Chemical composition of different plant part from Lactuca serriola L. – focus on volatile compounds and fatty acid profile

1 Introduction

The genus Lactuca belonging to the Asteraceae family, originated in the Mediterranean area and the most prevalent and widely dispersed species is Lactuca serriola L. In reference to the milky sap that the plant produces, the genus name Lactuca derives from the Latin word “lac” which means milk. L. serriola is generally known as prickly lettuce, compass plant or wild lettuce. Due to extensive introductions, this species is now found in numerous continental sites in Europe, Asia, Australia, Africa, a large portion of North America, and Central and Latin America. From a climatic point of view, the distribution limit of L. serriola in the northern hemisphere, where it is ubiquitous, is constrained by extremely cold temperatures. Its highest elevations have been recorded in Switzerland at 1560 m, in Turkey at 1750 m, in Afghanistan at 3100 m, in the northern Himalayas at 3600 m, and in the United States at 2358 m [1]. L. serriola, a C3 species, is an annual or biennial plant, with roots like a needle, stems 30–100 cm in height, and vertical bluish-green leaves. It typically grows in mountainous areas, on mountain slopes, in pebbly areas, along the beach, and in orchards [2]. L. serriola has been conventionally used in Turkish folk medicine. For example, the decoction of plants is used to treat liver problems and stomach pains while its infusion to reduce cholesterol and cure hemorrhoids. The leaves eaten raw are valued as a sedative agent. In Turkey, they are also eaten as a fresh salad [3].

Previous phytochemical studies on Lactuca species revealed the presence of a wide range of secondary plant metabolites, including coumarins, phenolics, sesquiterpene lactones, saponins, triterpenoids, lignans, and phytosterols [4]. Depending on the dose, the methanolic extract of L. serriola has been found to exhibit bronchodilator, vasorelaxant, spasmogenic, and spasmolytic activities. While the spasmolytic action at higher concentrations may be related to Ca⁺² channel blockers that can relax tracheal, gastrointestinal, and aortic smooth muscle, the spasmogenic activity may be related to some cholinergic elements. This may shed a light on the traditional use of L. serriola for treating intestinal, bronchial, and circulatory diseases involving spasms. Against Ehrlich ascites carcinoma, its extract has shown antitumor efficacy. For people with cardiovascular illnesses, L. serriola may be utilized as a cardioprotective medication. At doses of 300, 500, and 1000 mg/kg, its extract demonstrated a significant analgesic effect [4].

Taking into account the availability of a greater amount of chemical substances and their medicinal importance, the research on L. serriola is noteworthy. The main objective of this study was to identify the chemical components in the aerial parts and roots of L. serriola.

2 Materials and methods

3 Results and discussion

To the best of our knowledge, this is the first report on the composition of the volatiles obtained by the MSD-SPME technique from the roots, leaves, and seeds of L. serriola. Results of the chemical composition analysis are presented in Table 1 with their RRI values and relative percentage. A total of 67 molecules belonging to different chemical classes were identified. Generally, leaves were characterized by a greater number of compounds (45) than roots (26) and seeds (11). Most of the compounds obtained from the leaves were identified as aldehydes (16), terpenoids (9), and fatty acids (5). Among them, the most abundant were phenylacetaldehyde (18%), (E)-β-ionone (11%), (E,Z)-2,4-heptadienal (9.1%), (E,E)-2,4-heptadienal (8.2%), and benzaldehyde (4.2%). Furthermore, analysis of the fatty acid composition identified five constituents. The main ones were found to be non-adenoic (38.3%), palmitic (35.9%), and α-linolenic (14.3%) acids.

The classes of molecules extracted from the roots were mainly aldehydes (7), monoterpenes (3) and lactones (3). Specifically, 2-ethyl hexanol (36.9%), 2-propenoic acid, butyl ester (19.1%), benzaldehyde (6%), hexanal (4.8%), and nonanal (4.7%) were the most common chemical constituents.

