The Effect of Different Levels of Urea and Iron Fertilizer Resources on Yield and Quality of Lemon Verbena (Lippia citriodora H. B. K.)

Lemon verbena (Lippia citriodora H. B. k.) is a valuable medicinal plant because of the essential oils, such as neral and garanial, which are used in the pharmaceutical, food, cosmetic and sanitary industries. In different industries, using essential oil of medicinal plants depends on the chemical composition, which is influenced by environmental factors, harvest time and farming methods.

This experiment was conducted to investigate the effect of different levels of urea fertilizer and several iron fertilizers on yield and quality of lemon verbena at Fathabad garden of Kerman in 2017. The plants were planted based on factorial experiment in format of a randomized complete block design with three replications. Different levels of urea fertilizer (0, 100, 150 and 200 kg ha-1) and different sources of iron fertilizers included non-use, iron sulfate, nano iron chelate and ortho-chelate iron, were considered in four levels. The measured traits included plant height, fresh and dry weight of leaf per plant, number of branch, chlorophyll content and essential oil quantity and quality. Essential oil extraction was carried out using water distillation method using Clevenger's apparatus and identification of essential oil compounds was performed by gas chromatography–mass spectrometry (GC/MAS). The data were subjected to variance analysis using SAS software and means comparison were done with LSD at 5% level.

The results showed that different levels of urea and various iron sources had significant effect on all traits. Treatment with 150 kg ha-1 urea had the highest effect on measured traits compared to control. Among the types of iron sources, ortho-chelate iron had highest effect on quantity and quality compared to other iron sources and control. Generally, 150 kg ha-1 urea in combination with ortho-chelate iron produced the highest quantitative and qualitative yield. The same trend was observed in chlorophyll (35 mg/ml) and oil content (1.3%). In this study, a significant positive correlation was observed between chlorophyll content of leaves with vegetative traits and essential oil, which confirmed the positive effect of chlorophyll content on measured traits. Evaluation of quality of the leaves, 20 chemical compositions were detected in essential oils that were more than 85 percent of the total components. The neral and geranial were the predominant compounds in essential oils and the lowest levels of these compounds were observed in control (33.8%) and the highest levels were observed in the combination of 150 kg ha-1 urea with nano iron chelate (44.9%). The essential oils are terpenoid compositions that their constructive units need to NADPH and ATP, thus the presence of nitrogen is essential for the formation of these compounds. Moreover, iron acts as a cofactor for NADPH and ATP activity in nitrogen metabolism. Therefore, the increase in nitrogen and iron absorption by the plant has increased the content of essential oil. Due to the fact that geranial and neral are the most important compounds used in the essential oil of lemon verbena in various industries, especially pharmaceutical industry. The results of this experiment can be used to increase the quantity and quality of lemon verbena.

The current study demonstrated that different levels of urea and various iron sources had a significant effects on all traits. Generally, 150 kg ha-1 urea in combination with ortho-chelate iron produced the highest quantitative and qualitative yield. A significant positive correlation was observed between chlorophyll content of leaves with vegetative traits and essential oil. Twenty chemical compositions were detected in essential oils that the neral and geranial were the most predominant compounds.