Chemical composition and biological activities of Eruca vesicaria subsp. longirostris essential oils

Abstract

Context To date, there are no reports to validate the Tunisian traditional and folklore claims of Eruca vesicaria (L) Cav. subsp. longirostris (Brassicaceae) for the treatment of disease.

Objective Investigation of the chemical composition antimicrobial and antioxidant activity of essential oils from Eruca longirostris leaves, stems, roots and fruits.

Materials and methods The essential oils of E. longirostris from leaves, stems, roots and fruits were obtained after 4 h of hydrodistillation. Chemical compositions were determined using a combination of GC/FID and GC/MS. The in vitro antimicrobial activity of the volatile constituents of E. longirostris was performed in sterile 96-well microplates against three Gram-positive, four Gram-negative bacteria and one strain as yeast. The minimum inhibitory concentration (MIC) and minimum bactericidal concentration values were reported. Furthermore, the antioxidant activity was evaluated by DPPH and ABTS assays.

Results The main compound for fruits, stems and roots was the erucin (96.6%, 85.3% and 83.7%, respectively), while β-elemene (35.7%), hexahydrofarnesylacetone (23.9%), (E)-β-damascone (15.4%), erucin (10.6%) and α-longipinene (9.6%) constituted the major compounds in the essential oil of the leaves. The experimental results showed that in all tests, essential oil of fruits showed the better antioxidant activity than the others. On the other hand, the oils of stems, fruits and roots showed significant antimicrobial activity with MIC values ranging from 0.125 to 0.31 mg/mL against Candida species, Gram-positive and Gram-negative bacteria, mainly Salmonella enterica.

Conclusions The present results indicate that essential oils of E. longirostris can be used as a source of erucin.

Keywords:

• Antimicrobial activity
• antioxidant activity
• Brassicaceae
• chemical compounds identification

Introduction

The medicinal properties of plants have been investigated in the light of recent scientific developments throughout the world, due to their potent pharmacological activities, low toxicity and economic viability (Auddy et al. 2003).

Essential oils in plant are complex volatile mixtures that exist at low concentrations and are commonly found in aromatic plants (Edris 2007; Kahriman et al. 2012). Studies have demonstrated beneficial properties of essential oils in the prevention and treatment of cancer, cardiovascular diseases including atherosclerosis and thrombosis, as well as their bioactivity such as antibacterial, antiviral, antioxidants, antidiabetic, anti-inflammatory agents, local anaesthetic and immunomodulatory (Passos et al. 2007; Nazemiyeh et al. 2011; Ziaei et al. 2011).

Among the Eruca genus, rocket salads (leaves introduced as ingredients for salads), are widely used in the Mediterranean diet and well-studied as source of healthy phytochemicals (Schaffer et al. 2005; Bogani & Visioli 2007).

The name rocket is commonly used to indicate different species belonging to the large family of Brassicaceae (also called Cruciferae) that are mainly represented by Eruca sativa Mill. Rocket species are well-known in traditional medicine for their therapeutic properties as an astringent, diuretic, digestive, emollient, tonic, depurative, laxative, rubefacient and stimulant (Perry & Metzger 1978; Yaniv et al. 1998).

The tender young leaves are used in salads and sometimes cooked as a potherb. Ancient Egyptians and Romans both have considered the leaves in salads to be an aphrodisiac. It is also an antiscorbutic, stimulant and rubefacient (Boulos 1983). In terms of antioxidant compounds, rocket salad species are a good source of vitamins, such as vitamin C, carotenoids and polyphenols, which play a very important role among natural antioxidants (Martínez-Sánchez et al. 2008).

The essential oil extracted from the leaves of E. sativa contains 67 volatile compounds, which represents 96.52% of the oil (Mitsuo et al. 2002). The oil from seed of E. sativa has promising pharmacological efficacy and potential bio-active compounds as compared to different aerial and root plant extracts (Khoobchandani et al. 2010). The main active constituents found in leaves are 4-methylthiobutylisothiocyanate and 5-methylthio-pentanonitrile with 60.13% and 11.25%, respectively (Mitsuo et al. 2002).

