The genus Pelargonium is cultivated around the world for the production of essential oils and absolutes. Pelargonium graveolens L’Hér is an aromatic and hairy shrub from which oil is obtained from its leaves, flowers, and stems. Various factors such as cultivar, oil distillation method, distilled part of the plant, age of the material, growing location and seasonal changes in the region, as well as harvest season and time, affected the final essential oil composition of rose-scented geranium (Verma et al., 2013; Szutt et al., 2019). The main constituents of the essential oils of P. graveolens were reported as geraniol (14.1-34.6%), citronellol (15.2-31.3%), linalool (2.9-9.2%), citronellyl formate (4.4-9.2%), isomenthone (4.5-6.6%), 10-epi-γ-eudesmol (4.7-6.7%) and geranyl formate (3.8-6.2%) by Verma et al. (2013) citronellol (20.9-39.5%), geraniol (10.9-26.5%), linalool (2.9-14.2%), isomenthone (7.4-9.4%), citronellyl formate (5.5-9.1%) and 10-epi-γ-eudesmol (5.2-9.0%) by Singh et al. (2018); citronellol (22.3%), geraniol (15.5%), geranyl acetate (13.1%), limonene (9.3%), phenyl ethyl alcohol (5.9%) and linalool (5.6%) by Szutt et al. (2018); citronellol (27.0%), geraniol (20.7%), 10-epi-γ-eudesmol (13.1%), citronellyl formate (6.4%) and linalool (5.7%) by Ben ElHadj et al. (2020). According to ISO 4371-2012, P. graveolens essential oil from different geographical origins should have citronellol (18-43%), geraniol (5-20%), linalool (2--11%), citronellyl formate (4-12%), isomenthone (4-10%), geranyl formate (1-8%), (Z)-rose oxide (0.4-3.5%), menthone (0.0-2.5%) and geranyl tiglate (0.7-2.0%) as the main components (ISO, 2012). Considered one of the top 20 oils in the world, the essential oil of rose-scented geranium was extensively used as a flavoring agent in the food, soaps and beverages industry, cosmetic, perfumery, aromatherapy, traditional medicine, and pharmaceutical industries. Rose-scented geranium is famous for its strong rose-like pleasant fragrance; it is cultivated due to its high-value essential oil used in herbal medicine and aromatherapy and the production of high-quality perfumes and cosmetics. Also, P. graveolens essential oils or/and extracts are well known for their sensory attributes and pharmacological properties, antioxidant, antibacterial, antifungal, antimicrobial, insecticidal, allelopathic, anti-aflatoxin, anti-urease, anti-tyrosinase, therapeutic, repellent, fumigant and photoprotective effects (Lohani et al., 2019; Ben ElHadj et al., 2020; Kujur et al., 2020). Different drying methods have been developed for the quality product and high-quantity products, and it has been observed that the essential oil content and components depend on the drying conditions, drying method, and plant species (Özgüven et al., 2019). A previous report showed that drying methods and temperature had a significant effect on quality indicators such as organoleptic and sensory properties, oil content, and composition in medicinal and aromatic plants such as Laurus nobilis (Sekeroglu et al., 2007), Mentha longifolia (Saeidi et al., 2016), Mentha pulegium (Ahmed et al., 2018), Ocimum americanum (Bhatt et al., 2018), Thymus daenensis (Mashkani et al.,2018), Lippia citriodora (Aghdam et al., 2019), Lavandula angustifolia (Sałata et al., 2020) and Dracocephalum moldavica (Morshedloo et al., 2021). In most cases, it has been reported for many plants that increasing drying temperature lowers the essential oil content, while keeping the temperature below 30-35 °C preserves more aromatic compounds (Mashkani et al., 2018; Sałata et al., 2020). Drying techniques affect the essential oil yield and composition, so it is very important to determine an appropriate drying method to achieve higher active substances in medicinal and aromatic plants. The drying methods may differ from one aromatic herb and spice to another. For medicinal and aromatic plants which are sensitive to the drying process, optimum drying is required to obtain a high-quality product, as some bioactive compounds change during the drying process. Therefore, the optimization of quality requires studying each specific pre-drying and drying method for each type of herb (Thamkaew et al., 2021). Although a large number of herb-drying studies have been conducted in recent years, as far as we know, studies on the effectiveness of different drying methods on the quantity, quality, and composition of the essential oil of rose-scented geranium are scarce. The present study aimed to determine the influence of different drying methods, which included fresh plants (control), sun-drying, shade-drying, oven-drying at 30 °C, and oven-drying at 60 °C on dry herbage yield and essential oil content and composition of rose-scented geranium.