Finally, 11 fatty acids have been found in the seeds of L. serriola, including some essential ones. Linoleic (57.7%), oleic (16.1%), palmitic (10.3%), and nonadecanoic (9.4%) acids were present in higher quantities.

Of note, five fatty acids found in leaves overlapped with those found in seeds. In leaves, the ratio of saturated: polyunsaturated (S:P) fatty acids was 79.2/20.2, while in seeds, the ratio of saturated: monounsaturated: polyunsaturated fatty acids was 24.9/16.9/58.1.

Benzaldehyde, hexanal, octanal, nonanal, 6-methyl-5-hepten-2-one, and decanal were found to be present in both leaves and roots of L. serriola while hexanoic acid was found only in the roots. Previously, these compounds were also detected in the leaves of L. sativa [10]. α-Linolenic acid methyl ester has been detected in the ethanolic extract of L. runcinata DC [11]. The essential oil of the aerial parts of L. serriola growing in Egypt also contained hexahydrofarnesyl acetone, but in a higher percentage than that of the sample under examination (1.7% vs. 0.5%) [12]. Nonanal, decanal, β-cyclocitral (E)-β-ionone, hexahydro-farnesylacetone, and methyl palmitate were recently traced in the essential oil obtained by the hydro-distillation from aerial parts of L. serriola collected in Turkey [3]. However, a quantitative dissimilarity was present with respect to the main volatile of the samples from Azerbaijan, Turkey, and Egypt. Phenylacetaldehyde, identified as the major compound of L. serriola in the present study, was not found in the other two cases. The geographical area, the harvesting period, the particular climatic conditions, the extraction methods, and the analysis techniques could be some of the factors responsible for the discrepancy.

Based on the literature findings, the medicinal and industrial importance of some volatiles that are obtained from L. serriola has already been documented. Hexanal, a naturally occurring volatile compound with antibacterial activity, has been approved by the USFDA as a food additive and has been demonstrated to prolong the shelf life of fruit and preserve its original color [13]. The fragrance and perfume industries use hexanal as a synthesis intermediate [14]. Isobutyl isobutyrate is a flavoring ingredient in ice cream, soft drinks, and confections made with sugar and flour [15]. Under field circumstances, 2-pentylfuran greatly reduces fruit infestations by acting as an insect repellant against Drosophila suzukii [16]. Natural food flavor (E)-2-hexenal, a volatile compound found in green leaves, has strong antifungal effects on Aspergillus flavus. It has been shown that phenylacetaldehyde can be used to monitor (presence or absence) or sample (number per time per area) insects using traps easily available on the market [17]. In the perfume industry, phenylacetaldehyde is a fragrance ingredient used to create aromas resembling hyacinth or roses and can be used in the production of the polymers as a rate control additive during the ring-opening polymerization of polyesters. In the synthesis of pesticides, pharmaceuticals, and other fragrance substances like 2-phenylethanol, it can also serve as a building block [18]. The primary precursor in the production of vitamin A is β-ionone [19]. 2,4-Heptadienal resulted in significant growth suppression of Bacillus cereus, Vibrio parahaemolyticus, and Staphylococcus aureus [20]. The second most significant ingredient in the flavor industry, as per consumption, is benzaldehyde. The food, beverage, and fragrance industries use it extensively [21]. Benzaldehyde is mostly used in industrial settings as a precursor to other chemical molecules, ranging from medications to additives for plastics. Dimethylaniline and benzaldehyde are combined to create the aniline color malachite green. Additionally, it serves as a precursor to several acridine dyes [22]. Nonanal, a liquid aldehyde with a rose-like scent, is a common ingredient in flowery perfumes [23]. According to Li et al. [24], the growth of Aspergillus flavus can be effectively stopped when exposed to octanal, nonanal, and decanal. Among these, octanal is another common perfume ingredient with antimicrobial properties against Escherichia coli, Saccaharomyces cerevisiae, S. aureus, and Aspergillus niger as well as a powerful antioxidant agent [25]. Nonanal and decanal are crucial natural aromatic ingredients used to amplify floral and citrus notes in perfumery products [26]. 2-Ethyl-1-hexanol can be used in the manufacturing of plasticizers, antifoaming agents, dispersants, mineral dressing agents and petroleum fillers [27]. Butyl acrylate is used in the manufacturing of plastics, sealants, coatings, inks, textiles, elastomers, and adhesives [28].

