Effect of inoculation with plant growth-promoting microorganisms on some morphophysiological characteristics and essential oil and nutrients content of basil (Ocimum basilicum L.) under NaCl salinity stress

Basil (Ocimum basilicum L.) is an annual herbaceous plant belonging to the mint family (Lamiaceae). Its fresh and dried leaves and essential oils are used in the food, pharmaceutical, and cosmetic industries. In recent years, the use of symbiosis with plant beneficial rhizospheric microorganisms including rhizobacteria and endophytic fungi has been considered as a cost-efficient and sustainable strategy to alleviate the adverse effects of environmental stresses such as salinity. Therefore, the present study was conducted in the research greenhouse of Urmia University. It evaluated the effect of inoculation with growth-promoting microorganisms on some growth, physiological, and phytochemical characteristics of basil plants under salt-stress conditions. This research was conducted as a factorial experiment in a completely randomized design with three replications. The experimental factors were inoculation with microorganisms at three levels (control without inoculation, inoculation with Serendipita indica, and inoculation with a mixture of Pseudomonas areuginosa, P. putida, and P. fluorescens) and salinity stress at four levels (0, 40, 80 and 120 mM of NaCl). For inoculation, the germinated seeds were separately inoculated with S. indica suspension (5×105 spores per ml) and inoculum containing a mixture of Pseudomonas bacteria (1.61×109 cells per ml) and planted in prepared pots. The pots were irrigated with ordinary tap water until the plants were eight-leafed, and from this stage onwards, salinity stress treatments were applied by dissolving different concentrations of NaCl in the irrigation water and continued until the full flowering stage. In the full flowering stage, plant samples were collected and root colonization percentage by fungus, growth parameters (plant height, stem diameter, number and total length of lateral branches, leaf number and area, inflorescence length, fresh and dry weight of leaf and stem), leaf relative water content (RWC), photosynthetic pigments, concentration of leaf nutrients (N, P, K, Na and Cl), essential oil content and yield were evaluated. For essential oil extraction, shade-dried samples were hydro-distilled using a Clevenger-type apparatus. The results showed that due to salinity stress, the percentage of root colonization by S. indica, growth parameters, RWC, photosynthetic pigments, essential oil content, yield, N, P, and K content, and K/Na ratio decreased while Na and Cl content increased. In addition, all evaluated parameters in inoculated plants were higher than in non-inoculated plants except Na and Cl content. The highest and lowest rates of growth parameters, RWC (74.47 and 72.39%), essential oil yield (0.23 and 0.17 ml/pot), and N content of leaves (1.36 and 1.14%) were obtained in plants inoculated with S. indica and non-inoculated plants, respectively. The highest and lowest amounts of chlorophyll a (0.87 and 0.74 mg/g fw), chlorophyll b (0.41 and 0.37 mg/g fw), essential oil percentage (1.24 and 1.05%), K content (5.15 and 3.97%), P content (0.29 and 0.24%) and the K/Na ratio (10.05 and 4.07), were observed in plants inoculated with a mixture of Pseudomonas bacteria and non-inoculated plants, respectively. Also, the lowest Na (1.34%) and Cl (2.93%) accumulation was observed in plants inoculated with Pseudomonas bacteria and S. indica fungus, respectively. According to the results of this study, the use of plant growth-promoting microorganisms (S. indica fungus and a mixture of Pseudomonas bacteria) can alleviate adverse effects of salinity stress on the growth and essential oil production of the basil plant by increasing the absorption of water and nutrients, preserving photosynthetic pigments and reducing the accumulation of toxic ions.