Defensive response of radish seedlings to the oxidative stress arising from phenolic compounds in the extract of peppermint (Mentha × piperita L.)

Understanding the mode of action of allelochemicals, as the cause of oxidative stress and induction of antioxidant mechanism, is one of the challenging aspects in allelopathic studies. Accordingly, the aim of the present study was to analyze the effect of allelochemical stress, induced by peppermint water extract (WE), on radish (Raphanus sativus L.) growth. The peppermint WE concentrations were 0, 2, 4, 6, 8, and 10% (v/v). Some phenolic compounds of the peppermint WE determined by HPLC were trans-ferulic acid (10.8 mg/g), hesperidin (9.3 mg/g), ellagic acid (6.8 mg/g), and sinapic acid (4.2 mg/g). The maximum inhibitory effect of peppermint extract on germination and growth was obtained at 8% WE concentration. The results showed that the compounds in the extract of peppermint had significant effect on the amount of proline (PRO), soluble sugars and total phenols, as well as on the activity of antioxidant enzymes, and relative membrane permeability (RMP) (P < 0.05). However, with the exception of super oxide dismutase (SOD), all of the treatments had a significant (P < 0.05) positive effect on the activity of other enzymes such as ascorbate peroxidase (APX), catalase (CAT) and peroxidase (POD). The results suggest that oxidative stress caused by high levels of reactive oxygen species (ROS) resulting from peppermint allelochemicals inhibited the growth of the radish seedlings, and their ability to resist the phytotoxic allelochemicals of peppermint, which is associated with the activity of protective enzymes and the defensive function of radish plants.