The results showed that the drying methods had a significant effect on EO content (Figure 1). Fresh plant and samples dried by shade-drying and by oven at 30 °C showed high EO contents (1.98, 1.34, and 1.20%, respectively). In contrast, low essential oil content of 0.70 and 0.42% was obtained from sun-drying and oven-drying at 60 °C. The methods fresh sample, shade-drying, and oven-drying at 30 °C resulted in higher EO, while increasing temperature (from 30 to 60 °C) showed a decrease in EO content. Similarly, Çalışkan et al. (2017) in Mentha piperita, found higher essential oil content in shade-drying and oven-drying at 38 °C than sun-drying. Although not as much as the fresh plant material, both shade-drying at ambient temperature and oven-drying at 30 °C gave feasible results in terms of oil yield. The changes in essential oil content during the drying process depended on temperature, time, and drying method. Similarly, Sourestani et al. (2014) in Agastache foeniculum, found higher essential oil content at room temperature (25 °C) than oven-drying at 40 °C. Shade-drying and oven-drying (at 30 °C) methods are considered efficient to achieve the best EO quality and quantity (Saeidi et al., 2016). Some studies pointed out that increased drying temperature can damage glandular trichomes, decomposition of some essential oil components through high-temperature autoxidation and hydro peroxidation (Turek and Stintzing, 2006), and accelerated evaporation and decomposition of essential oil components (Mashkani et al., 2018) can cause a decrease in essential oil content.

Ninety-three components were identified in EO samples of rose-scented geranium by using different drying methods comprising 96.63 to 99.11% of total EO (Table 1). The majority of them consisted of oxygenated monoterpenes and sesquiterpene hydrocarbons. The main components in the EO in all drying methods were citronellol (24.0-39.9%), geraniol (4.2-17.1%), menthone (4.5-8.3%), and linalool (2.0-7.4%). Other main components in the oil were found to be β-caryophyllene, cis-rose oxide, geranyl tiglate, citronellyl butyrate, viridiflorene, 2-phenylethyl tiglate, β-bourbonene, δ-cadinene and nerol. These results are consistent with previous reports (Juliani et al., 2006), which demonstrated that commercial geranium oils are characterized by high citronellol (19-45%) and lower amounts of geraniol (less than 24%) and linalool (less than 14%) as the main components.

Citronellol and geraniol, which are oxygenated monoterpene, reached their highest values under natural shade-drying conditions; whereas the lowest value was achieved by artificial oven-drying at 60 °C. Since high temperatures cause a large loss in citronellol, geraniol, and linalool contents, these should be considered to be compounds which are sensitive to direct sunlight, high and low temperatures. The results demonstrated that the aerial parts of geranium with sun-drying, oven-drying at 30 °C, and oven-drying at 60 °C presented decreased citronellol content by about 12.03, 5.20, and 15.88% as compared to the shade-drying, with geraniol content by 5.91, 3.04, and 12.94% and linalool content by 4.54, 3.64, and 5.46%, respectively. Oxygenated monoterpenes with sweet rose-like (citronellol) and flowery rose-like (geraniol) odor in geranium oil are important reasons for the demand for perfumery.

The highest percentage of δ-cadinene, citronellyl butyrate, geranyl tiglate and 2-phenylethyl tiglate were obtained from samples dried in an oven 60 °C;, while the highest amount of linalool and β-caryophyllene were obtained from the sample dried by shade ambient temperature and the fresh sample. Increasing the drying temperature from 30 to 60 °C significantly reduced the contents in citronellol, geraniol, β-caryophyllene, menthone, and linalool in the dried aerial parts of geranium; whereas the contents in citronellyl butyrate, geranyl tiglate, 2-phenylethyl tiglate, δ-cadinene and spathulenol increased. Citronellyl formate, β-copaene, and β-selinene were detected only in oven-dried samples at 60 °C. The sun-drying method had a stimulative effect on some other compounds’ biosynthesis and accumulation such as α-pinene, cis-rose oxide, menthone, δ-cadinene, γ-cadinene, and caryophyllene oxide. The drying method affected the geranium’s chemical profiles and caused significant changes in the contents in citronellol, geraniol, linalool, menthone, and β-caryophyllene, which are the main compounds in the EO.

