Effect of water deficit stress and separate, dual and triple combined inoculation of some growth promoting bacteria on agromorphological traits of Lemon balm

Lemon balm (Melissa officinalis L.) is one of the most important plants in family Lamiaceae that is involved in the treatment of diseases such as insomnia, anxiety, depression, neurological diseases, migraine, nervous disorders of the stomach. The most antioxidant activity of the plant was related to leaves, stems and flowers, respectively. The essential oil of lemongrass leaves had more antioxidant activity than stem and flower essential oils. Nowadays, drought stress is considered as the most important factor in reducing production, especially in arid and semi-arid regions. Given the importance and role of medicinal plants in various industries, a significant point in the production of these plants is to increase their biomass production without the use of chemical inputs, especially in drought stress. It seems that application of plant growth promoting bacteria has positive effect on the yield and quality of medicinal plants under biological and non-biological stress conditions. Therefore, the present study was designed and conducted to investigate the effect of some plant growth-promoting bacteria on agromorphological traits of Lemon balm (Melissa officinalis L.) in water deficit conditions.

The factorial pot experiment was performed in a randomized complete block design with three replications in the open area of the research greenhouse of Shahrekord University, Faculty of Agriculture during 2009-2010 growing season. The experimental factors were water deficit stress at three levels (full irrigation, 75% and 50% of full irrigation) and bacterial inoculation treatments at eight levels (non-bacterial inoculation as control treatment, separate inoculation of Bacillus licheniformis, Bacillus megaterium and Pseudomonas putida, bacterial inoculation of B. licheniformis * B. megaterium, P. putida * B. megaterium and B. licheniformis * P. putida as dual inoculation and bacterial inoculation of P. putida * B. licheniformis * B. megaterium as triple inoculation). After harvest, plant height, leaf dry weight, stem dry weight, biological yield, root dry weight and root volume were measured. Leaf area was also measured using Digimizer software. Analysis of variance using SAS software, comparison of means by LSD test at 5% probability level and drawing graphs with Excel software were performed.

The results showed that the main effects of different levels of water deficit stress on all studied traits except plant height were significant at the level of 1% probability. So that water stress at the level of 100% full irrigation increased plant height, leaf dry weight, stem dry weight, root dry weight, biological yield, leaf area and water use efficiency compared to 50% of full irrigation by 17.6%, 124.5%, 79.1%, 65.6%, 102%, 384.4% and 43% respectively. The main effects of bacterial inoculation treatments on plant height, leaf dry weight, biological yield, leaf area, root dry weight, proline content and water use efficiency were significant at 1% probability level and on root volume at 5% probability level, while on dry stem weight trait no significant effect. Interaction effects of bacterial inoculation treatments and different levels of water deficit stress on leaf area, root dry weight, root volume and proline content at 1% probability level, on leaf dry weight, biological yield and water use efficiency at 5% probability level were significant and on plant height and stem dry weight had no significant effect.

According to the results, all bacterial inoculation treatments in non-stress conditions were able to improve the studied traits, effectively while in stress conditions, bacterial inoculation treatments at 75% stress and 50% irrigation levels, except in plant height and water use efficiency and proline content at 50% irrigation level had no significant effect on other traits.