Enhancing plant growth and stress tolerance: Exploring the effects of melatonin, salicylic acid, and bacterial inoculation in pepper and soybean under abiotic stress

The aim of this study, the potential negative effects of abiotic stress factors such as salt stress (SS) and boron toxicity (BT), which are frequently encountered in semi-arid and arid regions, on plant growth and soil microbial activity were investigated. To mitigate these negative effects, three separate experiments were conducted to examine the effects of foliar application of melatonin (MT) and salicylic acid (SA), as well as bacterial inoculation (BA) in pepper (Capsicum annuum) and soybean plants (Glycine max (L.) Merr.). In the first experiment, pepper and soybean plants were grown in a perlite medium, and control, TS (100mM NaCl), BT (2mM B), and TS+BT treatments were applied. The TS+BT treatment showed the most significant negative impact on plant growth (≅50%) and chlorophyll (≅60%) content, while it increased reactive oxygen species (≅250%), proline(≅400%), and malondialdehyde (≅320%) levels. Foliar application of 100 µM melatonin (MT) to alleviate the harmful effects of stress increased both the fresh and dry weights (≅50%) and chlorophyll (≅55%) content in both plants and reduced reactive oxygen species levels (≅25%) and malondialdehyde content (≅17%) (P≤0.05). Additionally, MT application enhanced antioxidant enzyme activity, aiding in stress management. The second experiment was conducted with soybean plants in a soil medium, and bacterial inoculation was applied in addition to the treatments from the first experiment. Nodule formation did not occur in plants without bacterial inoculation, but nodules formed with inoculation. However, under TS+BT treatment, plant growth (≅60%) and chlorophyll content (≅65%) decreased, while reactive oxygen species (≅300%), proline (≅120%), and malondialdehyde content (≅122%) increased. Soil microbial content, including dehydrogenase activity (DHA) (≅40%), microbial biomass carbon (MBC) (≅75%), and CO2 levels (≅75%), also decreased (P≤0.05). Both BA and MT+BA applications in this study reduced the negative effects of stress on the plant and improved stress tolerance. In the third experiment, 0.5mM salicylic acid (SA) was applied foliarly instead of melatonin, and the treatments from the second experiment were repeated. Similar to the second experiment, nodules formed with bacterial inoculation, while no nodules formed in plants without inoculation. Under TS+BT treatment, plant growth (≅40%) and chlorophyll content (≅70%) decreased, while reactive oxygen species (≅280%), proline (≅550%), and malondialdehyde content (≅220%) increased. Soil microbial content, including DHA ≅ (80%), MBC (≅60%~), and CO2 levels (≅25%), also decreased (P≤0.05). Both BA and SA+BA applications in this study reduced the negative effects of stress on the plant and improved stress tolerance. SA did not significantly affect soil microbial activity, but bacterial inoculation significantly increased microbial activity and improved plant resistance against stress (P≤0.05). Overall, our findings suggest that foliar application of MT and SA, as well as bacterial inoculation, can enhance plant growth and stress tolerance while also increasing soil microbial activity. These results could have important implications for mitigating production-related challenges caused by salt stress and boron toxicity in semi-arid and arid regions.