Eruca vesicaria (L) Cav. subsp. longirostris (Brassicaceae) is a wild plant common to Mediterranean regions (Alapetite 1981). It is an annual plant growing to 20–100 cm tall. The leaves are deeply pinnately lobed with 4–10 small lateral lobes and a large terminal lobe. The flowers are 2–4 cm diameter, arranged in a corymb, with the typical Brassicaceae flower structure; the petals are creamy white with purple veins, and the stamens yellow; the sepals are persistent after the flower opens. The fruit is a siliqua (12–25 mm) with an apical beak, and containing several seeds (Alapetite 1981). To the best of our knowledge, the antioxidant and antimicrobial activities of essential oil of E. longirostris have not been studied hitherto. Therefore, the main objective of this work was to determine the chemical composition of the essential oil from E. longirostris and evaluate their in vitro antioxidant and antimicrobial properties.

Materials and methods

Plant material

The aerial parts of E. longirostris were collected at the flowering stage on February 2012 from the area of Kasserin-Tunisia. The botanical identification was carried out by Dr. Fethia Harsallah-Skhiri, botanist in higher institute of biotechnology of Monastir, Tunisia. A voucher specimen (E. longirostris) (El 110) was deposited in the Laboratory of Medicinal Chemistry and Natural Products at the faculty of Sciences of Monastir, Tunisia. The flowers, leaves, stems and roots were divided into small pieces and weighed before the extraction of the volatile compounds.

Chemicals

2,2-Diphenyl-1-picylhydrazyl (DPPH) and butylatedhydroxytoluene (BHT) were purchased from Sigma Chemical Co. (St. Louis, MO). All other chemicals and solvents were of analytical grade. All solutions were freshly prepared in distilled water.

Essential oils extraction

Extraction was carried out by hydrodistillation for 4 h, using a Clevenger-type apparatus. The essential oil was collected by decantation, dried over anhydrous sodium sulphate, weighed and stored in sealed glass vials at 4–5 °C until analysis. Yield based on fresh weight of the sample were calculated.

GC-FID analysis

The essential oils were analysed using a Hewlett Packard (Palo Alto, CA) 5890 II GC equipped with flame ionization detector and HP-5MS fused silica capillary column (30 m × 0.25 mm ID, film thickness 0.25 μm). The temperature oven was programmed from 50 °C (1 min) to 280 °C at 5 °C/min (1 min). Injector and detector temperatures were set at 250 and 280 °C, respectively. Nitrogen was the carrier gas at a flow rate of 1.2 mL/min. Diluted samples (1/1000 in hexane, v/v) of 1.0 μL were injected manually and in the splitless mode. The identification of the components was performed by comparison of their retention times with those of pure authentic samples and by mean of their linear retention indices (LRI) relative to the series of n-hydrocarbons.

GC/MS analysis

GC/MS analyses were performed with a Varian CP-3800 (Palo Alto, CA) gas-chromatograph equipped with a HP-5 capillary column (30 m × 0.25 mm; coating thickness 0.25 μm) and a Varian Saturn 2000 ion trap mass detector. Injector and transfer line temperatures were set at 220 and 240 °C, respectively; oven temperature programmed from 60 °C to 240 °C at 3 °C/min; carrier gas helium at 1 mL/min; injection of 0.2 μL 10% hexane solution; split ratio 1:30. Identification of the constituents was based on comparison of the retention times with those of authentic samples, comparing their linear retention indices relative to the series of n-hydrocarbons, and on computer matching against commercial (NIST 98 and ADAMS) and home-made library mass spectra built up from pure substances and components of known essential oils and MS literature data (Stenhagen et al. 1974; Massada 1976; Jennings & Shibamoto 1980; Swigar & Silverstein 1981; Davies 1990; Adams 1995).

