Biochemical and Physiological Responses of Peppermint (Mentha piperita L.) Plant to Heavy Metal Stress in Urban Areas

Introduction Urban agriculture, as one of the basic features of urban planning, helps to increase the quality of urban life due to its cultural, economic and social benefits. However, pollution with heavy metals in cities causes the accumulation of these metals in different parts of planted plants and also the risk of consuming them for food in urban green spaces. Materials and Methods This study was conducted to assess the amount of heavy metal absorption and its effect on some biochemical and physiological properties of peppermint plant (Mentha piperita L.), in Mashhad city in 2021.The experiment was carried out as a factorial based on randomized complete block design with three replications. The first factor (location) were phase 4 Park (with high degree of contamination) and Nasim Park (with low degree of contamination). The second factor was the times of harvest (June15, July15 and August 15). At the time of every harvest fully developed leaves were collected to evaluate the traits. Results and Discussion The results of analysis of variance indicated that the effect of location was significant on all traits except for the yield of essential oil. Also, the effect of harvest time was significant on all traits except for peroxidase activity and the yield of essential oil. However, the interaction of location and harvest was significant only on phenol, flavonoid, proline, cadmium and lead concentration. The results indicated that the ascorbate peroxidase, catalase and peroxidase activities were higher in phase 4 Park. Moreover, the highest activities of ascorbate peroxidase, catalase and peroxidase were recorded in phase 4 + first harvest. Heavy metals cause the production of reactive free radicals and also increase the activity of antioxidant enzymes. However, the chlorophyll a, b, carotenoid and total chlorophyll contents were higher in Nasim. Thus, the highest contents of Chla, Chlb, Chltotal and carotenoid were observed in Nasim + first harvest. The higher amount of chlorophyll and carotenoids in the first harvest is due to the optimal growth conditions such as day length and sunlight and ambient temperature. In addition, total phenol, flavonoid, proline, Cd and Pb elements indicated a reducing trend in phase 4 compared to Nasim Park in different harvest times, but the amount of these traits were higher in the first harvest than in the subsequent harvests. Increased amount of total phenol in the first harvest can be related to the high air temperature at the first harvest which caused stressful conditions in this stage. Proline production also increases under heavy metal stress to help protect the plant against toxicity. However, the percentage of essential oil showed an increasing trend with enhanced absorption of Pb and Cd in phase 4 compared to Nasim Park. The higher percentage of essential oil in phase 4 may be attributed to reduced leaf growth due to the higher presence of heavy metals in that area. Conclusions Overall, while the concentration of Pb exceeded the global standard level in both parks, contamination with Cd and Pb (especially Pb) was greater in phase 4 than in Nasim Park, contributing to reduced growth traits in peppermint plants. Regarding harvest times, the first harvest exhibited better growth characteristics and higher absorption of heavy metals due to the plant's greater vigor. In contrast, the third harvest showed lower growth characteristics and weaker absorption of Pb and Cd, likely due to the energy expended for regrowth.