Taxonomical and Phytochemical Characterisation of 10 Stachys Taxa Recorded in the Balkan Peninsula Flora: A Review

The genus Stachys is one of the largest genera of the Lamiaceae, and it comprises about 300 species. Some species are highly polymorphic, with a number of infraspecific taxa. The aim of the present review is to summarise the available knowledge on 10 taxa belonging to the Balkan Peninsula flora (S. alpina L., S. germanica L., S. menthifolia Vis., S. obliqua Waldst. Et Kit., S. officinalis (L.) Trevis., S. palustris L., S. recta L. subsp. recta, S. recta L. subsp. subcrenata (Vis.) Briq., S. salviifolia Ten., and S. sylvatica L.) in order to enable insight into the identified biologically active substances and their possible application in intrageneric differentiation.


Introduction
Stachys L. (woundwort) comprises approximately 300 species, and is considered one of the largest genera of Lamiaceae, with nearly worldwide distribution. Most of these species occur in the warm temperate regions of the Mediterranean and southwest Asia, with secondary centres in North and South America and southern Africa; woundwort is not found in Australia and New Zealand [1]. The genus comprises 19 taxa in the flora of Croatia and neighbouring areas (Domac, 1973), among which eight species are endemic to the Balkan Peninsula or even narrower regions [2]. This article comprises phytochemical and taxonomical characterisation of 10 taxa: S. alpina L., S. germanica L., S. menthifolia Vis., S. obliqua Waldst. Et Kit., S. officinalis (L.) Trevis., S. palustris L., S. recta L. subsp. recta, S. recta L. subsp. subcrenata (Vis.) Briq., S. salviifolia Ten., and S. sylvatica L. among the presented species. Presented data also include chemotaxonomical guidance for highly polymorphic species of S. recta, with a proposal for the differentation of two of the presented subspecies [1][2][3][4][5][6][7]. S. menthifolia Vis. is not mentioned by Domac, but is listed in Flora Europaea as endemic species for Balkan Peninsula, and has been identified on several localities of Croatia and Montenegro [3,8,9]. The aim of present review is to find a scientific literature that contains research on the biologically active compounds of presented Stachys taxa belonging to the Balkan Peninsula flora and to report the findings on the chemotaxonomical composition that could help to differentiate within the Stachys spp. Presented data also include chemotaxonomical guidance for highly polymorphic species of S. recta, with a proposal for the differentation of two presented subspecies [1][2][3][4][5][6][7]10].

Taxonomy and Systematics
Genus Stachys is composed of annual or perennial herbs or subshrubs that are hispid or soft-pubescent. Leaves are sesille or stipulate, blade oblong to ovate, with serrate to crenate margins. Flowers are sessile or short-stalked, with two or more clustered in the axils of the leaves on the upper part of the stem. The calyx is bell-shaped, and has five lobes or teeth. Corolla has a narrow generally with a short, pouched spur on the lower side of the tube. The upper lip is erect or generally parallel to tube axis, concave, entire (notched), and generally hairy, and the lower lip is perpendicular to the tube axis or reflexed, three (two)-lobed, and glabrous to hairy. The nutlets are oblong to ovoid [2][3][4][5][6][7]10].
S. alpina L. (alpine woundwort) is a green, hirsute, glandular perrenial up to 100 cm in height. It grows in shadow on limestone soils from mountainous to subalpine areas [2,3,10] [ Figure 1].  S. officinalis L. (betony) is a perennial herb of 15-60 cm height, with sparse hairs, a short woody rhizome, well-marked basal rosettes, and erect, simple, or slightly branched stems. Betony occurs in open woods, stony grassy places, and dry meadows from the sea level up to 1600 m [2,3,10,11] [ Figure 2].
S. palustris L.(marsh woundwort) is a sparsely to densely hairy perennial plant of shady spots in marshes, bogs, ponds, lakes, and stream margins, damp places, roadside verges, and as a weed in cultivated fields growing up to 120 cm from a creeping rhizome [2,3,10] [ Figure 3]. S. officinalis L. (betony) is a perennial herb of 15-60 cm height, with sparse hairs, a short woody rhizome, well-marked basal rosettes, and erect, simple, or slightly branched stems. Betony occurs in open woods, stony grassy places, and dry meadows from the sea level up to 1600 m [2,3,10,11] [ Figure 2].  S. palustris L.(marsh woundwort) is a sparsely to densely hairy perennial plant of shady spots in marshes, bogs, ponds, lakes, and stream margins, damp places, roadside verges, and as a weed in cultivated fields growing up to 120 cm from a creeping rhizome [2,3,10] [ Figure 3].   S. recta L. subsp. recta (perennial yellow woundwort) is erect or ascending, subglabrous to sparsely hirsute, and usually an eglandular plant of up to 100 cm in height, which grows on different dry and stony habitats [2,3,10] [ Figure 4]. S. recta L. subsp. recta (perennial yellow woundwort) is erect or ascending, subglabrous to sparsely hirsute, and usually an eglandular plant of up to 100 cm in height, which grows on different dry and stony habitats [2,3,10] [ Figure 4].   S. recta L. subsp. subcrenata (Vis.) Briq. has a very similar appearance and distribution as compared to the S. recta L. subsp. Recta, but has narrower leaves and often a glandular calyx with unequal teeth [2,3,10,11] [ Figure 5].
