The effect of mycorrhizal fungi inoculation on the antioxidant response and nutrient absorption of Paulownia fortunei seedlings under drought stress

Background and objectives: Drought stress is one of the most important factors limiting plant growth in the world and one of the most common abiotic stresses. Drought stress is a multidimensional stress that stimulates a wide range of physiological, biochemical and molecular responses in plants. In arid and semi-arid areas, the establishment and growth of young seedlings is completely affected by water deficit and drought stress. In this situation, the use of symbiosis of microorganisms, especially mycorrhizal fungi, can reduce the effect of drought stress via increasing the absorption of water and nutrients. Therefore, the aim of this study was to determine the effect of mycorrhizal fungi symbiosis on the absorption of the on the absorption of some macro and micro nutrients as well as the activity of antioxidant enzymes of Paulownia fortunei (Seem.) Hemsl., known as a fast-growing species.
Methodology: This pot experiment was conducted in factorial form in a completely randomized design with three replications and two factors. The first factor included four levels of drought stress (no stress or control with full irrigation at 100% of crop capacity, mild stress with irrigation at 80% of crop capacity, moderate stress with irrigation at 60% of crop capacity and severe stress with irrigation at 40% of crop capacity) and the second factor which included two surfaces of using mycorrhiza (without and with arbuscular mycorrhizal fungi). Application of drought stress treatments was done based on the weight method. The investigated features included the measurement of soluble protein, catalase, peroxidase, superoxide dismutase enzymes, as well as the absorption of nutrients such as phosphorus, nitrogen, potassium, zinc, iron and manganese. The normal distribution and the homogeneity of the variance of the obtained data were checked using the Shapiro-Wilk test and Levene's test, respectively. Two-way analysis of variance was used to test the significance of the simple and interaction effect of the factors. Duncan's multiple range post-hoc test was used to compare the means.
Results: The results of analysis of variance showed that the interaction effect of drought stress and mycorrhiza was not statistically significant on any of the traits. The simple effect of mycorrhiza on the changes of superoxide dismutase enzyme was significant, but it was not significant on other measured traits. The results showed that drought stress significantly increased the amount of soluble protein, peroxidase and superoxide dismutase activity. The highest concentration of soluble protein was related to severe stress treatment with a mean value of 38.95 mg/g leaf wet weight, and the lowest is related to the control or no stress treatment with a mean value of 31.45 mg/g leaf wet weight. However, no significant difference was observed between mild, moderate and severe stress treatments. About catalase enzyme no significant difference was observed between moderate and severe stress treatments, as well as between mild and moderate stress treatments. The highest concentration of peroxidase enzyme was related to severe stress treatment with a mean value of 3.77 mg/min per mg of protein and the lowest mean value (3.21 mg/min per mg of protein) was related to non-stress treatment (control). No significant difference was observed between the treatments of moderate stress and severe stress, as well as between the treatments of mild and moderate stress in terms of the concentration of peroxidase enzyme. The highest concentration of superoxide dismutase enzyme was related to severe stress treatment with a mean value of 0.176 mg/mol/min per mg of protein and the lowest value (0.108 mg/min/mg of protein) was related to the control. The concentration of superoxide dismutase enzyme in the treatment with mycorrhiza with a mean value of 0.159 mg/mol/min per mg of protein was significantly higher than of it in the treatment without mycorrhiza with a mean value of 0.140 mg/mol/min per mg of protein. Drought stress also led to the reduction of all measured nutrients. While, inoculation with mycorrhizal fungi increased nutrients except nitrogen at all levels of drought stress.
Conclusion: It was concluded that although the absorption of macro and micro nutrients by the seedlings decreased with the increase of drought stress levels, the use of mycorrhizal fungi could increase the absorption of macro and micro nutrients in drought stress conditions due to the better absorption of water by the hyphae around the roots of Paulownia seedlings. Therefore, it seems that the mycorrhizal fungi are very important in the absorption and metabolism of elements required by Paulownia seedlings, especially in stress conditions. Also, the inoculation of mycorrhizal fungi led to an increase in the activity of the antioxidant enzyme superoxide dismutase (SOD), which can increase the resistance of seedlings against drought stress.