Effect of shadow and different levels of nitrogen on growth and essential oil content of peppermint (Mentha piperita L.)

Peppermint is an important medicinal and aromatic plant which its oil content varies from one region to another region and as a function of fertilization, shadow, irrigation, and photoperiod. In order to study the effect of different levels of shadow and nitrogen on growth and oil performance of this plant, a factorial experiment was conducted using a completely randomized design with three replications in the Faculty of Agriculture, University of Zanjan during the 2014-2015 cropping season. The experimental factors were, shadow factor in four levels (S0: without shadow, S25: 25%, S50: 50%, and S75: 75% shadow) and nitrogen factor as Urea in four levels (N1: without nitrogen, N2:60, N3:120, and N4:180 mg N/kg). The results showed that the highest number of leaves per plant (936), leaf surface area (1688mm2), chlorophyll index (51.32), essential oil content (2.455%), and essential oil yield (0.37 g/box) were obtained from the S25N3 treatment. The highest shoot dry weight was measured in the S50N4 treatment and the S25N3 treatment with 8.36% difference stood in the second place. The highest leaf dry weight was observed in S25N3 (11.55 g/box). Treatments applied to plant height and numbers of branches were not significantly affected. Therefore, to obtain the highest concentration and performance of essential oil in peppermint, as well as to increase the number of leaves per plant and leaf surface area for marketable and edible consumption, the use of 25% shadow and the application of 120 mg/kg nitrogen are recommended.