Secondary metabolites of peppermint change the morphophysiological and biochemical characteristics of tomato

Germination and growth of plants are influenced by allelochemicals that mostly cause crops, yield reduction. In the present study, the effect of stress arising from allelopathic compounds in the water extract (WE) of peppermint (Mentha×piperita L. CV. Mitcham) on the morphophysiological and biochemical characteristics of tomato (Lycopersicon esculentum Mill. CV. Rio Grande) was investigated. Different concentrations (0, 2, 4, 6, 8, and 10% (v/v)) of the WE were examined. Some phenolic compounds of the WE determined by the HPLC instrument 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 results showed that the maximum inhibitory effect on germination and growth (dry weight, and leaf area) was obtained at the concentration of 10% (v/v) extract, and its compounds had significant effect on the amount of proline (PRO), soluble sugar and starch, as well as on the activities of tomato,s antioxidant enzymes such as ascorbate peroxidase (APX), catalase (CAT), peroxidase (POD) and superoxide dismutase (SOD) at the 5% level. None of the treatments had a significant effect on the SPAD chlorophyll meter reading of tomato plants. It could be stated that the compounds present in the extract of peppermint must lead to high levels of reactive oxygen species (ROS), and subsequent oxidative stress inhibits the growth of the seedlings; however, more research is still required in this regard.