بهبود خصوصیات آگرومورفولوژیک بادرنجبویه (Melissa officinalis) تحت تنش خشکی از طریق تلقیح با باکتری‌های ریزوسفری ارتقاء‌دهنده رشد

Abstract
To assess the effectiveness of plant growth-promoting bacteria (PGPB) on the agromorphological characteristics of Lemon balm under drought stress in the second year of cultivation, an experiment was performed in the form of split plots in randomized complete block design with three replications. The first factor includes three levels of drought stress (full irrigation, 75% and 50% of full irrigation) and the sub-factor was inoculation with PGPB in 5 levels including control (no bacterial inoculation) and inoculation with four bacterial species (Azospirillum, Bacillus strain A, Bacillus amyloliquefaciens and Streptomyces rimosus). Based on the obtained results, the effect of different levels of drought stress on all investigated traits was significant. Bacterial treatment showed a significant effect on the number of sub-branches, leaf area index, stem dry weight, percentage and yield of essential oil and biological yield in the first cut and leaf dry weight in the second cut. The interaction effect of stress and bacteria was significant only on the number of branches in the second cut. With the increase in the intensity of drought stress, a decreasing trend was seen in all the analyzed traits including in plant height, number of sub-branches, leaf surface, stem and leaf dry weight, percentage and yield of essential oil and biological yield while inoculation of the plant with Azospirillum, Streptomyces rimosus and Bacillus amyloliquefaciens bacteria caused a 10-20% increase in the studied traits compared to the control. Overall, bacterial treatments reduced the adverse effects of drought stress on the studied traits. According to this research, in order to improve the quantitative and qualitative yield and some agromorphological characteristics of Lemon balm under drought stress, inoculation of the plant with Azospirillum, Streptomyces rimosus and Bacillus amyloliquefaciens bacteria is recommended.
Introduction
Lemon balm (Melissa officinalis L.) is one of the herbaceous perennial medicinal plants that has a variety of uses. Nowadays, due to the increasing popularity of medicinal plants, the area under these plants’ cultivation continue to increase. However, problems such as drought stress limit the development of these plants’ cultivation. Plants respond to water deficit stress at morphological, biochemical and physiological levels and use different adaptation mechanisms to withstand drought stress. Plant growth promoting rhizobacteria (PGPRs) are soil bacteria that can increase crop plant growth and protect them against biotic and abiotic stresses through different mechanisms. In this regard, this study was designed and conducted to investigate the effectiveness of plant growth-promoting bacteria (PGPB) on the agromorphological characteristics of Lemon balm under drought stress in the second year of Lemon Balm cultivation under water deficit stress.
Materials and Methods
This experiment was performed during 2019-2020 growing season at the research field of the faculty of agricultural, Shahrekord University in the form of split plots in randomized complete block design with three replications. The first factor includes three levels of drought stress (full irrigation, 75% and 50% of full irrigation) and the sub-factor was inoculation with PGPB in 5 levels including control (no bacterial inoculation) and inoculation with four bacterial species (Azospirillum, Bacillus strain A, Bacillus amyloliquefaciens and Streptomyces rimosus). The amount of irrigation water was calculated using plant water requirement. In order to apply water stress, a moisture meter (Delta-T, SM-150T) was used to measure soil moisture content and the plants were irrigated when the respective MAD threshold was reached. Leaf area was measured with Digimizer software. After harvesting, first the plant height was measured and then the number of main and sub-branches was counted and the leaf dry weight, stem dry weight and biological yield were determined. The essential oils were extracted by hydro-distillation method using a Clevenger equipment. SAS software implemented for statistical analysis and the means were compared using LSD test.
Results and discussion
The results showed that the effect of different levels of drought stress on plant height, number of sub-branches, leaf area, stem dry weight, leaf dry weight, essential oil percentage, biological and essential oil yield was significant. Bacterial treatment showed a significant effect on the number of sub-branches, leaf area index, stem dry weight, percentage and yield of essential oil and biological yield in the first cut and leaf dry weight in the second cut. The interaction effect of stress and bacteria only on the number of branches in the second cut was significant. With the increase in the intensity of drought stress, a decreasing trend was seen in all the analyzed traits including in plant height, number of sub-branches, leaf surface, stem and leaf dry weight, percentage and yield of essential oil and biological yield while inoculation of the plant with Azospirillum, Streptomyces rimosus and Bacillus amyloliquefaciens bacteria caused a 10-20% increase in the studied traits compared to the control. The highest essential oil yield, in both cuts was observed in inoculated treatment of Bacillus amyloliquefaciens.
Conclusion
The results of this study showed that increasing drought stress causing significant reduction of all studied traits. However, bacterial treatments reduced the adverse effects of drought stress on the studied traits compared to non-inoculated at all drought stress levels. Among these bacterial treatments, Bacillus amyloliquefaciens had the most effect in improving the adverse effects of stress in all investigated traits and both cuts. According to this research, in order to improve the quantitative and qualitative yield and some agromorphological characteristics of Lemon balm under drought stress, inoculation of the plant with Azospirillum, Streptomyces rimosus and Bacillus amyloliquefaciens bacteria is recommended.