effect of Different Levels of Nitrogen on Yield, Nitrate Content, Essential Oil Yield and some Physiological Traits in Four Cultivars of Sweet Basil (Ocimum basilicum L.)

Sweet basil (Ocimum basilicum L.) is one of the oldest spices belongs to the Lamiaceae family. It is a herbaceous annual plant, which is native to subtropical regions of Asia, Africa, America. Basil is an important economic crop and widely used in the culinary arts, food processing and pharmaceutical industries. Nitrogen is one of the most important nutrients for plant production that plays a major role in photosynthetic activities and crop yield capacity and its availability affects plants growth and biochemical processes. Nitrogen deficiency is often a limiting factor in vegetable production such as sweet basil. Nitrogen usage significantly can be increased the herb yield of basil grown in different environmental conditions. The nutritional and environmental factors are the most important factors for growth of the medicinal plants. In the present research, changes in yield, nitrate content, essential oil yield and some physiological traits have been investigated in four cultivars of sweet basil under different levels of nitrogen.

A field experiment was carried out in the Agricultural Research Station of Khorramabad during 2016 growing season. The experimental factors were arranged as a factorial-split-plot in time experiment based on randomized complete blocks design with three replications. Experimental factors were cultivars of sweet basil (O.basilicum var Italian Large Leaf، O. basilicum var Mobarakeh, O. basilicum var Sweet Thai and O. basilicum var Cinnamon) and nitrogen fertilizer (0, 100 and 200 kg ha-1 urea) assigned to the main plots as factorial and different cuttings (three harvests from each plot) considered as sub-plots. The half amount of nitrogen (as urea; 46% N) was distributed in experimental plots before planting the seeds and the rest used after the first harvest. Seeds of sweet basil were planted at May, 25th 2016 as five rows with 2m length and each main plot area was 5 m2. The space between rows was 50 cm. All plots were irrigated immediately after sowing. Subsequent irrigations were carried out every four days. The control of weeds conducted manually during the experiment time. The sweet basil plants were harvested three times in early flowering stage in July 12, August 15 and September 25. Samples of 1 m length were taken from the center of two rows located in the middle of each plot. Plants were cut from above ground and transferred to the lab for measuring total dry yield. The measured traits in this study included branches per plant, inflorescences per plant, plant height, leaf percentage, total dry yield, essential oil yield, chlorophyll a, chlorophyll b, total chlorophyll, carotenoid and nitrate content. SAS (version 9.1) and MSTAT-C statistical software were used to conduct analysis of variance (ANOVA) and comparison of means, respectively. The Duncan's multiple range test, at 5% probability level, was used to rank the differences among means.

The results indicated that the application of nitrogen fertilizer significantly improved all the studied traits. The highest and the lowest essential oil yield belonged to 100 kg ha-1 urea and control, respectively. Nitrogen fertilizer by providing the nitrogen needed for the basil plant during the growing season provides a favorable environment for the production of further biomass. Total dry weight of the all basil cultivars increased with application of nitrogen fertilizer in all harvests. The highest total dry weight and nitrate concentrations were achieved by Italian largel cultivar with application of 200 kg ha-1 urea in the second harvest. The increment in total dry weight of basil by application of nitrogen fertilizer could be attributed to the increment in chlorophyll content and better growth of plants and subsequently the better canopy development which ultimately leads to the better usage of solar irradiance, higher photosynthesis and finally higher dry weight in basil plants. Application of nitrogen chemical fertilizer could increase essential oil yield of sweet basil, mainly due to the increasing of total dry weight. Among basil cultivars, Italian large leaf cultivar had the highest leaf percentage and essential oil yield compared to the other cultivars. The lowest number of branches per plant, number of inflorescences per plant, leaf percentage, essential oil yield and total chlorophyll belonged to Mobarakeh cultivar. All the evaluated parameters were lower than the first harvest except leaf percentage and chlorophyll content.

According to the results, application of nitrogen fertilizer had a positive effect on growth, and consequently led to increase the plant vegetative yield. Due to the fact that in terms of more of the studied traits there was no significant difference between application of 100 and 200 kg ha-1 urea fertilizer and also reduce the consumption of chemical fertilizers generating pollution, Italian large leaf cultivar and application of 100 kg ha-1 urea are recommended to access an acceptable growth and quantitative and qualitative yield in sweet basil under the environmental conditions similar to Khorramabad.