Essential Oil Enriched with Oxygenated Constituents from Invasive Plant Argemone ochroleuca Exhibited Potent Phytotoxic Effects

Invasive species are considered as one of the major threats to ecosystems worldwide. Although invasive plants are regarded as a foe, they could be considered as natural resources for valuable bioactive compounds. The present study aimed to characterize the chemical composition of the essential oil (EO) from the invasive plant Argemone ochroleuca Sweet, collected from Saudi Arabia, as well as to evaluate its phytotoxic activity. Seventy-four compounds were characterized via GC-MS analysis of EO representing 98.75% of the overall mass. The oxygenated constituents (79.01%) were found as the main constituents, including mono- (43.27%), sesqui- (17.67%), and di-terpenes (0.53%), as well as hydrocarbons (16.81%) and carotenoids (0.73%). Additionally, 19.69% from the overall mass was characterized as non-oxygenated compounds with mono- (1.77%), sesquiterpenes (17.41%), and hydrocarbons (0.56%) as minors. From all identified constituents, trans-chrysanthenyl acetate (25.71%), γ-cadinene (11.70%), oleic acid, methyl ester (7.37%), terpinene-4-ol (4.77%), dihydromyrcenol (2.90%), α-muurolene (1.77%), and γ-himachalene (1.56%) were found as abundant. The EO of A. ochroleuca showed significant phytotoxic activity against the test plant Lactuca sativa and the noxious weed Peganum harmala. The EO attained IC50 values of 92.1, 128.6, and 131.6 µL L−1 for seedling root growth, germination, and shoot growth of L. sativa, respectively, while it had IC50 values of 134.8, 145.7, and 147.9 µL L−1, respectively, for P. harmala. Therefore, this EO could be used as a bioherbicide against weeds, while further study is recommended for the characterization of the authentic materials of the main compounds in the EO as well as for the evaluation of potency of this oil on a field scale and the determination of its biosafety.


Introduction
Plants are promising natural resources for essential oils (EOs) with a complex mixture of secondary metabolites, including mono-, sesqui-, and di-terpenoids, in addition to hydrocarbons [1,2].

EO Chemical Profile
The hydrodistillation extraction of above-ground parts of A. ochroleuca provided 0.031 ± 0.001% (v/w) of a colorless oil. The GC-MS analysis of EO was performed, and the chromatogram, including representation of the main constituents over main peaks, is shown in Figure 1. The chemical profile of this plant was described here for the first time. The full chemical profile is presented in Table 1, which is composed of 70 compounds, representing 98.75% of the total mass. Among the overall identified mass, terpenes were found as the main compounds with a concentration of 80.65%, including mono-, sesqui-, and diterpenes.
Diterpenes were rarely characterized in the EOs derived from the plant kingdom [37]. Phytol, a common oxygenated diterpenoid in EOs of plant species, was the only identified compound in EO of A. ochroleuca with a low concentration (0.53%). The identification of this compound in the EO of this plant was in agreement with that reported in A. mexicana [38].
The biosynthetic pathways of the identified compounds might describe the relationship between all the constituents due to the similar starting and/or intermediate compounds as well as the pathway itself. The plausible biosynthetic pathways of the two major compounds, γ-cadinene, and patchouli alcohol, as examples, were united at the start with intermediate-compound, farnesyl pyrophosphate that biosynthetically transformed to germacryl cation [40,41] as described in Figure 4. From this brief overview, we can detect the theory that the biosynthetic of the constituents of EOs might establish the relationships between these constituents during the growth of the plant. medicinal compound with various activities such as anti-influenza [33], anti-tumor [34], and antiinflammatory [35]. Rifai and Soekamto [36] described the purification and abundance of patchouli alcohol in EO derived from Pogostemon cablin. Diterpenes were rarely characterized in the EOs derived from the plant kingdom [37]. Phytol, a common oxygenated diterpenoid in EOs of plant species, was the only identified compound in EO of A. ochroleuca with a low concentration (0.53%). The identification of this compound in the EO of this plant was in agreement with that reported in A. mexicana [38].
The biosynthetic pathways of the identified compounds might describe the relationship between all the constituents due to the similar starting and/or intermediate compounds as well as the pathway itself. The plausible biosynthetic pathways of the two major compounds, γ-cadinene, and patchouli alcohol, as examples, were united at the start with intermediate-compound, farnesyl pyrophosphate that biosynthetically transformed to germacryl cation [40,41] as described in Figure 4. From this brief overview, we can detect the theory that the biosynthetic of the constituents of EOs might establish the relationships between these constituents during the growth of the plant.