S. sylvatica L. (hedge woundwort) is an erect and hirsute perennial found in shady spots in woodland, forests, roadsides, alpine meadows, and grasslands, growing up to 120 cm. It differs from marsh woundwort in habitat preference, the broader leaves, and its characteristic, unpleasant smell when crushed, but the two can hybridise where they co-exist [2,3,10,11] [ Figure 6]. S. recta L. subsp. subcrenata (Vis.) Briq. has a very similar appearance and distribution as compared to the S. recta L. subsp. Recta, but has narrower leaves and often a glandular calyx with unequal teeth [2,3,10,11] [ Figure 5]. S. sylvatica L. (hedge woundwort) is an erect and hirsute perennial found in shady spots in woodland, forests, roadsides, alpine meadows, and grasslands, growing up to 120 cm. It differs from marsh woundwort in habitat preference, the broader leaves, and its characteristic, unpleasant smell when crushed, but the two can hybridise where they co-exist [2,3,10,11] [ Figure 6].   S. sylvatica L. (hedge woundwort) is an erect and hirsute perennial found in shady spots in woodland, forests, roadsides, alpine meadows, and grasslands, growing up to 120 cm. It differs from marsh woundwort in habitat preference, the broader leaves, and its characteristic, unpleasant smell when crushed, but the two can hybridise where they co-exist [2,3,10,11] [ Figure 6].  S. obliqua Waldst. et Kit. is an erect lanate-hirsute perennial with oblong-lanceolate leaves that is native to the Balkan Peninsula [12] [ Figure 7].
S.germanica L. (downy woundwort, german hedgenettle) is a densely tomentose and ascendent or erect biennial or perennial with well-marked basal rosettes, growing up to 120 cm and found in light forests, shrubby sunny slopes, rocks, and cut-over areas [11,12] [ Figure 8].
S. salviifolia Ten. (Mediterranean woundwort) is a densely tomentose or lanate-tomentose perennial growing up to about 80 cm that is widely distributed on rocky places in the littoral areas [2,3,10,11] [ Figure 9].  S.germanica L. (downy woundwort, german hedgenettle) is a densely tomentose and ascendent or erect biennial or perennial with well-marked basal rosettes, growing up to 120 cm and found in light forests, shrubby sunny slopes, rocks, and cut-over areas [11,12] [ Figure 8].   S.germanica L. (downy woundwort, german hedgenettle) is a densely tomentose and ascendent or erect biennial or perennial with well-marked basal rosettes, growing up to 120 cm and found in light forests, shrubby sunny slopes, rocks, and cut-over areas [11,12] [ Figure 8].  Stachys menthifolia Vis. is an endemic species that was first described by Roberto Visiani (1829) at the locality of Kotor (Montenegro), who also identified this species in habitat near Dubrovnik (Croatia) [13]. According to the Flora Europaea, the species is endemic of the Balkan Peninsula, and spread in Albania, Greece, and Yugoslavia [3]. In 2002, Šilić and Šolić identified new localities in the Stachys menthifolia Vis. is an endemic species that was first described by Roberto Visiani (1829) at the locality of Kotor (Montenegro), who also identified this species in habitat near Dubrovnik (Croatia) [13]. According to the Flora Europaea, the species is endemic of the Balkan Peninsula, and spread in Albania, Greece, and Yugoslavia [3]. In 2002, Šilić and Šolić identified new localities in the eastern part of the Biokovo Massif [9]. S. menthifolia Vis. is a calciphilic plant, with the distinctive feature of a short, glandulary, and pilose stem, and is mostly present in the fissures of limestone or in the open limy rocky grounds, as in the Croatian localities. In the majority of the Montenegrin localities, the plants grow individually or in small groups, often in the strata of thermophilic woods and the underbrush of pubescent oak (Quercetalia pubescentis), in stone fissures, or in open surfaces, while the habitats of the new Croatian localities are mainly open limy rocky grounds with denser populations composed of larger individuals [9,14,15].

