Introduction
The diverse and magnificent plant kingdom of the world is widely known for its medicinal importance. Natural substances obtained from medicinal plants and their plant extracts are used for the treatment of a lot of diseases in human pathology as well as plant pathology. Therefore, it is important to understand the relationship between medicinal plant species and their environment in terms of producing optimal secondary metabolites. Salvia officinalis L., which belongs to Lamiaceae family, as the main herb in the world is cultivated in many countries. Essential oil from S. officinalis aerial parts is used as raw material in pharmaceutical and cosmetics industries. It has been known that different harvest times have a great effect on both quality and quantity of essential metabolites. Furthermore, the drying process, which is one of the post-harvest processes, is very expensive, but if it is done correctly, it significantly increases the efficiency and components of the essential oil after harvesting. In large-scale agriculture, S. officinalis raw materials needs a lot of space and high cost for drying process due to the large volume of material at harvest time. In this case, if it is possible to directly extract the essential oil with the desired quantity and quality from the fresh plant, the production cost will be significantly reduced from the economic point of view. For these reasons, it is necessary to determine optimum harvesting time(s) and plant material kinds affecting essential oil quantity and quality. The purpose of this study was to evaluate the yield and quality of valuable medicinal plant essential oil of S. officinalis in different harvesting times of fresh and dried plants throughout the year in Fars province to determine the yield and quality of essential oil.
 
Materials and Methods
For these purposes, aerial parts of S. officinalis in full flowering stage were collected from selected plants from a 3-year farm located in Saadatshahr city of Fars province. A factorial experiment based on a completely randomized blocks design with three replications and two factors include three different harvesting times (5^th of May, 26^th of July and 5^th of November, 2020) and two types of plant materials (fresh and dry) was performed in the farm. The essential oils of different S. officinalis samples were extracted by hydro-distillation using Clevenger apparatus and with three replications. The yields were calculated based on dry weight and the oils were analyzed by a combination of GC-FID and GC-MS techniques, to check for chemical variability based on British Pharmacopoeia. At the end of the experiment, data analysis was performed using SAS software (version: 9.4). The means were compared by Duncan's multi-range test at a statistical level of 1%.
 
Results and Discussion
The results of the current study revealed that the essential oil content of S. officinalis aerial parts harvested at different times and plant materials ranged from 0.19% to 1.18% (w/w). It was found that the interaction effects of harvesting times and plant materials on the essential oil yield was significant at the 1% probability level, and the comparison of the average interaction effects of harvesting times and plant materials showed that the highest essential oil yield of S. officinalis was related to the second harvesting time (26^th of July, 2020) and the dry plant material (1.18 % w/w). Meanwhile, the lowest yield of essential oil (0.19% w/w) was obtained in the first harvesting time (5^th of May, 2020) and in fresh plant material. 34 different constituents have been identified in which 22 compounds were common. Monoterpene hydrocarbons were the main group of compounds in all studied plant samples. The key compounds of essential oil were 1,8-cineol, α-thujone, β-pinene and camphor. The comparison of the average interaction effects of harvesting times and plant materials on the main compounds of the essential oil demonstrated that the interaction effects on α-thujone and camphor constituents were significant at the 1% probability level and for β-pinene were were significant at the 5% probability level. The results of this research showed that the yield of essential oil in dry plant materials was higher than in fresh plant materials and different harvesting times had a significant effect on the quantitative and qualitative performance of essential oil.
 
Conclusion
Eventually, the results obtained from this study showed that the quantity and quality of essential oil in S. officinalis were influenced by the different harvesting times and plant materials. In order to achieve the maximum quantity and quality of essential oil in this species, the best type of plant materials was dry plant material and the best harvesting times was the summer harvesting in the full flowering stage. Due to the fact that the performance of essential oil at different harvesting times fluctuates in terms of quantity and quality during the growing season, it is necessary to have the essential oil certificated from each harvest in order to successfully enter the market.