Antimicrobial activity

Strains

The used microorganisms were as follows: three Gram-positive bacteria [Staphylococcus aureus (ATCC 6538) Bacillus subtilis (ATCC 6633) and Bacillus amyloliquefaciens (ATCC 15841)]. Four Gram-negative bacteria [Esherichia coli (ATCC 8739), Pseudomonas aeruginosa (ATCC 9027), Salmonella enterica (CIP 8039) and Salmonella typhimirium (ATCC Ames TA98)] and Candida albicans (ATCC 90028) as the yeast strain. The culture medium used for bacteria was Muller-Hinton agar, whereas Sabouraud agar was used for growing the yeast. The incubation conditions used were 24 h at 37 °C for the bacteria and 48 h at 28 °C for the yeast.

Preparation of resazurin solution

For antimicrobial assay, the resazurin solution used as a redox indicator was prepared by dissolving 270 mg of resazurin (Sigma-Aldrich, St. Louis, MO) in 40 mL of sterile distilled water. A vortex mixer was used to ensure that it was a well-dissolved and homogenous solution.

Antimicrobial assay

To screen for antimicrobial activity, plates were prepared under aseptic conditions. A sterile 96-well plate was labelled. A volume of 100 μL of test material in 10% (v/v) DMSO or sterile water (usually a stock concentration of 1 mg/mL for essential oil) was pipetted into the first row of the plate. All other wells, 100 μL of nutrient broth or normal saline was added. Serial dilutions were performed using a multichannel pipette. To each well 10 μL of resazurin indicator solution was added. Finally, 20 μL of bacterial suspension (5 × 10⁶ cfu/mL) was added to each well to achieve a concentration of 5 × 10⁵ cfu/mL. Each plate was wrapped loosely with cling film to ensure that bacteria did not become dehydrated. Each plate had a set of controls: a column with a broad-spectrum antibiotic as a positive control (usually gentamicin in serial dilution), a column with all solutions with the exception of the test compound, and a column with all solutions with the exception of the bacterial solution adding 10 μL of nutrient broth instead (Satyajit et al. 2007).

Resazurin is a blue-coloured redox indicator. Reactions indicating positive results were represented by blue-coloured solution, indicating inhibition of microbial growth. When the oxygen within the medium is limited, indicating microbial growth, the resazurin is reduced and the colour changes from blue to pink. Furthermore, the change in colour from blue to pink will permit the determination of the minimum inhibitory concentration (MIC). The MIC value was defined as the lowest concentration of oil which no growth of microorganisms was observed after incubation. The minimum bactericidal concentration (MBC) is usually an extension from the MIC, where the organisms quantitatively indicate the minimum concentration when no viable organism appears in the culture.

Antioxidant activity

Scavenging effect on DPPH^·

The hydrogen atoms or electrons donation ability of the corresponding samples were measured from the bleaching of purple-coloured methanol solution of DPPH^· (Cuendet et al. 1997). The effect of essential oils on DPPH^· radical was estimated according to Hatano et al. (1988). 0.5 mL of each sample concentration (0.03, 0.062, 0.125, 0.25, 0.5 and 1 mg/mL) and butylated hydroxytoluene (BHT) were mixed with the same volume of DPPH^· methanolic solution. The mixture was shaken vigorously and allowed standing for 30 min in darkness and at a temperature of 25 °C. The absorbance of the resulting solution was measured at 520 nm with a UV spectrophotometer. Lower absorbance of the reaction mixtures indicated higher free radical-scavenging activity. All measurements were performed in triplicate. A mixture of 0.5 mL of DPPH^· solution and 0.5 mL of methanol were taken as a control. The ability to scavenge the DPPH^· radical was calculated with the following equation: where A_Control is the absorbance of the control reaction (containing all reagents except the test compound) and A_Sample is the absorbance of the tested sample.

ABTS radical scavenging activity assay

Antiradical activity was done by using the ABTS^·+free radical decolourization assay developed by Re et al. (1999) with some modifications. The preformed radical monocation of ABTS was generated by reacting ABTS solution (7 mM) with potassium persulphate (2.45 mM). The mixture was allowed to stand for 15 h in the dark at room temperature (Re et al. 1999). The solution was diluted with methanol to obtain the absorbance of 0.7 ± 0.02 units at 734 nm. Samples were separately dissolved in methanol to yield the following concentrations: 0.03, 0.062, 0.125, 0.25, 0.5 and 1 mg/mL. In order to measure the antioxidant activity of samples and BHT, 10 μL of each one at various concentrations was added to 990 μL of diluted ABTS^·+.