Essential Oils
Similar to many other representatives of the family Lamiaceae, Stachys species produce essential oil. Essential oils are complex mixtures and in spite of the large size of the genus Stachys, the composition of the volatile compounds is known in only a small number of species.
Isocaryophyllene and b-caryophyllene, which are dominating constituents of the previously investigated essential oil from one population of S. officinalis in Montenegro [16], were totally absent in that from a population in Serbia, with germacrene D being the main compound [17]. Another study conducted in Serbia also confirmed sesquiterpene hydrocarbons as being the main fraction and identified germacrene D, β-caryophyllene, and α-humulene as the main constituents [18].
In a study conducted on samples from Croatia, germacrene D and (E)-caryophyllene were the main compounds of S. officinalis, with isocaryophyllene being totally absent [1].
Concerning the prior investigations of S. sylvatica, it is noteworthy that germacrene D is present in a large amount in the sample from Italy [19], and was also present in high proportions in the essential oil from Serbia [17], while the monoterpene fraction is present in all of the samples in a very low percentage only. S. sylvatica essential oil from Croatia was also rich in germacrene D, but in contrast to the oil from Italy and Serbia, monoterpene hydrocarbons were found in a large amount with α-pinene and β-pinene being the most abundant constituents of this fraction. The investigation of the chemical composition of essential oils from S. sylvatica collected from three different wild populations in Kosovo [20] showed that the leaves and flowers of S. sylvatica were characterised by three main constituents: α-pinene, β-pinene, and germacrene D, which is aligned with the findings on the main constituents that were identified in a study conducted in Croatia. Essential oil obtained from S. sylvatica sample collected in Turkey contained high proportions of germacrene D, α-pinene and β-caryophyllene, but the β-pinene constituent was not identified [21]. Another study conducted on a S. sylvatica sample collected in Turkey also implicated monoterpene hydrocarbons as the major constituents of essential oil, but with difference in a composition, with γ-muurolene, α-cedrene (11.2%), and limonene being the most abundant components [22].
Sesquiterpene hydrocarbons were the main group of essential oil constituents of S. salviifolia and S. palustris sample from Croatia, with germacrene D being the main constituent of S. salviifolia, in contrast to S. palustris, which contained a significant aldehyde fraction and a high amount of alcohols [1]. The essential oil from the aerial parts of S. palustris from both samples from Italy was characterised mainly by carbonylic compounds, fatty acids, and their esters, along with sesquiterpenoidic compounds and phenols [23,24]. The composition of the essential oil from one population of S. recta from Serbia [17] was similar to the oil from a population in Turkey [31] by the preponderance of 1-octen-3-ol. In contrast to the oil from one population in Greece [25] and to the oil from Turkey, in the oil from Serbia, linalool was present in very low percentage only. An analysis of S. recta oil from another population in Serbia revealed a different composition compared to other oils; germacrene D and E-caryophyllene were the main constituents of the essential oil, while 1-octen-3-ol was present in very low proportions, and linalool was totally absent [17]. The reason for these variations in the chemical composition of the essential oil was probably because there are different subspecies of S. recta, which can differ in their essential oil composition.
Two subspecies of S. recta (S. recta subsp. recta and S. recta subsp. subcrenata) that were analysed in the Croatian study were collected at the contiguous habitats. Although both subspecies had a high amount of sesquiterpene hydrocarbons, their composition differed; germacrene D was the main component of S. recta subsp. subcrenata essential oil, but its level is low in the S. recta subsp. recta oil. S. recta subsp. subcrenata was also characterised by a high percentage of oxygenated sesquiterpenes, whereas S. recta subsp. recta had a very low amount of these compounds. On the other hand, S. recta subsp. recta was rich in aldehydes, and had a rather high amount of ketones and fatty acids, which were present in the essential oil of S. recta subsp. subcrenata in a very low percentage only [1].
Stachynone and stachynene are constituents of the essential oil of S. officinalis and S. recta, as reported by Maly (1985), and were absent in all of the essential oils that were studied. One reason may be that these compounds decompose in the gas chromatographic injector [29].
Sesquiterpene hydrocarbons were the main group of constituents found in S. menthifolia, as demonstrated for several other Stachys species [25]. Abietatriene and 13-epi-manoyl oxide were present in high amounts in S. menthifolia from Greece [25], while samples collected in two different habitats in Croatia (Biokovo and Dubrovnik) differed in their essential oil composition. The major constituent found in the essential oil from the sample collected in Dubrovnik was, as in the sample from Greece, diterpenoid abietatriene (11.7%), with significant amounts of sesquiterpene hydrocarbons α-bisabolene (8.4%) and β-caryophyllene (7.4%). Oxygenated sesquiterpenes were the most abundant volatiles in the sample from Biokovo, followed by diterpene hydrocarbons. The content of sesquiterpene hydrocarbons was much higher in the sample from Dubrovnik, which also showed no presence of 4'-methoxyacetophenone. The authors attributed the differences in chemical oil composition to the different harvesting dates of the samples [14,15].