As for the fatty acids, α-linolenic acid (44.4%), palmitic acid (21.3%), and linoleic acid (20.2%) were previously detected in the leaves of L. canadensis [29] and L. sativa along with stearic and oleic acids [30]. Furthermore, linoleic, oleic and palmitic acids, in addition to n-tetradecane, were also the main volatile compounds of L. sativa leaf essential oil [31]. In our analysis, α-linolenic acid is the third most common fatty acid found in the leaves of L. tatarica. Various plant foods, including walnuts, rapeseed (canola), several legumes, flaxseed, and green leafy vegetables are characterized by high content. It is the precursor of three longer-chain n-3 fatty acids, docosapentaenoic acid (DPAω3 22:5ω3), eicosapentaenoic acid (EPA 20:5ω3), and docosahexaenoic acid (DHA 22:6ω3), which play vital roles in cardiovascular health, inflammatory response and brain development [32]. Oleic acid – which is the second most abundant fatty acid in L. tatarica seeds – has the ability to permeate the stratum corneum by disrupting the intercellular lipid structures, which may account for how it affects skin physiology and drug absorption [33]. Thus, it could be used in transdermal drug delivery systems. Nonadecanoic acid demonstrated the inhibitory activity for HL-60 cell proliferation [34]. Both linolenic and linoleic acids showed antiplasmodial activity [35]. Palmitic acid is an essential compound of cell membranes, transport and secretory lipids and it plays vital roles in protein palmitoylation [36]. Methyl nonadecanoate displayed interesting antisickling activity with a normalization rate higher than 70%. It possesses the potential bioactivity to be used as a medicine for the treatment of sickle cell anemia [37]. Methyl palmitate can be considered a universal macrophage inhibitor. It aims to diminish inflammation by both increasing the production of an anti-inflammatory cytokine and blocking the synthesis of pro-inflammatory cytokines. Methyl palmitate had also the ability to suppress macrophages in general and acts by reducing inflammation and fibrosis, possibly through decreasing NF-kB. Consequently, a promising anti-inflammatory and antifibrotic medication could be a methyl palmitate-like molecule [38]. Moreover, this compound demonstrated a strong acaricidal activity [39]. Methyl linolenate exhibited a depigmenting effect and could be used as a cosmetic lightening agent or as a potential therapeutic treatment for depigmentation [40]. Together with methyl linoleate, it could be a suitable candidate for the treatment of melanoma [41]. The latter is also often used as an ingredient in biodiesel, fabrics, emulsifiers and lubricants [42]. Finally, methyl stearate is utilized in agriculture as a lipid carrier and solvent [43]. Due to the crucial roles that commercial esters like methyl oleate play in the oleochemical, pharmaceutical, food, biodiesel, and cosmetic sectors, there has been a sharp increase in demand for these substances. Numerous commercial products, including emulsifiers, wetting agents, detergents, and intermediate stabilizers are made using this molecule [44].

4 Conclusions

In this research, the chemical investigation of L. serriola leaves, roots, and seeds volatiles was performed using the GC-MS/FID technique. To our knowledge, plant volatiles of L. serriola were first extracted and analyzed using the MSD-SPME technique. This process is fast, convenient, and economical in terms of raw resources. It also shortens the extraction period while allowing the maximum amount of volatiles to be extracted. In addition, L. serriola has further emerged as a source of useful ingredients in various fields, including the pharmaceutical one, and therefore its multiple bioactive characteristics deserve other investigation.

Abbreviations

NIST

National Institute of Standards and Technology

PDMS-DVB

Polydimethylsiloxane/Divinylbenzene

USFDA

U.S. Food and Drug Administration