The absorbance was measured spectrophotometrically at 734 nm after 20 min. All measurements were performed in triplicate. The percentage decrease of absorbance at 734 nm was calculated for each point and the antioxidant capacity of the test samples was expressed as percent inhibition (%). The percentage scavenging of ABTS^·+was calculated by the following formula: where A_x and A_o were the absorbance at 734 nm of samples with and without extract, respectively.

Statistical analysis

The results were given as the average ± SE for at least three replicates for each sample. The data were subjected to ANOVA and Duncan’s multiple range test was used to compare means. Statistical analyses were performed with the SPSS statistical software program (SPSS v.16). p Values <0.05 were regarded as significant.

Results and discussion

GC–MS analysis of essential oil

The yields (w/w) of the essential oils from E. longirostris obtained by hydrodistillation of dry material ranged from 0.003% (stems) to 0.018% (fruits). The composition of the oils was determined by GC and GC–MS. The percentage composition with LRI calculated for each compound is reported in Table 1.

Table 1. Chemical composition of the essential oils isolated from the stems, leaves, roots and fruits of E. longirostris.

LRI	Compound	Roots (%)	Stems (%)	Leaves (%)	Fruits (%)
949	5-Methyl-hexanenitrile	0.2			0.1
985	Heptanenitrile	0.1
1006	1-(Methylthio)-hexane	0.1
1043	β-Isophorone				0.1
1099	Pentyl isothiocyanate	0.1
1120	Isophorone				0.1
1165	4-Methylpentyl isothiocyanate	0.5	1.2
1193	3-Hydroxy-2-ethyl-γ-pyrone (=2-ethylpyromeconic acid)	0.2	0.6		0.1
1202	5-(Methylthio)-pentanenitrile	13.5	6.8		2.3
1285	(E)-Anethole	0.1
1312	3-Methylthiopropyl isothiocyanate	0.2	0.4
1314	4-Vinyl guaiacol				0.1
1320	4-Hydroxy-3-methylacetophenone	0.1
1351	α-Longipinene			9.6
1383	(E)-β-Damascenone				0.1
1392	β-Elemene			35.7
1412	(E)-β-Damascone			15.4
1431	Erucin (=4-methylthiobutyl isothiocyanate)	83.7	85.3	10.6	96.6
1486	(E)-β-Ionone			2.7	0.1
1562	Ledene alcohol	0.1	1.4
1582	Caryophyllene oxide		0.5
1716	Pentadecanal	0.1
1765	Tetradecanoic acid	0.1	0.3
1781	1-Pentadecanol	0.1	0.3
1817	Hexadecanal		0.3
1843	Hexahydrofarnesylacetone		0.2	23.9	0.1
	Sesquiterpene hydrocarbons	0	0	45.3	0
	Oxygenated sesquiterpenes	0.1	1.9	0	0
	Apocarotenoids	0	0.2	42	0.5
	Sulphur and/or nitrogen compounds	98.4	93.7	10.6	99
	Phenylpropanoids	0.1	0	0	0
	Non-terpene derivatives	0.6	1.5	0	0.2
	Total identified	99.2	97.3	97.9	99.7
	Yield (%)	0.009	0.003	0.016	0.018

Note: LRI: linear retention index determined on the polar column DB-5 rel. to a series of n-alkanes.

The compounds are listed according to their elution order on the apolar HP-5 MS capillary column. They were separated into six classes, which were sesquiterpene hydrocarbons, oxygenated sesquiterpenes, apocarotenoids, sulphur and/or nitrogen compounds, phenylpropanoids and non-terpene derivatives. In general, most chemical compositions found in the three essential oils from fruits, stems and roots are isothiocyanate, where the highest is the erucin (96.6, 85.3 and 83.7%, respectively), followed by 5-(methylthio)-pentanenitrile (2.3, 6.8 and 13.5%, respectively). However, the major component of the essential oil from the leaves was β-elemene (35.7%) followed by hexahydrofarnesylacetone (23.9%), (E)-β-damascone (15.4%), erucin (10.6%) and α-longipinene (9.6%). Qualitatively, the chemical composition of the essential oils from the roots, stems and fruits is rather close, being dominated by erucin with a value ranged from 83.7% to 96.6%.

