Effects of nitrogen fertilizer and harvest time on agronomy, essential oil and menthol of Mentha piperita L.

Mint (Mentha piperita L.)، belonging to Labiatae family، is one of the most important plants، which is used widely in terms of pharmaceutical، food and health. Essential oils and compounds are important components of the plant and are severely affected by agricultural agents. In this study، the effects of nitrogen fertilizer and harvesting time on yield and yield components، essential oil and menthol were studied. A split-plot design with three replications was used. The measured traits included leaf length، leaf width، plant height، fresh weight per square meter in the first and second harvest، fresh weight of 100 leaves، essential and oil menthol of the first second harvest، shoot yield and essential oil yield per hectare. Variance analysis results showed that different levels of nitrogen and harvesting time had a significant effect on all traits except for leaf length. The interaction of nitrogen and harvesting time showed a significant difference for all traits except leaf length and width. Mean comparison of traits showed that the plant height، fresh weight per square meter، fresh weight of 100 leaves، essential oil of the first harvest، menthol of the first and second harvest، shoot yield per hectare and essential oil yield increased with increasing of nitrogen intake. Mean comparison of different harvesting times showed that the highest shoot yield and essential oil yield were obtained at budding stage. The interaction between different levels of nitrogen fertilizer and harvesting time also showed that the highest shoot yield per hectare was obtained by the application of 200 kilograms of nitrogen fertilizer per hectare at budding stage. The highest amount of menthol was obtained at consumption level of 300 kg nitrogen fertilizer per hectare and at 50 % flowering. In addition، according to the obtained results، essential oil percentage of the second harvests showed a significant decreasing trend. Overall، results showed that the highest fresh yield per hectare and essential oil were obtained at budding stage and consumption level of 200 kg/ha nitrogen fertilizer.