3.1. Plant growth parameters
Plant growth parameters of E. purpurea under different culture media and NO3-/NH4+ ratios at the full flowering stage are shown in Table 2 and Figures 1 to 3. Overall, plants grown in the 50% perlite + 50% peat moss medium with perlite particle size less than 0.5 mm and 90:10 NO3-/NH4+ ratio had the highest height (mean 105 cm) (Figure 1), fresh leave weight (mean 30 g/plant), fresh root weight (mean 65 g/plant) (Figure 2), total biomass (mean 96 g/plant), and leaf area (mean 60 cm2) (Figure 3). Decreasing perlite percentage of culture media and perlite particle size improved all the morphological properties (Table 2). There were significant differences in the plant morphological properties at different NO3-/NH4+ ratios. Increasing nitrate (NO3-) proportion in the nitrogen nutrition of E. purpurea caused to increase in plant height and root weight considerably (Table 2).
The pH of growing media in recirculating systems is influenced by the NO3-/NH4+ ratio (Zheng et al., 2006). It found that an increase of NO3-/NH4+ ratio leads to significant increase of E. purpurea phytochemical properties due to improving root growth and root/shoot ratio (Russo et al., 2019).
Based on open hydroponic cultivation system in the present experiment, decreasing perlite particle size, increased the retention time of nutrient solution in the culture media. Increasing nutrient accessibility for plant roots by increasing retention time improves nutrient uptake and plant growth. However, the pure perlite culture system (100% perlite, < 0.5 mm) has a very low air-filled porosity (AFP) of 33% and water holding capacity (WHC) of 56% in comparison with other fine-perlite culture media (Table 3). Accordingly, the lowest growth parameters were obtained in pure perlite medium (Table 2), which can be attributed to the rapid withdrawal of nutrient solution from the culture medium and the inability of the medium to maintain the nutrient solution. Due to the high porosity of peat mass and nutrient solution retention capability, an increase of the plant morphological parameters is expected in the presence of peat moss in various cultural media (Table 2). The noticeable increase in chlorophyll content by reducing perlite particle size implies the significant effect of culture media on photosynthesizing pigments (Table 2). It has been reported that the application of nitrogen fertilizers in the fine perlite culture media increased N content of the plants, thereby increasing their chlorophyll content, subsequently, and their ability to absorb sunlight and produce photosynthates, which resulted in their higher leaf area, and growth and yield (Coelho et al., 2020).
3.2. Essential oil analysis
The flower head, leaves, and root essential oil compositions of E. purpurea grown at the 50% perlite + 50% peat moss medium with perlite particle size less than 0.5 mm growing medium at different NO3-/NH4+ ratios (90:10 and 70:30) are shown in Tables 3 and 4, respectively. The essential oils were separated into 51 components, 38 of them were identified, comprising 92.8% of the total essential oil yield (Tables 3 and 4).
The content and composition of the essential oil exhibited a variable pattern at different plant organs at different NO3-/NH4+ ratios (Tables 4 and 5). The essential oil was characterized by a higher percentage of terpene hydrocarbons, especially the monoterpenoids, which constituted 60 to 70% of the essential oil composition. The major terpene hydrocarbons found are α-pinene, myrcene, β-caryophyllene, 1-Pentadecene, and germacrene D. The percentages of these terpene hydrocarbons were higher in flower head than leave and root at both NO3-/NH4+ ratios. The most abundant terpene found in the essential oil was germacrene D, which showed a remarkable rise from 1.5% in root to 51% in flower head and 0.95% in root to 47% in flower head at 90:10 and 70:30 NO3-/NH4+ ratios, respectively. Variability was also obtained in the concentration of other compositions. The results (Tables 4 and 5) indicate that the various components of the essential oil of E. purpurea are specific to the plant organs, which influence their concentration.
The variations in the concentrations of various essential oil compositions at different NO3-/NH4+ ratios (Tables 4 and 5) may be due to supply different amounts of N-NO3- to the plant. The presence of nitrogen as a key factor can affect the production of essential oils in aromatic plants (Oniszczuk et al., 2019). Nitrogen is critical factor in biosynthesis pathway of essential oil in medicinal and aromatic plants (Baričevič and Zupančič, 2002). Nitrogen increases photosynthetic efficiency and plays an important role in increasing the amount of essential oil by increasing the number and area of leave and providing a suitable condition for receiving sunlight energy and also participating in the structure of chlorophyll and enzymes involved in photosynthetic carbon metabolism (Hosseinpour et al., 2020). Nitrogen is an essential nutrient in plants used to synthesize many organic compounds in plants such as nucleic acids, enzymes, proteins, and amino acids, which are necessary for essential oil biosynthesis pathway (Banica et al., 2020). Besides, essential oils are terpenoids compounds whose constituent units (isonoids) such as isopentenyl pyrophosphate and dimethyl ally pyrophosphate are strongly formed into ATP and NADPH, and due to the effect of nitrogen in the production of these compounds, the amount of essential oil increased (Sitarek et al., 2017). Nitrogen increases the essential oil content of plants by increasing the dry weight (Nyalambisa et al., 2016). Comparing of the results in Tables 4 and 5 indicated that increase of NO3- concentration could increase the percentage of essential oil composition due to its effect on essential oil biosynthesis as demonstrated in previous researches (Diraz et al., 2012).
Germacrene D, myrcene, α-Pinene, β-Caryophyllene, and 1-Pentadecene were the major compositions of essential oil of E.purpurea grown in very fine-grade (< 0.5 mm) perlite with 50:50 v/v perlite to peat moss ratio (Tables 4 and 5). The compositions have a valuable beneficial effects in medicine and agriculture industries (Maggini et al., 2019).
Germacrene D is a natural hydrocarbon, belongs to sesquiterpenes, which is found in aromatic plants (Kaya et al., 2019). The hydrocarbon is a useful bioactive phytochemical compound in human health Maintains healthy blood pressure is one of the important roles of germacrene compounds in humans (Balciunaite et al., 2020). The antimicrobial properties of germacrene D were reported in previous researches (Sharifi‐Rad et al., 2018). Anti-inflammatory, antimicrobial, and antioxidant effects of germacrene D are also well known (Balciunaite et al., 2020). The anti-insect influence of germacrene D has been reported in previous studies (Sharifi‐Rad et al., 2018).
Myrcene is a terpene with anti-inflammatory and anti-depressant effects (Mousavi et al., 2019). Regulating the efficiency of other terpenes and cannabinoids by increasing of myrcene is recognized previously (Maggini et al., 2019).
Pinene has a several of potential benefits, including anti-inflammatory, antimicrobial, antitumor, antioxidant, and neuroprotective effects. It may also help counteract the short-term memory issues that many people experience. Beta-caryophyllene is also known for antioxidant and anti-inflammatory medicinal effects. It is especially useful to decrease pain and anxiety (Holla et al., 2005).
The most abundant terpenes including, germacrene D, myrcene, α-Pinene, β-caryophyllene, and 1-pentadecene were found in chemical composition of E. purpurea extract by previous researchers. Comparing of the results in present study with other researches shows the noticeable increase in essential oil composition by using novel growing media and nutrition pattern (Table 6), which is related to improve physical properties of growing media (50% perlite + 50% peat moss medium with perlite particle size less than 0.5 mm and 90:10 NO3-/NH4+ ratio).