The chemical compositions of the three essential oils suggest the possibility of using them as an erucin source. In addition, the essential oils of E. longirostis may be have a reduced risk of cancer of the lung, stomach, breast, prostate, pancreas, colon and rectum, which has been attributed to isothiocyanate content (Heber 2004; Higdon et al. 2007). The major component found in the essential oil from leaves was β-elemene (35.7%). It is known that in China, elemene emulsion has been approved by the State Food and Drug Administration of China to treat malignant effusions and some solid tumours. This indicates that elemene is a promising agent for the treatment of tumours. β-Elemene, the major active component of elemene, has been shown to be effective against various tumours such as lung cancer, prostate cancer and glioblastoma (Wang et al. 2005; Yao et al. 2008; Li et al. 2010).

We can notice essentially that as shown in Table 1, the main compounds of the different essential oils of E. longirostris were classified as sulphur and/or nitrogen compound. This observation confirms the pungent taste of our plant like those of Brassicaceae family.

Previous studies have highlighted rocket as a rich source of glucosinolate compounds (Kim et al. 2004). Virtually all other members of the Brassicaceae contain glucosinolates as secondary metabolites that act as part of plant defense mechanisms. Glucosinolates and their hydrolysis products (isothiocyanates, thiocyanates, nitriles and sulphates) have also been implicated in giving rocket its characteristic pungent aromas and flavours (Bennett et al. 2002) and volatiles (such as isothiocyanates and indoles).

In the same way, Federica et al. (2011) showed that aroma intensity, pungency, crunchiness and juiciness were strong determinants of overall rocket salad flavour perception. Visual traits also characterized sensory components. Bitterness, usually considered a negative flavour trait, was moderately perceived in E. vesicaria (L.) Cav. subsp, without negatively affecting typical flavour perception. In the range of the examined material, glucosinolate content did not contrast with typical flavour, demonstrating that good taste and putative health-promoting properties may coexist.

Antimicrobial activity

A microbroth dilution assay was employed to study the antimicrobial activities of essential oils of E. longirostris against Gram-negative, Gram-positive bacteria and Candida species. As it can be seen in Table 2, the antimicrobial activity of E. longirostris essential oil was more pronounced against Gram-positive than Gram-negative bacteria. This is a general observation derived from studies with essential oils from many other species (Burt 2004). Generally, the higher resistance among Gram-negative bacteria could be ascribed to the presence of their outer phopholipidic membrane, almost impermeable to lipophilic compound (Sela et al. 2013). The absence of this barrier in Gram-positive bacteria allows the direct contact of the essential oils hydrophobic constituents with the phospholipids bilayer of the cell membrane, where they bring about their effect, causing either an increase of on permeability and leakage of vital intracellular constituent, or impairment of the bacteria enzyme (Burt 2004). The results revealed that the selected essential oils showed antibacterial activity with varying magnitudes. Essential oils from stems, fruits and roots were active against all of the microorganisms tested, showing the lowest MIC (mg/mL) values (0.125 and 0.25 mg/mL) against S. enterica (CIP 8039) and B. amyloliquefaciens ATCC and B. subtilis ATCC 6633, respectively. On the other hand, the in vitro antimicrobial activity of the essential oil of E. longirostris of roots, fruits and stems shows that S. aureus is totally inhibited (bactericidal effect) at 0.62 mg/mL (Table 2), while P. aeruginosa and Escherichia coli are totally inhibited at 2.5 mg/mL.

Table 2. Antibacterial activity of the essential oils isolated from the roots, stems, leaves and fruits of E. longirostris.