Phytotoxic Activity of the A. ochroleuca Essential Oil
The EO from the above-ground parts of A. ochroleuca showed significant phytotoxic activity on seed germination and seedling development of L. sativa ( Figure 5a) and P. harmala (Figure 6a).
At the highest concentration of the EO (250 µL L −1 ), the seed germination, shoot growth, and root growth of the L. sativa seedling were reduced by 88.6, 86.9, and 97.0%, respectively (Figure 5a).

Phytotoxic Activity of the A. ochroleuca Essential Oil
The EO from the above-ground parts of A. ochroleuca showed significant phytotoxic activity on seed germination and seedling development of L. sativa ( Figure 5a) and P. harmala (Figure 6a). Plants 2020, 9, x 7 of 13 According to IC50 values, the root was the most inhibited with an IC50 value of 92.1 µL L −1 (Figure 5b), followed by germination (128.6 µL L −1 ) and finally the shoot (131.6 µL L −1 ). On the other hand, the EO of A. ochroleuca at the lowest concentrations (50, 100, and 150 µL L −1 ) revealed significant phytotoxic activity against the germination, shoot growth, and root growth of the noxious weed P. harmala, while at the highest concentrations (200 and 250 µL L −1 ) it revealed a sharp increase in the phytotoxicity ( Figure 6). Based on the IC50, the root of P. harmala showed the lowest value (134.8 µL L −1 ), while the seed germination and shoot growth attained IC50 values of 145.7 and 147.9 µL L −1 , respectively ( Figure  6).   According to IC50 values, the root was the most inhibited with an IC50 value of 92.1 µL L −1 (Figure 5b), followed by germination (128.6 µL L −1 ) and finally the shoot (131.6 µL L −1 ). On the other hand, the EO of A. ochroleuca at the lowest concentrations (50, 100, and 150 µL L −1 ) revealed significant phytotoxic activity against the germination, shoot growth, and root growth of the noxious weed P. harmala, while at the highest concentrations (200 and 250 µL L −1 ) it revealed a sharp increase in the phytotoxicity ( Figure 6). Based on the IC50, the root of P. harmala showed the lowest value (134.8 µL L −1 ), while the seed germination and shoot growth attained IC50 values of 145.7 and 147.9 µL L −1 , respectively ( Figure  6).   At the highest concentration of the EO (250 µL L −1 ), the seed germination, shoot growth, and root growth of the L. sativa seedling were reduced by 88.6, 86.9, and 97.0%, respectively (Figure 5a). According to IC 50 values, the root was the most inhibited with an IC 50 value of 92.1 µL L −1 (Figure 5b), followed by germination (128.6 µL L −1 ) and finally the shoot (131.6 µL L −1 ). On the other hand, the EO of A. ochroleuca at the lowest concentrations (50, 100, and 150 µL L −1 ) revealed significant phytotoxic activity against the germination, shoot growth, and root growth of the noxious weed P. harmala, while at the highest concentrations (200 and 250 µL L −1 ) it revealed a sharp increase in the phytotoxicity ( Figure 6). Based on the IC 50 , the root of P. harmala showed the lowest value (134.8 µL L −1 ), while the seed germination and shoot growth attained IC 50 values of 145.7 and 147.9 µL L −1 , respectively ( Figure 6).
It is clear that the roots of both L. sativa and P. harmala seedling were more affected with the EO than shoots, and this could be attributed to the direct contact with the EO as well as the permeability of the root membrane [24,42]. In the present study, the potent phytotoxic activity of the A. ochroleuca EO could be ascribed to the presence of high oxygenated compounds, particularly the major compounds such as trans-chrysanthenyl acetate, γ-cadinene, oleic acid-methyl ester, patchouli alcohol, and terpinene-4-ol. The oxygenated compounds of the EOs were reported to have stronger biological activities than non-oxygenated ones [1,2,43,44]. The bicyclic sesquiterpene γ-cadinene was reported to have larvicidal activity against malaria, dengue, and filariasis mosquitoes [45], and it is also reported to have antimicrobial activity [46]. In addition, the EO of Annona salzmannii showed potent trypanocidal and antitumor activities due to its high content of γ-cadinene [47]. The fatty acid methyl esters have been reported to possess larvicidal activity [48] and antibacterial activity [49].
In addition, the EO rich in trans-chrysanthenyl acetate has been reported to have antimicrobial and antioxidant activities [54,55].
The major compound γ-cadinene has been reported as major constituent (18.4%) in the essential oil of Eupatorium adenophorum, which showed a phytotoxic activity against Phalaris minor and Triticum aestivum [56]. Additionally, the EO of Schinus lentiscifolius showed a phytotoxic effect on lettuce due to the presence of high content of γ-cadinene [57]. Although the other major compounds have not been reported as allelochemicals, they could participate in the phytotoxic effect of the A. ochroleuca EO, particularly the patchouli alcohol, and terpinene-4-ol, which are oxygenated terpenes [58].
Several modes of action of EOs as allelochemicals were reported, including the inhibition of permeability, cell division, photosynthesis, respiration, enzyme activities, and genomic materials [59,60]. However, the specific mode(s) of action of the major identified terpene compounds in the present study, either alone or in combinations, needs further investigation.
It is worth mentioning that the weed P. harmala is considered a noxious weed in several countries, including Saudi Arabia, where it is widely distributed in the northern regions [61]. It is hard to control and needs powerful herbicides or manual uprooting; no reported biological control methods for the P. harmala is available [62]. In this context, the present study revealed the potentiality of A. ochroleuca EO to control P. harmala, this noxious weed, as an eco-friendly bioherbicide, where this oil showed strong phytotoxicity against this weed.

