عنوان مقاله [English]
Introduction: Spearmint (Mentha spicata L.) is an aromatic plant belonging to the Lamiaceae family. The ground fresh biomass and dried leaves of the plant are used as spice and herbal tea, and cultivated commercially in the entire world. Spearmint oil also has economic importance and is used in perfumery, confectionary and pharmaceutical preparations. Major components in cultivated spearmint oil are carvone, limonene, and 1,8-cineole. Essential oil compositions of aromatic plants depend on their genetic structure, the climatic factors and the agronomical practices as well as harvest and post-harvest managements. A climate with adequate and regular rainfall and good sunshine during its growing period ensures a good yield. Climatic condition is one of the most important factors influencing the growth and accumulation of active ingredients that its effect is concealed at the harvesting time of plants. In addition, harvest time (cut) and phonological stages can affect herbage yield and oil content and composition of perennial medicinal plants.
Materials and Methods: Plant materials were harvested from a farm in Safiabad city, Khuzestan province, Iran. Five samples from each ecotype of spearmint (Kashan and Shushtar) were collected during growth season at the beginning of flowering stage, from May to the end of November. Plants were hand-harvested by cutting the herbage at 5 cm above the soil. Then the plants were dried at room temperatures (approximately 40 °C). Spearmint essential oil was extracted via steam distillation using a Clevenger apparatus. The distillations were carried out on a sample size of 50 g of dry aerial parts with a distillation time of 180 min. The oil was dried over anhydrous sodium sulfate and was kept in a refrigerator until further analysis. The essential oil’s chemical composition was analyzed by GC and GC/MS.
Results and Discussion: The results showed that temperature and humidity conditions prevailing in the area at the time of harvest had significant effect on plant growth and ultimately leaf dry weight, oil content and composition. The highest leaf dry weight of the Kashan (250.67 g.m-2) and the Shushtar (290.23 g.m-2) were observed at the third and fifth harvest time, respectively. There was no significant difference between the first to fourth harvest times of the Kashan and the first, third, fourth and fifth harvest time of the Shushtar. The highest (3.33%) and the lowest (1.72%) amounts of essential oil of the Kashan ecotype were obtained from the third and the first cuts, respectively. The Shushtar ecotype had lower oil than the Kashan ecotype. The second harvest time of the Shushtar ecotype had the highest (2.53%) amount of essential oil and the lowest (1.36%) amount was observed at the first harvest time that had no significant difference with the third and fifth harvest times. The highest amount of oil yield of the Kashan (8.34 g.m-2) and the Shushtar (4.89 g.m-2) ecotypes was recorded at the third and the fourth cuts, respectively. It seems that plants during the second to the fourth harvests encouraged with hot stress and they produced more essential oil in response to high temperature. Limonene, cis-dihydrocarvone, carvone and caryophyllene were the main oil components at five harvest times. Both ecotypes were similar in regards of main oil components but the Shushtar had more oil composition than the Kashan. In the Kashan ecotype, limonene was at the lowest amount (12.22%) and reached to the maximum amount (20.55%) at the second harvest time. In contrast, cis-dihydrocarvone decreased from 4.95% to 2.34% at the second harvest time and finally the maximum amount (11.14%) was recorded at the fifth harvest time. Carvone content was 68.94% at the first harvest and after that increased to the highest level (74.21%) and finally decreased to the minimum level (65.15%) at the fifth harvest time. In the Shushtar, limonene increased from the first until the fourth harvest and ultimately declined in the last cut. The maximum amount of carvone (72/67%) was observed in the first harvest and then was relatively constant during the second to fourth harvests, and reached to the lowest level (56/58% ) at fifth harvest. The Kashan ecotype had more carvone than the Shushtar ecotype. The fluctuation of carvone and cis dihydrocarvone were in opposite of each other and it seems that lower temperatures in the fifth harvest caused to carvone convert to cis-dihydrocarvone. However, the amount of carvone was more than 50 % at all harvest times.
Conclusion: Although harvest times had a significant effect on the traits, both ecotypes showed a good oil content and yield at five cuts. Oil composition was also changed during different harvest times. Nevertheless, Carvone content was up to 50% at all harvest times and it seems that we can recommend harvesting of plants from May to the end of November.