Mentha spicata is one of main cultivated crops in temperate, Mediterranean and sub-tropical regions. This plant is an enriched source of vitamins and mineral nutrients. Also, it is used for various medicinal purposes such as treatment of gastrointestinal and respiratory diseases. Nowadays, among the plant stresses, heavy metals toxicity is considered as dominant and global issue damaginh the environment. Heavy metals toxicity is mainly caused by human activities such as mining, irrigation with wastewater and using the products with high content of heavy metals. Heavy metals can negatively affect soil and water quality and human health even in low concentrations. Copper (Cu) is one of most dangerous heavy metals which threatens the human health and reduces the crops quality. Pseudomonas putida and Pseudomonas fluorescens bacteria are plant growth-promoting bacteria. They can stimulate growth and enhance plant resistance to different environmental stresses such as heavy metals toxicity. The aime of this study was to alleviate the adverse effects of Cu heavy metal toxicity on some of most important characteristics of Mentha spicata through the application of bacterial treatments.

In order to evaluate the influence of bacterial treatments including Pseudomonas putida, Pseudomonas fluorescens and putida + P. fluorescens on copper chloride toxicity stress at three levels including 0 (control), 100 and 200 mg kg-1 soil, a factorial experiment was carried out based on completely randomized design with four replications, at Department of Horticultural Science and Engineering of Maragheh University, during 2019. Some of most important characteristics of Mentha spicata such as plant height, aerial part fresh weight, root fresh weigh, photosynthetic pigments content, malondialdehyde (MDA) content, free proline content and antioxidant enzymes activity were measured under stress conditions.

The results of the current study showed that copper chloride toxicity stress had a negative effect on plant growth parameters such as plant height, fresh weight of aerial parts and root as well as photosynthetic pigments content, while the use of Pseudomonas putida and Pseudomonas fluorescens bacteria significantly alleviated these negative effects through increasing the activity of antioxidant enzymes and proline content. The highest fresh weight of aerial part (5.65 g) was recorded in plants treated with putida + P. fluorescens under control condition. The lowest weight of aerial part (2.95 g) was measured in plants treated with P. fluorescens under stress level of 200 mg kg-1 copper chloride. According to reports, the growth-promoting bacteria improve plant growth parameters by reduction of ethylene concentration in plant tissues under heavy metals stress conditions. Also these bacteria, directly increase plants growth rate through the increasing solubility of nutrients such as N and P in soil and biosynthesis of plant growth regulators and phytohormones. The highest SOD activity (35.55 U g-1 min-1) was measured in plants treated with P. putida + P. fluorescens under stress level of 100 mg kg-1 copper chloride.

Based on the results of the present study, copper chloride stress decreased stem height, fresh weight of aerial parts, fresh weigh of roots and the leaf photosyntheyic pigments content (chlorophyll a and b), and increased malondialdehyde and proline content in Mentha spicata plants. However, the use of P. putida and P. fluorescens bacteria alleviated the negative effects of heavy metal stress by increasing the activity of antioxidant enzymes (SOD and GPX) and proline content and reducing the amount of MDA (MDA is considered as indicator for cell membrane damages rate). Therefore, using P. putida and P. fluorescens bacteria treatment could be introduced as efficient and widely used technique to enhance tolerance rate of Mentha spicata against copper chloride toxicity stress.