Factors such as plant species, drying method, drying conditions and time, amount of water evaporated during drying, temperature, the chemical structure of the compounds, oxidation, chemical reactions, degradation, isomerization, cyclization, dehydrogenation, glycoside hydrolysis, autoxidation of terpenoids, esterification and/or other processes could significantly change the chemical profiles of EO and some of the EO compounds may be lost, reduced and/or increased (Ahmed et al., 2018; Bhatt et al., 2018; Beigi et al., 2018; Özgüven et al., 2019; Thamkaew et al., 2021). One of the most important chemical changes is due to the autoxidation of oil components that affect the deterioration process of terpenoids, and increasing drying temperature and exposure to direct sunlight causes further loss in aroma components and degradation of aroma quality (Başer and Demirci, 2011; Thamkaew et al., 2021). Compared to shade drying, especially in an oven at 60 °C and sun drying, the volatile profile of the EO changed due to the formation of secondary aroma compounds such as terpene esters, sesquiterpenes, alcohols, aldehydes, and others. During drying, the EO composition and content increased, decreased and the production of new compounds occurred. During the drying process, the EO compositions of the plants changed, which may be a result of the release of components from the rupture of their cell walls, oxidation reactions, or hydrolysis of glycosylated volatile compounds (Xing et al., 2018).

Among the compounds identified in EO, the percentage of oxygenated monoterpenes (OM) was the highest, ranging from 38.9 to 73.2%. EO extracted from plants dried at 30 and 60 °C and sun-dried contained more sesquiterpenes and fewer monoterpenes compared to fresh samples and drying in shady, natural conditions. Sun and oven drying most reduced the contents in compounds from the OM groups, while fresh and drying in shady, natural conditions reduced the contents in compounds from the SH and OS groups. Drying of Pelargonium graveolens in the shade was most suitable for a high percentage of OM. Aghdam et al. (2019), who found high monoterpene content in fresh lemon verbena plants and sesquiterpene content in oven drying, presented similar conclusions.

Drying in direct sun resulted in a reduction in the contents in EO, citronellol, geraniol, linalool, and β-caryophyllene compared to fresh samples, shade-drying, and oven-drying at 30 °C, so it may not be an appropriate drying method for geranium. The shade-drying method preserved the EO content and the major volatile components in geranium better than the oven-drying and sun-drying methods. However, although the drying time of shade-drying is longer than sun-drying, it can provide advantages in terms of preserving light-sensitive substances and minimizing light-dependent chemical reactions such as oxidation. In terms of EO content and components, it was found that drying with hot air at 60 °C is not appropriate, but low drying temperature (30 °C) is appropriate for oil content and preservation of heat-sensitive compounds in geranium.

The PCA-biplot created to visually evaluate the changes in terpene classes according to drying methods is shown in Figure 2. The PCA biplot in this study captured 97.7% of the variations due to drying method and drying method by terpene group interactions. Polygons were created to evaluate drying methods in the biplot, and drying methods are presented with vectors. Terpenes, which had a positive relationship with drying methods, were located close to each other. In the three sections on the biplot, drying methods and terpene classes, which are positively related, formed three groups. Oxygenated sesquiterpenes (OS), sesquiterpenes (S), sesquiterpenes hydrocarbons (SH) and others (O) increased with oven-drying at 60 ^oC, monoterpenes (M) and oxygenated monoterpenes (OM) increased with sun-drying, monoterpenes hydrocarbons (MH) increased with shade-drying, fresh plant and oven-drying at 30 °C methods (Figure 2).

The quality of medicinal dried herbs is defined by the content in bioactive compounds. The drying methods had a significant impact on the essential oil content and composition, which is the quality indicator of Pelargonium graveolens L’Hér. While high temperature reduced the rate of EO in oven-drying, fresh plant and shade-drying were found to be more appropriate in terms of oil content and components compared to the other methods. A significant difference was noted in the percentage of main constituents such as citronellol, geraniol, linalool, β-caryophyllene geranyl acetate, geranyl tiglate, citronellyl butyrate and viridiflorene between the different natural and artificial drying methods, as well as between them and fresh samples. While the highest oxygen monoterpene contents were identified in shade-dried and fresh plants, oven-drying had a stimulating effect on the biosynthesis and accumulation of sesquiterpene compounds. The data can be used by pharmaceutical and perfumery industries in their post-harvesting programs. The results of this study showed that drying Pelargonium graveolens in natural shade is more suitable for high oil yield and oxygenated monoterpene content while drying this plant in the oven at 30 °C can be recommended to shorten the drying process.