Plant Materials Collection, Identification, and Preparation
The above-ground parts of A. ochroleuca were collected from a roadside habitat, Al Assir village, Taif, western Saudi Arabia (21 • 11 27.2" N 40 • 40 05.9" E). The plant specimen was identified according to Chaudhary [63] by Dr. Abdulaziz Assaeed, Professor of Range Ecology, Department of Plant Production, College of Sciences, King Saud University, Saudi Arabia. A voucher specimen of the collected plant is released in the herbarium of King Saud University, with code: KSU-0160115001. The above-ground parts of the healthy plants were collected in paper bags and transferred to the laboratory. The plant materials were dried in shade at room temperature (28 ± 3 • C) for two weeks (until complete dryness), ground into a fine powder, and packed in a paper bag.

Essential Oil Extraction, GC-MS Analysis, and Constituents' Identification
The EOs were extracted by hydrodistillation from two samples of A. ochroleuca above-ground parts via a Clevenger-type apparatus for three hours. The oil layer was collected, and water was removed by 0.5 g of anhydrous Na 2 SO 4 ; they were stored in a dark glass vial at 4 • C till further analysis. The yields of the extracted EOs were calculated via the equation 100× (V/W), where V: volume of extracted EO, and W: weight of the plant material used in extraction. The chemical composition of the EO samples was analyzed and identified separately by gas chromatography-mass spectrometry (GC-MS) as described in our previously documented work [2,4].
In brief, GC-MS analysis was carried out at the Department of Medicinal and Aromatic Plants Research, National Research Center, Giza, Egypt, using the GC-MS instrument which has TRACE GC Ultra Gas Chromatographs (THERMO Scientific™. Corporate, Waltham, MA, USA) and Thermo Scientific ISQ™ EC single quadrupole mass spectrometer. The GC-MS system is equipped with a TR-5 MS column with dimensions of 30 m × 0.32 mm i.d., 0.25 µm film thickness. At flow rate of 1.0 mL min −1 , helium was used as carrier gas with split ratio of 1:10. The temperature program was 60 • C for 1 min, rising by 4.0 • C min −1 to 240 • C and held for 1 min. A diluted sample in hexane (1 µL) at a ratio of 1:10 (v/v) was injected, and the injector and detector were held at 210 • C. Mass spectra were recorded by electron ionization (EI) at 70 eV, using a spectral range of m/z 40-450. The identification of the chemical constituents of the EOs was achieved using Automated Mass spectral Deconvolution and Identification (AMDIS) software, Wiley spectral library collection, NIST library database, retention indices relative to n-alkanes (C 8 -C 22 ), or appraisal of the mass spectrum with authentic standards.

Phytotoxic Bioassay of the A. ochroleuca Essential Oil