	Leaves		Fruits		Stems		Roots		GM
	MIC	MBC	MIC	MBC	MIC	MBC	MIC	MBC	MBC
Gram-positive bacteria
S. aureus ATCC 25923	na	na	0.31	0 .62	0.31	0 .62	0.31	0 .62	15.62
B. subtilis ATCC 6633	na	na	0.25	0.25	0.25	0.25	0.25	0.25	nd
B. amyloliquefaciens ATCC 15841	na	na	0.25	0.25	0.25	0.25	0.25	0.25	nd
Gram-negative bacteria
E. coli ATCC 8739	na	na	2.5	2.5	2.5	2.5	2.5	2.5	3.9
P. aeruginosa ATCC 9027	na	na	2.5	2.5	2.5	2.5	2.5	2.5	500
S. enterica CIP 8039	na	na	0.125	0.125	0.125	0.125	0.125	0.125	nd
Yeast
C. albicans ATCC 90028	na	na	0.25	0.25	0.25	0.25	0.25	0.25	nd

Notes: MIC: minimum inhibitory concentration (mg/mL); MBC: minimum bactericidal concentration (mg/mL); GM: gentamicine (μg/mL); nd: not determined; na: not active.

Finally, the essential oils of E. longirostris of roots, fruits and stems exhibited moderate activity against C. albicans with the minimum fungicidal concentration (0.25 mg/mL). High antimicrobial activity of E. longirostris of roots, fruits and stems has been attributed to their isothiocyanate and nitrogen components such as erucin and 5-(methylthio)-pentanenitrile. The weak antimicrobial activity of the essential oils from the leaves in E. longirostris has been attributed to the interaction between essential oils compound and the low content of erucin (Khoobchandani et al. 2010). The interaction between essential oils, compound can produce four possible types of effects: indifferent, additive, antagonistic or synergistic effects (Nestor et al. 2012). In our case, the high antimicrobial activity of E. longirostris roots, stems and fruits essentials oils has been attributed to the synergistic effects of the sulphur and the nitrogen compounds. On the other hand the low content of the sulphur and the nitrogen compounds and the presence of important quantities of the sesquiterpene hydrocarbons and apocarotenoids in the leaves testament to the antagonistic effect.

Antioxidant activity

DPPH^· radical-scavenging activity

The antioxidant activity of the essential oils from E. longirostris was assessed by employing DPPH^· free radical-scavenging and the results are shown in Table 3. The DPPH free radical is considered a simple and very fast method for determining antioxidant activity. The effect of antioxidant on DPPH^· radical scavenging was tough due to their hydrogen donating ability or radical scavenging activity. The method is based on the reduction of alcoholic DPPH^· solution in the presence of a hydrogen donating antioxidant (Viuda-Martos et al. 2008). The essential oils of E. longirostris from fruits, stems and roots were able to effectively reduce the stable free radical DPPH^· with a percentage of inhibition [PI (%)] values ranging from 80.66 ± 0.01% to 83.56 ± 0.01%. These results suggest that the presence of isothiocyanate compounds in essential oils of these plant parts may be the main cause of their considerable radical-scavenging activity (Khoobchandani et al. 2010; Ramandeep et al. 2011). An average PI (%) of DPPH^· in the presence of the essential oil from the leaves was observed showing their moderate scavenging ability [PI (%) = 56.28 ± 0.02%]. According to Mata et al. (2007), the absence of antioxidant activity observed for the terpene compounds in the DPPH^· reduction can be explained by the fact that they are not capable of donating a hydrogen atom and the low solubility provided by them in the reaction medium of the assay, because this test utilizes methanol or ethanol as solvent.

Table 3. Antioxidant activities of E. longirostris essential oils on DPPH^·, ABTS test.

	PI (%) of DPPH^· scavenging	PI (%) of ABTS scavenging
Fruits	83.56 ± 0.01	90.56 ± 0.092
Stems	80.66 ± 0.01	87.31 ± 0.1
Leaves	56.28 ± 0.02	74.18 ± 0.1
Roots	81.76 ± 0.02	89.43 ± 0.1
BHT	94.7 ± 0.01	95.2 ± 0.6

Note: PI (%): percentage of inhibition.

