## 1 Introduction  
Essential oils (EOs) are aromatic and volatile liquids produced from different parts of the plants--flowers, roots, stems, banks, leaves, seeds, peels or fruits--or from whole plants [1] and stored in their secretory cells, cavities, canals, epidermic cells or glandular trichomes [2]. EOs represent multicomponent, water-insoluble mixtures of polar and nonpolar natural compounds including terpen (monoterpenes and sesquiterpenes), aromatic compounds (aldehydes, alcohols, phenols, methoxy derivatives) and terpenoids (isoprenoids) [3]. The rich chemical compositions of EOs determine their broad spectrum of biological activities, such as antimicrobial, antiviral, antioxidant, antiparasitic, insecticidal and many others, which find pharmaceutical, cosmetic, fragrance, agricultural and food industry applications [4]. _Hypericum perforatum_ L., widely known as St John's Wort (SJW), is a perennial herbaceous plant that belongs to the _Hypericaceae_ family. It is largely distributed in Europe, Asia, North America and Northern Africa. SJW prefers sunny locations with well-drained, limey soil, mostly at roadsides, slopes, wood borders and stone quarries, and reaches a height of 50-100 cm. SJW flowers are bright yellow, star-shaped, clustered in a trio and possessing five petals. SJW leaves have specific tiny, transparent oil glands resembling perforations [5]. The genus _Hypericum_ unifies approximately 480 species worldwide, of which 61 are spread throughout Europe. Twenty-two species of this genus are found in Bulgaria, five of which are endemic to the country. _H. perforatum_ L. is among the most popular and commonly used herbs in Bulgarian folk medicine. Its extracts possess strong antioxidant, anti-inflammatory, neuroprotective and gastroprotective properties [6]. SJW is a popular herbal remedy recommended by Traditional Chinese Medicine for the treatment depressive disorders [7]. It was found that the antidepressant and anxiolytic properties of SJW have been attributed to one of its major phytocomponents, known as hyperforin, which plays the role of a reuptake inhibitor for neurotransmitters such as dopamine, norepinephrine, serotonin and glutamate [8; 9]. According to some authors, several flavonoids, such as quercetin, hyperoside, quercetin, isoquercitrin, muqielianin and others, also contribute to the antidepressant activity of SJW [10]. SJW preparations in the form of oil macroates or timctures have been used for the topical treatment of skin disorders, such as superficial wounds, abrasions, sunburns, dermatitis and eczema, due to their anti-inflammatory and wound healing effects [11; 12]. EO obtained from SJW is colourless or light yellow, possessing a distinctive odour and liquid consistency at room temperature. While SJW EO is generally found in large amounts in the roots, leaves and flowers of the plant, it is presented in small quantities in other plant organs, such as bark and stems [6]. The most commonly detected chemical components in SJW EO are monoterenes, sesquiterpenes and their oxygenated derivatives, but their phytochemical profile is also comprised of naphthodianthrones (hypericin and pseudohypericin), hyperforin, proanthocyan, flavonoids, biflavonoids, xanthones, phenylpropanes, phenolic acids and volatile compounds [13]. The phytochemical profile of the EO of _Hypericum_ spp. vary widely, influenced by the characteristics of geographical region, some environmental factors and phenological cycle, as well as the type of plant organ in which the EO is produced [14]. Although _Hypericum_ spp. extracts have been extensively studied in terms of their biological properties, the number of studies conducted with their EOs is still limited.