**3. Results**

#### *3.1. Essential Oil Content*

The results demonstrated that the EO content underwent profound changes that seemed to strongly depend on the collection time (*p* < 0.01). EOs of the fresh aerial parts showed two intermittent ascendant trends during the year. The plants collected in spring and autumn contained a higher amount of EOs than the other seasons (Figure 1). The oil content of these samples began to increase from January and reached the highest amount (0.22%) at the beginning of the heat period (March). After that, the accumulation of EO gradually decreased from April, and the lowest amount was obtained in July (0.09%). There was a significant difference between the oil content of the plants collected from August to December. Similarly, the accumulation of oils increased again with a slight slope in August and September and showed the second peak in October (0.18%). Then, the percentage of EO declined from November and eventually reached its lowest level in December (0.10%). The dried aerial parts of plants contained more EOs than fresh ones varying in the range of 0.21 to 0.45%. Moreover, the essential oil content of the dried aerial parts significantly changed during the different seasons (*p* < 0.01) and showed a pattern similar to the one reported for the fresh aerial parts (Figure 1). The maximum amount of oils was detected in March (0.45%), followed by October and November (0.39%) while the minimum was recorded in August (0.21%).

**Figure 1.** The seasonal variation of the essential oil content extracted from fresh and dried aerial parts of Arabian lilac. Values are the mean ± standard deviation (SD) of three replications (*n* = 3).

#### *3.2. Essential Oil Composition*

The EO composition of the fresh and dried aerial parts of Arabian lilac, along with their relative percentages and chemical classes in different collection periods are listed in Table 2. According to the GC-MS analysis, forty-one components were identified during different months, representing 97.02 to 98.92% and 97.03 to 99.96% of the fresh and dry oils, respectively. The major components of oils were classified into five groups; monoterpene hydrocarbons ( α-pinene and sabinene), sesquiterpene hydrocarbons (β-caryophyllene and laurenene), oxygenated sesquiterpenes (caryophyllene oxide and (5E,9Z)-farnesyl-acetone), diterpene hydrocarbons (phytane and abietadiene) and oxygenated diterpenes (phytol, manool oxide, (6Z,10Z)-Pseudo phytol, manool, and 7 α-hydroxy-manool).

The predominant compounds of the fresh and dried plant's oils were β-caryophyllene (22.60–35.03% and 25.14–32.43%, respectively), sabinene (7.24–16.73% and 11.04–18.38%, respectively), caryophyllene oxide (3.78–6.38% and 4.48–7.29%, respectively), (6Z,10Z)-Pseudo phytol (0.00–15.02% and 0.00–14.12%, respectively), laurenene (3.29–7.32% and 2.80–6.90%, respectively), (5E,9Z)-farnesyl-acetone (3.03–4.76% and 3.22–5.01%, respectively).

Sabinene, caryophyllene oxide, (6Z,10Z)-Pseudo phytol, (5E,9Z)-farnesyl-aceton, phytol, and α-pinene in fresh oils were less than in dried ones. In contrast, the fresh oils contained higher amounts of the β-caryophyllene, laurenene, phytane, manool oxide, and abietadiene compared with dried aerial parts. The most abundant chemical groups of the total fresh and dried sample's oils were sesquiterpenes, diterpenes, and monoterpenes. However, the fresh oils were richer in sesquiterpene and diterpene hydrocarbons, and the dry oils contained higher amounts of the oxygenated types of diterpenes and sesquiterpenes.

The data presented in Table 2 clearly shows a high variability in the percentages of the EO composition throughout the year. The biosynthesis of β-caryophyllene, as a major constituent of Arabian lilac's oil, markedly varied in different months and its annual fluctuation trend was the same for both fresh and dried oils. β-Caryophyllene in fresh oils had an ascendant trend from January, and the highest level was recorded in March (35.03%). Then, there was a clear decline during April and May. Moreover, the lowest level was obtained in June (22.60%). The amount of β-caryophyllene of dry oils increased continuously from February and declined again after rising in September. Furthermore, the maximum and minimum percentage of β-caryophyllene in dry oils were recorded in September (32.43%) and February (25.14%), respectively.

Sabinene, the second main component of Arabian lilac's oil, was highly increased in spring and autumn in fresh and dried material's oils. In contrary, there was a grea<sup>t</sup> decrease in its amount when oils were harvested during winter and summer. The highest content of sabinene in fresh and dried samples was found in March (16.73%) and October (18.38%), respectively. Additionally, the lowest amounts were obtained in July (7.24%) and August oils (11.04%), respectively (Table 2). The variation pattern of laurenene, manool oxide, manool, α-pinene, and abietadiene of fresh and dried aerial parts' oils was similar to that exhibited for β-caryophyllene and sabinene.

