of Peribacillus simplex bacterium on the growth of Mentha piprita L. in different watering regimes

Growth promoting bacteria can support host plants against water stress as the most common abiotic stress, especially in arid and semi-arid regions. Peppermint is one of the valuable medicinal plants that can lose yield under water stress conditions. To end this, a factorial greenhouse experiment as randomized complete blocks with three replications was conducted. Treatments included: inoculation and non-inoculation of the Peribacillus simplex 54-1 bacterium on peppermint seedlings and three rates of watering: normal, moderate water stress with stopping watering 10 days before harvesting and severe water stress with stopping watering 20 days before harvesting. The results showed that P. simplex inoculation could increase the fresh and dry yield of the peppermint by 59% and 47%, respectively, in normal watering, as well as by 74% and 58% in moderate water stress and by 64% and 39% in severe water stress. The dry weight of the root was 84% increased under normal watering conditions and averagely 42% increased under water stress conditions in respect to their controls. Bacterium inoculation was able to decrease the production of malondialdehyde as an index of oxidative stress by 2.26 and 4.18 times in moderate and severe stress, respectively, in respect to non-inoculation conditions. Improving the peppermint antioxidant system by increasing the rate of polyphenol oxidase and catalase could be among the possible reasons for this situation. Therefore, this bacterium can be used as a suitable candidate in the preparation of biological fertilizer to deal with water stress in peppermint and increase its production.