The phytotoxicity of the A. ochroleuca EO was performed against L. sativa L. as a standard test plant, where it is known to be very sensitive to allelochemicals [64] as well as the noxious weed P. harmala. In brief, the seeds of lettuce were purchased from the Agriculture Research Center, Cairo, Egypt, while the seeds of P. harmala were collected from Taif, southeast Saudi Arabia. Seeds with uniform size and color were carefully chosen and surface-sterilized via sodium hypochlorite (0.3%). Prior to the experiment, the viability of the seed was performed by the germination of seeds in the Petri plate lined with a filter paper (Whatman No. 1) using distilled water at 25 • C with adjusted light conditions of 16/8 h light/dark cycle. The germination percentage was 97.23 ± 0.5% for L. sativa and 90.12 ± 1.02% for P. harmala.
To assess the phytotoxicity of the EO, various concentrations (50,100,150,200, and 250 µL L −1 ) were prepared by dilution using 1% Tween ® 80 (Sigma-Aldrich, Darmstadt, Germany) as an emulsifying agent. In Petri plates, 20 sterilized seeds of either L. sativa or P. harmala were spread on sterilized filter paper (Whatman No. 1), 4 mL of each concentration or control (Tween ® 80) was poured, and the plates were sealed with Parafilm ® tape (Sigma, St. Louis, MO, USA). Five plates were prepared per each concentration, and the experiment was repeated three times. A total of 180 plates (2 plants × 6 treatments [5 concentrations + control] × 5 plates as replications × 3 times) were prepared and incubated in a growth chamber at 25 • C with adjusted light conditions of 16 h/8 h light/dark cycle. After 5 days of incubation for L. sativa and 7 days for P. harmala, the reduction in seed germination, shoot growth, and root growth of the seedlings were calculated based on the following equation: where N is the number of germinated seeds and L is the length of seedling root or shoot.

Statistical Analysis
The experiment of bioassay was designed as a completely randomized design and repeated three times with five replications per each treatment. The data of seed germination and seedling growth inhibition were subjected to one-way ANOVA and followed by Duncan's HSD post hoc test at a probability level of 0.05.

Conclusions
The chemical composition of A. ochroleuca EO was characterized with highly oxygenated constituents (79.01%), including mono-, sesqui-, di-terpenoids, carotenoids, and hydrocarbons. The trans-chrysanthenyl acetate, γ-cadinene, oleic acid, methyl ester, patchouli alcohol, and terpinene-4-ol were determined as the main compounds. The A. ochroleuca EO exhibited significant phytotoxic activity. The high oxygenation of the EOs constituents was deduced to be correlated with the increase of the phytotoxicity. Therefore, the substantial phytotoxicity of A. ochroleuca EO could be ascribed to its high content of the oxygenated compounds (78.28%), and thereby it could be used as an eco-friendly bioherbicide. However, further study is recommended for characterization of the main identified compounds, either singular or in combination at field scale level, as well as for the evaluation of their modes of action and biosafety.