Foliar Application of Fe, Zn, and Mn as a Practical Strategy to Alleviate the Soil Cu Toxicity and Stimulate the Physiological and Biochemical Properties of Peppermint (Mentha piperita L.)

The current comprehension of the connection between foliar application (FA) of microelements and its impact on reducing soil copper toxicity in medicinal plants is limited. Therefore, this investigation assessed whether micronutrient FA (Fe, Zn, and Mn) could mitigate soil Cu toxicity in Mentha piperita L. Four FA treatments (deionized water spray as control or 0, a spray of 1% Fe, 1% Zn, or 1% Mn) were applied to peppermint grown under soil Cu toxicity conditions (0 as control, 5, and 25 mg kg−1 soil). The dry weight of shoots and roots were negatively affected under 25 mg kg−1 Cu toxicity and 0 FA (1.22 and 0.35 g, respectively), in contrast to 0 Cu toxicity and 0 FA (2.11 and 1.77 g, respectively). However, the application of FA containing Fe, Zn, and Mn enhanced these parameters at all levels of Cu toxicity. Under 25 mg kg−1 Cu toxicity, FA with Fe, Zn, and Mn reduced the leaf proline content by 35.1%, 24.9%, and 25.2%, respectively, compared to 0 FA. Under 25 mg kg−1 Cu toxicity, FA of Mn and Zn increased the activity of ascorbate peroxidase (APX) by 28.4% (for leaves) and 23.7% (for roots), respectively, in comparison to 0 FA. The predominant essential oil (EO) components identified under all levels of Cu toxicity included menthofuran, menthol, menthone, menthyl acetate, pulegone, and limonene. FA of Fe, Zn, and Mn can provide a cost-effective and practical approach to amending Cu-contaminated soils. To enhance peppermint EO production, an application of 5 mg kg−1 Cu toxicity can be recommended.