Evaluation of Nitrogen and Harvest Time Interaction on Yield, Quantity and Quality of Essential Oil of Four Cultivars of Sweet Basil (Ocimum basilicum L.)

Sweet basil (Ocimum basilicum L.) is an annual herbaceous plant from labiatae family. The amount and chemical composition of essential oil of sweet basil depends on genetics, growing season, environmental factors and plant growth stage. Nitrogen is one of the most important nutrients required by plants. This element plays an essential role in the synthesis of amino acids and proteins and is part of the structure of leaf chlorophyll and some plant hormones. Nitrogen application can significantly increase the growth and yield of medicinal plants in different climatic conditions and affect the quantitative and qualitative properties of essential oils. The results of various studies have shown that nitrogen significantly alters the amount of essential oils in basil. Despite the positive effect of nitrogen on improving plant growth and yield, excessive use of nitrogen fertilizer can have a negative effect on the absorption of other elements. On the other hand, high amounts of water-soluble nitrogen lead to groundwater pollution. Therefore, determining the appropriate amount of nitrogen fertilizer in proportion to the nutritional needs of crops, in addition to improving the quantity and quality of yield, reduces damage to agricultural systems and the environment. Therefore, the aim of the present study was to evaluate the effect of different amounts of nitrogen fertilizer on vegetative yield and quantitative and qualitative characteristics of essential oil of four basil cultivars and to introduce the best cultivar in Khorramabad climate.

This experiment was performed as a factorial split plot in time in a randomized complete block design with three replications during 2016 growing season in the Agricultural Research Station of Khorramabad. Experimental treatments included four sweet basil cultivars (Italian Large Leaf, Cinnamon, Sweet Thai and Mobarakeh), three levels of nitrogen fertilizer (0, 100 and 200 kg ha-1 chemical nitrogen fertilizer) and three harvests. Nitrogen fertilizer (from urea source) was added to the plots in two stages (half of the fertilizer before seeds planting and the rest after the first harvest) based on the treatments. Basil plants were harvested three times at the beginning of flowering. Traits measured in this study included chlorophyll index, leaf/stem ratio, leaf dry weight, total plant dry weight, percentage and yield of essential oil and determination of chemical composition of essential oil. To extract the essential oil, water distillation method and Clevenger apparatus were used. To determine the main constituents of essential oil, all treatments related to a given repetition were chosen in second harvest were used. Gas chromatography with mass spectrometer was used to identify the compounds of basil essential oil. Analysis of variance of data was done using SAS ver. 9 and mean comparison was performed based on the Duncan's multiple range test at 5% probability level using MSTAT-C software. Figures were drawn by excel software.

The results of the present research showed that the highest leaf/stem ratio (1.8) was obtained from applying 100 and 200 kg ha-1 of urea fertilizer in Italian Large Leaf cultivar in the first harvest. Among the studied basil cultivars, Italian Large Leaf cultivar had the highest leaf dry weight, total dry weight, percentage of essential oil and essential oil yield. The maximum leaf chlorophyll index was related to Sweet Thai and Cinnamon cultivars and the minimum chlorophyll index, percentage of essential oil and essential oil yield was related to Mobarakeh cultivar. Consumption of urea fertilizer was associated with a significant improvement in vegetative growth and as a result, basil yield increased. Although the essential oil percentage of basil cultivars under control treatment was higher than plants that were fed with nitrogen fertilizer, but the highest essential oil yield was obtained from application of 100 kg ha-1 urea fertilizer, which shows the greater effect of dry yield on essential oil yield compared to the percentage of essential oil. In all studied traits, the maximum value was related to the second harvest and the minimum amount (except the essential oil percentage) was allocated to the first harvest. In chemical analysis of essential oils obtained from young leaves and shoots of basil cultivars under different fertilization treatments, 29 to 35 compounds were identified. The maximum concentration of major constituents of essential oils (except 1-8-cineol, methyl cinnamate and methyl chavicol) was related to the control treatment (no fertilizer application).

Since there was no significant difference between the levels of 100 and 200 kg ha-1 of urea in terms of total dry weight and the highest essential oil yield was obtained from the treatment of 100 kg ha-1 of urea fertilizer, Italian Large Leaf cultivar and consumption of 100 kg ha-1 of urea fertilizer can be used in environmental conditions similar to Khorramabad.