Investigating the effect of different soil textures on morphological characteristics and the amount of essential oil of Lippia citriodora medicinal plant

Introduction
The physical properties of soil are essential because of their role in supporting plant growth. These characteristics determine how the plant interacts with the soil, absorption of water and nutrients, root penetration, soil temperature, and the activity of microorganisms. Among the physical properties of soil, soil texture has a significant impact. On the other hand, with the increase in the population and the pharmaceutical industry's need for medicinal plants as the primary raw material for drug production, attention and research on the Lippia citriodora plant are essential. L. citriodora is effective in treating colds, asthma, colic, fever, diarrhea, dyspepsia, insomnia, and anxiety. L. citriodora tea is relaxing and soothing to the nerves. Its analgesic, anti-inflammatory, and antioxidant effects have been proven. Plants containing essential oil, such as L. citriodora may differ in yield and quality of oil according to the type of soil texture, so in order to achieve the highest yield of essential oil and the best quality, the soil should be adapted to the type and needs of the plant for better growth and development be provided. Therefore, the purpose of this research is to investigate the effects of different soil textures on the growth and development and the amount of essential oil of L. citriodora in greenhouse conditions.
 
Materials and Methods
This research has been carried out in pots in the research greenhouse of Yazd University in the crop year of 2017-2018. The experiment was conducted as a randomized complete block design in three replications for five months. Plastic pots with drains of 18 heights, opening diameter of 17 cm, and drain diameter of one cm were used for growing seedlings. The pots with different soil textures (loamy, sandy loam, sand, and silty clay loam) based on the calculation of the weight percent of soil moisture at the field capacity (FC) were irrigated. The length of the stem and roots were measured with a ruler with mm accuracy. A caliper was used to measure the thickness of the stem. The samples of shoots and roots were kept in an oven at 70 ^°C for 48 hr. A digital scale with an accuracy of 0.001 gr was used to measure the dry weight of the shoots and roots of the plants. In order to measure the root volume after separating and washing the roots, the roots of each replicate were placed inside a graded cylinder with a certain amount of water, and the volume of the root was measured in mm based on the change in the water volume inside the cylinder. To measure the plant leaf surface, five leaves of different sizes were randomly separated from each repetition and different parts of the plant, and using the ADC Bioscienticltd model (AM200) device, the total surface size of each of five leaves was measured in each repetition. The essential oil of L. citriodora was extracted by distillation method with water from a Cloninger machine. In this way, in order to increase the contact surface of the water with the plant in the process of extracting essential oils, first the dry leaves of the L. citriodora, were turned into powder by an electric mill and immediately transferred into a 2000 ml flask, then 500 ml of water was added to it. After boiling in water for three hr, the heat source was cut off and the resulting essential oil was poured into opaque glass containers and stored in a refrigerator at 4 ^°C. To check the statistical effects of each treatment on the study variables, the normality of the data was checked using the Kolmogorov-Smirnov test at the 95% confidence level. One-way analysis of variance (ANOVA) was also used and Duncan's test was applied to compare means. Pearson's correlation method was performed to check the correlation between soil texture and measured traits.
 
Results and Discussion
The results showed the significant effects of soil type on the amount of L. citriodora essential oil (P≤0.001). It has shown significant effects on plant height and root volume (P≤0.01) and on the thickness of the stem, the number of leaves per plant and the dry weight of the leaves (P≤0.05). While on some other traits, such as the number of plant branches, fresh weight of aerial parts, fresh weight of leaves, leaf area, root length fresh and dry weight of roots, no significant difference was observed. The results of Pearson's correlation coefficients showed that there is a positive and significant relationship between different soil textures and measured traits. The highest correlation for root length and leaf area was observed with coefficient of 0.89 and 0.80, respectively (P≤0.01). The results of the mean comparison showed that the highest length of the plant was in loamy texture and the lowest was related to silty clay loam texture, the highest number of leaves and dry matter in the aerial part was reported as well as root volume related to loamy texture. To investigate the effect of soil texture on the percentage of essential oil of L. citriodora showed that the highest percentage of essential oil is in silty clay loam texture and, the lowest percentage is related to sandy soil texture.
 
Conclusion
The results of investigating the effect of different soil textures on the morphological characteristics of L. citriodora medicinal plant showed that the maximum amount of stem thickness, number of leaves per plant, dry leaf weight, plant height, shoot dry weight, and root volume in L. citriodora was in sandy loam soil texture. Moreover, the highest percentage of essential oil is in silty clay loam texture and the lowest percentage is related to sandy soil texture. According to the obtained results, the L. citriodora plant needs a medium amount of nutrients for growth and development. Since the number of leaves produced by the L. citriodora plant in loam and sandy loam soil texture was more than in other textures. Therefore, it can be concluded that the performance of L. citriodora medicinal plant for producing essential oil in each plant is higher in loamy and sandy loam soils.