Abstract
In order to study the effect of different amounts of nitrogen and plant density on growth, yield, the percentage of essential oil in leaf and essential oil yield of peppermint (Mentha piperita L.), an experiment was conducted in 2008 at the Experimental Field of the Agricultural Faculty of Bu-Ali Sina University. The experiment was split plot based on randomized complete block design with three replications over two cuts. The main plots included the amounts of 100, 150 and 200 kg nitrogen/ha in which half of fertilizer were used for the first cut and another half for the second. The sub-plots were consisted of plant densities as 8, 12 and 16 plants/m2. Morphophysiological characteristics including plant height, node and leaf number per plant, leaf area index, fresh and dry yield, essential oil percentage in leaf and essential oil yield over two cuts, and also the growth of dry matter, leaf area index and crop growth rate in the first cut were measured. The results showed that, nitrogen treatment significantly affected the properties measured, so that, their highest rates were obtained with the application of 100 and 200 kg nitrogen/ha from the first and the second cut, respectively. In addition, total dry matter, leaf area index and crop growth rate increased with increasing the amounts of nitrogen. Plant density also affected the plant height; node number, leaf number and leaf area index, essential oil percentage in leaf and essential oil yield in the first cut significantly, and in the second cut, plant density effect on leaf number and leaf area index was significant as well.

Keywords: Peppermint, Nitrogen, Plant density, Yield, Growth indices, Essential oil