The S. alpina sample from Croatia showed the presence of high amounts of oxygenated sesquiterpenes, with a significant aldehyde fraction portion [1]. S. germanica essential oil samples from Serbia and Italy also showed oxygenated sesquiterpenes as the main constituents, with a sample from Serbia having a large amount of monoterpene hydrocarbons as well (11,3%) [17,24].
A similar composition was found for S. obliqua samples collected in Turkey, which had a significant portion of monoterpene hydrocarbons and a very high percentage of oxygenated sesquiterpenes with germacrene D as the main constituent [26][27][28].
A detailed overview of the main components (sesquiterpenes and monoterpenes) in the essential oil of Stachys taxa reviewed is presented in Tables 1 and 2.

Other Biologically Active Substances
Similar to most of the plants belonging to the Lamiaceae family, the Stachys taxa have been submitted to several investigations in order to determine the content of the biologically active compounds. These investigations have reported the presence of flavonoids, phenolic acids, iridoids, and terpenoids.

Discussion and Conclusions
The presented studies revealed some differences between Stachys taxa, indicating the existence of a chemical polymorphism. The taxonomic relationships within the taxa are complex, and have been the objective of several investigations. Although different types of biologically active substances were determined within the investigated Stachys species, the main differentiation is based on essential oil and fatty acid composition. Tundis et al. (2014) focused on taxonomy within Stachys and Betonica subgenera, as well as infrageneric differentiation within Stachys species focusing on these chemotaxonomical markers. Based on the collected data on essential oil, three groups of Stachys species were observed: species mainly characterised by monoterpene hydrocarbons (to which S. sylvatica was assigned), species mainly characterised by oxygenated monterpenes (to which S. recta was assigned), and species that contain similar amounts of sesquiterpene hydrocarbons and oxygenated sesquiterpenes [35].
The essential oil approach, including its fatty acid composition, was taken by Bilušić Vundać (2006), who through principal components (PCA) and hierarchical cluster analyses (CA) differentiated seven Stachys taxa to two distinct groups, first comprising S. alpina, S. recta subsp. Recta, and S. palustris, and second comprising S. officinalis, S. sylvatica, S. recta subsp. subcrenata, and S. salviifolia. The species within the first group showed a high content of aldehyde components, and in the case of S. alpina and S. palustris, a high content of alcohol, while in the species within the second group, germacrene D was identified as the main component.
The essential oil and fatty acid approach was also used in chemotaxonomic differentiation by Kiliç at al. (2017), who defined germacrene D/β-caryophyllene the essential oil chemotype and 6-octadecanoic acid fatty acid chemotype for S. sylvatica [21].
The chemical investigations of Stachys species have determined the presence of monterpenes, sesquiterpenes, diterpenes, iridoids, phenylethanoids, flavonoids, and fatty acids as the main biological constituents. The content of these components and their occurrence in investigated species is variable, due to many external factors that impact the secondary metabolite composition of the plant species [1,35].
The differentiation of the presented Stachys species based on the available data on their chemical composition requests further chemotaxonomical and molecular research in order to present a key for infrageneric classification. Througout history, basic plant species and subspecies identification has relied mostly on morphological traits. For morphological characterisation, taxonomists have formulated a descriptor list, such as leaf shape and colour, flower colour, etc. With the development of scientific methods, chemotaxonomy research increased in order to enable separation within the taxa based on the presence and quantity of a specific chemical compound [38]. In recent studies, the authors have tried to resolve the classification of Stachys taxa based on the prevalent essential oil compositions from the aerial parts [10,21,26]. Nevertheless, the results vary, since different geographic localities, seasons, harvest periods, properties of soils, and climatic conditions strongly affect the secondary metabolite composition of plant species, especially their essential oil composition [1,21,35]. Therefore, traditional evaluation methods and chemotaxonomy should be combined with molecular markers that are not influenced by the environmental factors, for a better distinction among Stachys taxa.
Taking all of the above into consideration, a potential suggestion for differentiation can be suggested within the presented investigations only for two S. recta subspecies, which were collected at identical conditions and habitat (time of collection, harvest period, soil, and climate), which could support the opinion of Visiani (1829), who described S. recta subsp. subcrenata as separate species and named it S. subcrenata [10].
Although many constituents of this genus have been already identified, further investigations of Stachys spp. are required in order to clearly identify the chemotaxonomical markers for specific taxa and clarify their taxonomic relationships.
Funding: This research received no external funding.