Influence of Vermicompost and Nitrogen Treatments on the Enhancement of Morphophysiological Traits and Essential Oil Composition of Spearmint ( Mentha spicata L.) for Double Harvesting Seasons

This research investigates the impact of organic and chemical fertilizers on the total production and morphophysiological characteristics of Mentha spicata L., a medicinal plant, during two growing seasons in 2021. The study was conducted in a randomized complete block design at Zanjan University's research farm, employing treatments of vermicompost (at rates of 5, 10, and 15 tons per hectare), urea (at rates of 50, 100, and 150 kilograms per hectare), and a control treatment. Significant differences were observed in all measured indices between the fertilizer treatments and harvest times. Particularly, the application of vermicompost at a concentration of 10 tons per hectare significantly enhanced fresh and dry weights, stem height and diameter, number of branches, percentage and yield of essential oils, chlorophyll a, total chlorophyll, carotenoids, total phenols, flavonoids, and antioxidant capacity compared to other treatments. The urea treatment also significantly increased chlorophyll b, leaf width, and length compared to other treatments. Enzyme measurements conducted during one season revealed the highest levels of the APX enzyme in the vermicompost treatment (15 tons per hectare), the CAT enzyme in the vermicompost treatments (10 and 15 tons per hectare), and the SOD enzyme in the vermicompost treatment (10 tons per hectare) during the second harvest. Conversely, the control treatment exhibited the lowest levels across all measured traits. Overall, the results highlight the beneficial effects of using organic and environmentally friendly fertilizers in improving essential oils content, physiological traits, and the overall yield of M. spicata L., suggesting their potential for enhancing sustainable agriculture practices and medicinal plant productivity.