The Comparison of the Essential Oil Percentage Content and Composition of African Basil (Ocimum gratissimum L.) in Spring and Fall Harvests

Basil genus (Ocimum) contains 30 to 150 species which grown in tropical and subtropical regions of Asia, Africa, Central and South America and found as a wild plant in these areas. In India, around 25,000 ha is under cultivation of Ocimum spp., with an annual production of about 250–300 tonnes of essential oil. Ocimum gratissimum L., a dicotyledonous shrub plant, which belongs to the Lamiaceae family, stands out for the quality, quantity and chemical diversity of the essential oils. These oils have been used in the pharmaceutical, cosmetic and food industries. Some of the essential oil compounds have antibacterial, insecticidal and antioxidant properties with high demand on the international market of the fine perfumery industry. It is also popularly used in herbal medicine for treating several diseases, such as upper respiratory tract infection, fever, cough, diarrhea and pneumonia. Being a short-duration economically viable medicinal and aromatic crop, clove basil has huge potential for large scale cultivation. Plant genetic has an important role in determining the type and amount of secondary metabolites of medicinal plants. Moreover, the recognition of species and genotypes with high genetic capability in the production of desired metabolites has been at the top of the plant breeding plans of medicinal plants. In addition, essential oil composition of plants may be affected by harvest time which is due to the impact of weather conditions on plant growth and development. The present study was aimed to evaluate the oil composition of two genotypes in two harvests.

The research was conducted in the research farm of the college of agriculture, shahid Chamran University, Ahvaz, Iran during 2019. Two valuable genotypes of Ocimum gratissimum L. (278 and 296), with two different essential oil profiles, were investigated in two harvests; spring and autumn seasons. The aboveground parts of the plants were collected on June and November and dried on shade at room temperature. The essential oils of the plants were extracted by water distillation through Clevenger apparatus and the quantity and quality of the essential oils were analyzed by GC and GC-MS.

The results of present study showed that the essential oil content of two genotypes was not affected by the harvest season while its amount was different in two genotypes. The essential oil content of genotype 296 was 2-fold of 278. According to the qualitative analysis of the essential oils, fifty compounds were identified in the essential oils of 278 and 296 genotypes. More than 98% of the identified compounds (in the essential oils of these two genotypes) were classified into five chemical classes; including hydrocarbon and oxygenated monoterpens, and hydrocarbon and oxygenated sesquiterpene and phenylpropanoids. The major constituent of the essential oil of genotype 278 was oxygenated monoterpene, thymol, on June (35.48 %) and November (45.85 %), which was not found in genotype 296. Gamma-terpinene was also significantly increased from June (13.15 %) to November (25.80 %). P-cymene (11.31-3.56 %), alpha- thujone (4.76-2.94 %), Germacrene D (3.73-2.76 %), caryophyllene E (3.66-1.51 %), myrcene (2.93-3.01 %), alpha-terpinene (2.63-3.38 %) and bourneol (2.28-0.71 %) were the remains of oil composition. Dihydro eugenol, which belongs to the chemical class of phenylpropanoids, was identified as the main essential oil components of genotype 296 which its amount was not affected by the harvest time. The other oil constituents were Beta (Z)-Ocimene (11.89-3.40 %), Germacrene D (3.58-2.80 %), and caryophyllene E (0.52-2.68 %).

Terpenoids such as thymol are synthesized via the mevalonic acid pathway, and phenylpropanoid compounds such as dihydroeugenol and eugenol are synthesized via the shikimic acid pathway. The metabolite diversity across different species could be explained by the differential gene expression pattern. According to the results of the present study, thymol was identified as the main oil components of genotype 278. This may be due to the increased expression of mevalonate enzymes. The monoterpene was replaced by phenylpropanoid; dihydrogenugenol, in the oil of genotype 296 which might be due to more expression of the enzymes of the phenylpropanoid pathway. In the other hand, Thymol, P-cymene and gamma-terpinene in genotype 278 varied significantly in different harvesting times, indicating the effect of temperature on the activity of enzymes involved in the synthesis of essential oil components. On the contrary, the amount of dihydrogenugenol in genotype 278 on June and November is not affected by the environmental conditions in two seasons. With regard to the conclusions to the proper growth of genotype 278 and 296, as well several harvests annually, essential oil content and thymol and dihydrogenugenol, therefore, it is suggested that further research should be carried out for developing plant cultivation in Khuzestan and southern provinces which is not suitable for basil growth.