Effect of Organic Fertilizers and Urea Fertilizer on Phenolic Compounds, Antioxidant Activity, Yield and Yield Components of Peppermint (Mentha piperita L.) under Drought Stress

The shortage of water in Iran is always considered as a limiting factor in the production of crops. Among them, its effect on growth, absorption of nutrients and the metabolic processes of plants is different and can reduce the growth. The amount of this decrease is always associated with significant changes in the qualitative properties of plants, including medicinal plants. Hence, this investigation was carried out with the aim of improving the essential oil yield of peppermint through integrated nutrient management under the drought stress conditions.
Matherials and

This experiment was carried out at the field research station of the Agricultural Research Center of Tarbiat Modares University in Karaj during 2018. The experiment was conducted as factorial arrangement based on a randomized complete block design with three replications. Treatments included six fertilizer rates (F1: control, F2: 140 kg ha-1 urea, F3: 110 kg ha-1 urea + 3.5 ton ha-1 vermicompost, F4: 70 kg ha-1 urea + 7 ton ha-1 vermicompost, F5: 35 kg ha-1 urea + 110 ton ha-1 vermicompost and F6: 14 ton ha-1 vermicompost) and three irrigation regimes (irrigation was suppressed until 75 (control), 60 (mild stress) and 45% (sever stress) soil moisture was reached). According to the recommended N requirements (140 kg ha-1), the vermicompost was broadcast by hand and incorporated immediately into the soil using a rototiller three days before planting. The urea was applied at two stages. Because of sandy soil, plots were irrigated frequently (interval of 1-2 days) with drip irrigation system to avoid wilting during the plant growth. The plants were harvested at floral initiation by cutting plants about 10 cm above the soil surface. Data of growth parameters were measured as plant height (cm), LAI, dry weights of herbage, essential oil percentage and oil yield. (which were collected at floral initiation). Analysis of variance (ANOVA) of the data from each attribute was computed using the SAS package (SAS Institute, 2009). Significant of differences among irrigation regimes, various fertilizers and their interaction for variables were compared by LSD test (P≤0.05).

The results showed that drought stress reduced leaf chlorophyll content, DPPH, plant height, leaf area index, shoot dry weight and oil yield. Although the amount of flavonoids, carotenoids, total phenols and oil percentage first increased and then decreased by increasing drought stress. Increasing the rate of vermicompost in fertilizer treatment increased the physiological and vegetative traits of peppermint plant. According to the results of interaction between two treatments, the highest plant height, shoot dry weight and essential oils yield were resulted under normal irrigation and 14 tons ha-1 vermicompost. The highest total phenol content was obtained in moderated drought stress and combined fertilization of 70 kg of urea with 7 tons ha-1 of vermicompost. Also, the highest percentage of essential oil and flavonoids content were observed in moderated drought stress treatment with 14 tons ha-1 of vermicompost fertilizer. In general, the results showed that among different treatments of drought, vermicompost fertilizer had a positive effect on vegetative and physiological functions of plants. Application of vermicompost combined with chemical fertilizer increased plant height, essential oil percentage and essential oil yield.

The results revealed that organic fertilizer treatment improves essential oil yield and physiological traits of peppermint by improving various chemical, physical and biological characteristic of the soil. As a result, organic and chemical fertilizer combination reduces the requirement for synthetic fertilizer and improves performance and environmental sustainability.