The effect of seed bio-priming treatments on morphological indices of Moldavian balm (Dracocephalum moldavica L.) under water deficit stress

Introduction Dragonhead or Moldavian Balm (Dracocephalum moldavica L.) is an annual medicinal herbaceous and aromatic plant that its essential oil is used widely in medicine, food, cosmetic and health industries, which is belong to Lamiaceae family. Due to the importance and role of medicinal plants in various industries, there is an increasing demand for herbal medicines. But nowadays water deficit is known as an important environmental factors that affecting plant growth, yield and production, especially in arid and semi-arid regions. In order to reduce the adverse effects of drought stress in arid and semi-arid regions, it is necessary to provide strategies to reduce the drought damages. Seed bio priming is one of the techniques which enhance the growth of plants and their resistance against adverse environmental conditions. Some studies show that the use of some microbial species that are collectively designated as plant growth promoting rhizobacteria (PGPR), can improve yield and quality of medicinal plants. Given the importance of medicinal plants, adverse effects of drought stress and beneficial effects of some seed microbial inoculation as seed bio priming, this study aimed to investigate the effect of PGPRs on the morphological indices of Moldavian Balm under water deficit conditions. Materials and methods This pot experiment was conducted in a factorial experiment based on completely randomized block design with three replications. Traits involving three levels of drought stress (100 % of full irrigation (as control), 75 and 50 % of full irrigation) and seed inoculation with 8 levels of PGPRs (without inoculation (as control), and inoculating with Mycobacterium sp., Rhodococcus sp., Pseudomonas putida, Bacillus sp., Azotobacter sp., Pseudomonas fluorescence and Corynebacterium sp.). Bacteria were obtained from the Biotechnology research center of Shahrekord University, Iran. One seed lot of Moldavian Balm cv. SKZ-1 was used in this experiment. Seeds were surface-sterilized in 1.5% NaOCl for 5 minutes. For each of bacterial treatments, surface-sterilized seeds were soaked in the relative bacterial suspension (approximately 5 × 108 cfu.ml-1 suspensions) for two hours. Ten seeds sowed into each similar size pots were filled with 5000 gr field soil. A sample of soil was sent to laboratory for determining of chemical and physical characteristics. After germination and at 3-4 leaf stage, seedlings were thinned down to 3 plants per pot. Drought stress treatments were implementing from 3-4 leaf stage and continued to harvest. Results and discussion The results showed that the effects of drought stress and bacterial treatments on all studied traits and their interactions on some traits such as leaf area, leaf dry weight, shoot dry weight, root volume and area was significant. Overall, with increasing of drought stress severity plant height, stem diameter, number of lateral branches, number of flowering branches, leaf area and leaf dry weight, root volume and area was decreased, significantly. But this decline showed lower rate in bacterial treatments. The bacterial treatments showed a significant increase in plant height, stem diameter, root dry weight and length, leaf area and leaf dry weight compared to control. The most plant height was found in Mycobacterium sp. and Pseudomonas putida, inoculated treatments that increased plant height by 29.3 and 26.3 percent compared to control. All bacterial treatments could increase the number of lateral branches and number of flowering branches compared to control. Although difference between Corynebacterium sp. inoculated treatment and control was not significant. The highest number of lateral branches was found in Mycobacterium sp. and Pseudomonas fluorescence inoculated treatments that increased the number of lateral branches by 21 and 15 percent compared to control. The highest number of flowering branches was found in Mycobacterium sp. inoculated treatment that increased the number of flowering branches by 29.6 percent compared to control. The effects of bacterial treatments on leaf area and leaf dry weight were affected by different levels of drought stress. Overall, in the absence of the bacterial inoculation the highest leaf area was observed in full irrigation treatment and in 75 and 50 % of full irrigation treatments decreased by 27.5 and 47.9 percent compared to full irrigation treatment, respectively. The reduction trend of leaf area under drought stress treatments was also observed in other bacterial inoculation treatments. But the difference was that bacterial treatments could mitigate the effects of drought stress. Conclusions In conclusion, this experiment results showed that bacterial treatments had positive effects on all morphological studied traits, especially under water deficit stress conditions. Mycobacterium sp., Bacillus sp., Pseudomonas fluorescence and Azotobacter sp. were more affective in morphological studied traits. These bacterial treatments could reduce the harmful effects of drought. Since, based on these results, the use of Mycobacterium sp., Bacillus sp., Pseudomonas fluorescence and Azotobacter sp. bacterial inoculation treatments, as seed bio-priming, can be recommended for improving of morphological indices of Moldavian Balm especially under water deficit stress conditions.