A quite different trend in the amount of caryophyllene oxide in fresh and dried sample's oils from other components was found. The amount of this constituent in the fresh plant's oils of summer and winter seasons was higher than in spring and autumn seasons and the maximum content was obtained in January (6.38%). The maximum of caryophyllene oxide of dried sample oils was observed in June (7.29%) whereas the least amount was obtained when plants were collected in September (4.48%). Similarly, the variations in the amount of (5E,9Z)-farnesyl-aceton, (6Z,10Z)-Pseudo phytol, phytol, and <sup>7</sup>α-hydroxy-manool throughout the year was the same as those of caryophyllene oxide changes.





#### *Foods* **2019**, *8*, 52

#### *3.3. Total Phenolic Content*

The total phenolic content (TPC) (*p* < 0.01) significantly varied in different months of the year ranging from 9.48 to 13.69 mg GAE/g of dry weight (DW) (Figure 2). The TPC gradually increased in December and reached its maximum amount in January (13.69 mg GAE/g DW). Then, a slight drop in TPC occurred in February (12.49 mg GAE/g DW) and March (11.19 mg GAE/g DW). Although, there was no significant difference among TPC of March, April, and May extracts, it increased slightly in spring, and the highest content was obtained in early summer (13.52 mg GAE/g DW). TPC dramatically declined from to September (9.48 mg GAE/g DW). The samples collected in autumn contained a lower value of TPC than those collected in other seasons. However, the least amount was recorded in September (9.48 mg GAE/g DW).

**Figure 2.** The seasonal variation of total phenol and flavonoid contents of Arabian lilac. Values are the mean ± SD of three replications (*n* = 3). DW: dry weight; GAE: gallic acid equivalents.

#### *3.4. Total Flavonoid Content*

The quantitative analysis of total flavonoid content (TFC) showed a significant difference in the diverse seasons (*p* < 0.01). As shown in Figure 2, there was a high level of TFC when the plants were harvested in the winter and summer, while its amount was low during the spring and autumn seasons. The maximum amount of TFC was found in December (11.31 mg QUE/g DW) and remained constant in January and, then, clearly decreased from the end of the winter season (February) and finally reached the minimum level in the middle of the spring (March) (9.40 mg QUE/g DW). Afterwards, a considerable rise occurred in June (11 mg QUE/g DW) which followed on July (10.92 mg QUE/g DW). However, this rate remained unchanged, and it began to increase again from September to December after a sharp drop in August (9.64 mg QUE/g DW).

#### *3.5. Total Flavone and Flavanol Contents*

The seasonal variation had marked impact on the amount of total flavone and flavanol contents (TFFC) (*p* < 0.01). The lowest concentration of TFFC was recorded in January (2.10 mg QUE/g DW). The amount of the TFFC was continuously increased from February, and the maximum concentration appeared in July (2.32 mg QUE/g DW). Thereafter, an opposite trend was observed and markedly declined from August to December (2.29–2.16 mg QUE/g DW) (Figure 3).

#### *3.6. Total Anthocyanin Content*

Total anthocyanin content (TAC) significantly differed (*p* < 0.01) from one month to another (1.26–2.81 mg C3G/100 mL). The pattern of seasonal variation of the TAC was interestingly quite reverse with respect to the TFFC pattern. The plants collected in winter contained higher levels of TAC compared with other seasons, and the maximum amount was observed in January (2.81 mg C3G/100 mL). Conversely, the anthocyanins were in lower concentrations during spring and especially summer. As shown in Figure 3, it declined from February to August with relatively sharp slope and the least level was obtained when the plants were harvested in August (1.26 mg C3G/100 mL), which was 2-fold less than that obtained in January. Then, the TAC increased during autumn from September to November.

**Figure 3.** The seasonal variation of total flavone and flavanol, and anthocyanin content of Arabian lilac. Values are the mean ± SD of three replications (*n* = 3). QUE: quercetin equivalents.

#### *3.7. Total Antioxidant Activity*

Although the extracts wholly exhibited an effective reducing power of the radical species target DPPH•, their scavenging effect was strongly dependent on the time of collection (*p* < 0.01). The strongest antiradical potential was detected in January (91.02%) which was not constant, and it significantly decreased from February, and the extracts collected in March showed lower total antioxidant activity (TAA) (86.90%). The TAA of the extracts began to increase during spring and summer and peaked in July (90.15%). After that, a significant decrease trend was observed in the ability to scavenge free radicals of the extracts harvested in August and September. The TAA increased again from October to January (Figure 4). Eventually, the results obtained from the evaluation of DPPH• reducing power indicated that the extracts collected in winter and summer seasons were more effective than the ones harvested in spring and autumn.

**Figure 4.** The seasonal variation of antioxidant activity of Arabian lilac. Values are the mean ± SD of three replications (*n* = 3).