Radical cation ABTS^+· scavenging activity

The ABTS method gives a measure of the antioxidant activity of essential oils by determining the reduction of the radical cation as the percentage of inhibition [PI (%)] of absorbance at 734 nm. Re et al. (1999) reported that the decolourization of the ABTS^·+cation reflects the capacity of an antioxidant to donate electrons or hydrogen atoms in order to inactivate this radical species. Table 3 shows the antioxidant activity of all essential oils from different parts of E. longirostris. The essential oil of E. longirostris from fruits, roots and stems showed potent antioxidant activity, which could be due to high amount of erucin essential oil with a value ranged from 87.31 ± 0.1% to 90.56 ± 0.092% compared to BHT [PI (%) = 95.2 ± 0.6%]. As shown previously by Marina and Antonio (2014), we note that the isothicyanate and nitrogen compound is responsible for the high antioxidant activity of essential oils from fruits, roots and stems. The moderate antioxidant activity of essential oil from leaves may be due to the low content of erucin.

In general, the antioxidative effectiveness of essential oil depends on the content of phenolic compounds and the reaction activity of the phenol towards the chain-carrying peroxyl radicals and on the stability of the phenoxyl radical formed in the reaction (Lopez-Luts et al. 2008). In addition, this observation is certainly associated with the low content of phenolic, isothyocyanate and nitrogen constituents in the three investigated oils (Germano et al. 2002; Khoobchandani et al. 2010; Ramandeep et al. 2011). In addition, it is known that the synergistic or antagonistic effect of a compound present in minor percentage in a mixture has to be considered as well (Germano et al. 2002).

In the Brassicaceae family, the polyphenol compounds have been studied in relation to their content and antioxidant capacity. Among the diverse functions of these compounds, the antioxidant properties are very important and depend on the stability of compounds in different systems, as well as number and localization of hydroxyl groups (Rice et al. 1997). However, in plant foods, flavonols are not found free but rather as complex conjugates with sugar residues. In fact, alteration in the arrangement of the hydroxyl groups and degree of substitution by glycosylation decrease the antioxidant activity (Rice et al. 1997).

Recently, a comparative study of antioxidant compounds, flavonoids and vitamin C, and also antioxidant activity was carried out in four species of Brassicaceae vegetables used for salads (Ascension et al. 2007). Indeed, it has been shown that, the antioxidant activity evaluated by different methods (ABTS, DPPH, and FRAP assays) showed a high correlation level with the content of polyphenols and vitamin C.

Conclusion

In recent years, great importance has been attached to the consumption of fresh-cut vegetables for health reasons. The beneficial effects have been attributed to the antioxidant vitamins such as ascorbic acid, β-carotene and α-tocopherol present in vegetables. Moreover, other different compounds such as polyphenols have been studied in relation to their content and antioxidant capacity. The most studied cruciferous vegetables belong to the Brassica genera including broccoli, cabbage, kale, etc. However, other new leaf species belonging to the Brassicaceae family have been recently introduced as ingredients for salads, such as rocket, watercress and mizuna.

Furthermore, from this work, we have investigated the chemical composition, antimicrobial and antioxidant activities of essential oils from Eruca longirostris leaves, stems, roots and fruits. We can notice essentially that the E. longirostris volatile compounds from stems, roots and fruits are rich in sulphur compound and show an interesting antioxidant and antimicrobial activity. In this context, further in vivo study would be necessary and mandatory to confirm antimicrobial and antioxidant activities of E. longirostris volatile compounds which may be useful for preservation and/or extension of the shelf-life of raw and processed foods as well as pharmaceuticals and natural therapies of infectious diseases in the human, and management of plant diseases.

Acknowledgements

The authors are grateful to Dr. Fethia Harzallah Skhiri (High Institute of Biotechnology of Monastir, Tunisia) for the botanical identification.

Disclosure statement

The authors declare that there are no conflicts of interest. This research was supported by grants from the Tunisian Ministry of High Education.