The Effect of Mycorrhizal Fungus (Piriformospora indica) on the Morphological, Physiological and Biochemical Traits of the Medicinal Plant Stevia (Stevia rebaudiana) under Drought Stress

Stevia (Stevia rebaudiana) is one of the medicinal plants of the Asteraceae family that contains natural compounds, especially stevioside and ribaodioside A, which are estimated to be 150 to 400 times sweeter than sucrose. Plants are exposed to various environmental stresses during growth and development under natural and agricultural conditions. Among these, drought is one the most severe environmental stresses affecting plant productivity. About 80–95% of the fresh biomass of the plant body is comprised of water, which plays a vital role in various physiological processes including many aspects of plant growth, development, and metabolism. Stevia is susceptible to various environmental stresses but the major effects are contributed by drought. Today, the fungal species Stevia rebaudiana is used as a biofertilizer and increases the production of secondary metabolites of economically valuable plants and also increases the growth and seed production of many plants. This fungal endophyte produces a significant amount of acid phosphatase for mobility in a wide range of insoluble or complex forms of phosphate, enabling the host plant to have adequate access to inactive phosphorus reserves in the soil. However, medicinal plants that are cultivated have often been reported to have lower abundance of arbuscular mycorrhizal fungi in the rhizosphere, which significantly reduces plant survival. Considering the coexistence role of mycorrhizal fungi in modulating the effects of drought stress, the aim of this study was to investigate the morphological, physiological and biochemical traits of stevia in response to the effects of mycorrhizal inoculation and drought stress.
 
This experiment was conducted to investigate the effect of P. indica endophytic fungus under water stress conditions on vegetative characteristics, physiological parameters and micronutrients of stevia. A factorial experiment was employed based a completely randomized design with four replications in the research greenhouse of Islamic Azad University, Mahabad Branch in 2017. The first factor was drought stress at four levels (25, 45, 60 and 80% of field capacity) and the second factor was inoculation of seedlings with fungus at two levels (no inoculation and inoculation with P. indica). Water stress was applied based on a combination of plant appearance symptoms (no wilting to severe wilting) and soil moisture. Investigated traits included root colonization, dry weight, leaf number, plant height, stem diameter, chlorophyll a, b, total chlorophyll, carotenoids, proline, soluble sugars, antioxidant power and micronutrients including copper, iron, zinc and manganese. To analyze the data variance, SAS 9.1 statistical software was used to analyze the variance of the data.
The results showed that the evaluated traits in the present study were affected by the main treatments of fungus and drought stress. Seedlings inoculated with P. indica endophytic fungi had the highest percentage of root colonization, growth parameters, photosynthetic pigment content, soluble compounds and micronutrients compared to no inoculation. Drought stress increased soluble sugars, proline content and antioxidant power of stevia leaves and decreased the other traits by increasing the stress level from 25 to 80%. The highest rate of root colonization (26.90%), stem diameter (3.21 mm) and carotenoid content (1.71 μg/ml) was observed in the treatment of plant inoculation with fungi and 25% drought stress. While the highest antioxidant power was found in the treatment of plant inoculation with fungi and 80% drought stress. According to the results of the present study, use of P. indica fungus had the most positive effect on the quantitative and qualitative characteristics of stevia medicinal plant compared to no fungus inoculation.
 
This study showed the positive effect of P. indica endophyte inoculation on quantitative and qualitative characteristics of root colonization, dry weight, number of leaves, plant height, stem diameter, chlorophyll a, b, total chlorophyll, carotenoids, proline, soluble sugars, antioxidant power and The micronutrients of calcium, iron, zinc and manganese showed stevia, and drought stress reduced the studied traits except for proline content, soluble sugars and antioxidant power. Inoculation of stevia seedlings with P. indica endophytic fungi at drought stress levels had the highest rate of root colonization, stem diameter, carotenoid content and antioxidant power compared to non-fungal inoculation. Therefore, due to the effect of biological compounds of natural origin and the production of plants with healthier active secondary compounds, the use of P. indica endophytic fungi can be recommended.