Physiological and biochemical assay of drought stress responses in eggplant (Solanum melongena L.) inoculated with commercial inoculant of Azotobacter chroococum and Azotobacter vinelandii

The aim of this study is to examine the effects of commercial rhizobacteria inoculant on eggplant plants grown under drought stress conditions. Commercial inoculant containing Azotobacter chroococum and Azotobacter vinelandii rhizobacteria species was applied to eggplant plants by root inoculation and the plants were exposed to different levels of drought stress (moderate drought stress-MS and severe drought stress-SS). To determine the growth-promoting ability of inoculation with rhizobacteria, changes in plant morphology (shoot-root fresh and dry weights, shoot length and diameter) and physiology (relative water content-RWC, stomatal conductivity-gₛ, K, Ca, Mg and Na accumulations in shoot and root, photosynthetic pigment contents) were investigated. To determine the impacts of the inoculant on the potential of increasing the drought tolerance of eggplant, besides the enzyme activities of superoxide dismutase (SOD), catalase (CAT) and glutathione reductase (GR), non-enzymatic antioxidant activities such as antochiyanin, total phenolic substance, proline were investigated. In addition, H₂O₂ and malondialdehyde (MDA) contents were analyzed to resolve whether drought stress causes oxidative damage in eggplant. The increase in the severity of drought caused a decrease in plant growth and shoot-root fresh and dry weights. Nevertheless, these adverse effects of drought stress were alleviated by inoculation. Decreased RWC, gₛ values of plants under drought stress, and especially K, Ca and Mg accumulations and protein contents in the root increased significantly with inoculation. On the other hand, Chlorophyll (Chl) (Chl a, Chl b, Chl a + b) and carotenoid contents were significantly increased in leaves under uninoculated MS and SS. Inoculation with rhizobacteria reduced the increase in photosynthetic pigment contents. Depending on the severity of stress, higher levels of total phenolic compounds and proline were accumulated in inoculated plants compared to uninoculated plants. However, higher SOD, CAT, and GR enzymatic activities were observed in inoculated stressed plants, and membrane lipid peroxidation was reduced. These results were found to be important in that the commercial bacterial inoculant has the potential to diminish the negative effects of drought stress in eggplant and supports the stress tolerance of the plant by mitigating the drought-related oxidative damage.