Essential Oil Composition of Species in the Genus Achillea
Abstract Based on the present knowledge of essential oil composition of species belonging to the genus Achillea the factors, which may influence the composition with regard to plant biology, production and application are discussed. According to studies from the last 15 years, a mean of 54 compounds have been identified in samples of different species. Among them, the largest number of components (149 compounds) were found in the oils of A. millefolium, A. pannonica and A. collina. The monoterpenes, 1,8-cineole, camphor, bomeol, α- and β- pinenes are among the five most abundant components. Beside chamazulene, the most frequently identified sesquiterpenes are β-caryophyllene and its oxide. The presence of chamazulene seems to remain a characteristic, but it is not ubiquitous to the members of the Millefolium group. The heritance mechanism of sesquiterpenes, especially chamazulene, seems to be established, while we know relatively less about the genetic regulation of the monoterpene compounds. During ontogenesis, major differences could be found between the stages before and after flower development. The phenological phase assuring the highest level of azulenes seems to be during flowering. Composition and compositional changes of an essential oil within the Achillea genus in different plant organs seems to depend on the species. In several cases a dominance of sesquiterpene components above the monoterpenes was found in the vegetative organs. The most important difference seems to be the lack or low amount of chamazulene as artefact in the extracts compared to the distillates. Key Word Index Achillea millefolium, Achillea sp., Asteraceae, yarrow, essential oil composition, taxonomy, chemotype, ontogenesis, morphogenesis, ecological factors, 1,8-cineole, camphor, borneol, chamazulene. Introduction Yarrow (Achillea) species may be mentioned as "evergreen" tools in therapeutic practice. Both in the ethno-pharmacology and in the up-to-date phytotherapy they assure a valuable source of natural remedies. The name of the genus might originate from the name of the Greek hero Achilles, who used this plant for curing his wound. Yarrow species were mentioned in ancient books of the Middle Ages and throughout the centuries. The majority of the species are used in their source countries as one of the most important medicinal plants against different complaints. Today, several therapeutic applications are approved by scientific experimental results. The whole overground parts but primarily the inflorescences are effective as anti-inflammatory, spasmolytic, choleretic drugs. Essential oil and extracts of the plants are used for preparation of cosmetics, stomachic and digestive teas, creams, etc. As yarrow species are widely known and utilized, they have also been the topic of several pharmacological, anatomical and biological investigations. In this article we want to give an overview of the state of knowledge on the essential oil composition of species of the genus Achillea and the factors which may influence it in practice. Taxonomical Aspects & Difficulties The genus Achillea consists of more than 120 perennial herb species being widespread in the Northern Hemisphere. A large number of species are endemic and restricted to certain regions, in contrast to other species from the genus growing over a wide geographical range. Concerning the pharmacological significance of yarrow, the most important species belong to the group Millefolium (Table I) According to the Flora Europaea (1) this group consists of eight species, some of which (A. distans W. et K., A. millefolium L.) are divided into subspecies. The taxonomical status of species in group Distans might be questionable, belonging (1,2) or not (3,4) to the same group. Sometimes even further species may be mentioned as members of the Millefolium group (4,5). According to Ehrendorfer (6), A. ceretanica also belongs to the diploid basic species of this group. In particular cases some species had formerly been known as cytotypes or subspecies of other species (e.g. A. pratensis Saukel et Lnger). North-American or Asian species (A. lanulosa Nutt., A. borealis Bong. ) may also be identified as members of this group (7). The group is a polyploid complex (n=9) with species from the diploid to the octoploid level. Although the chromosome number of the six major species of the group already seems to be well defined, aneuploids also often occur (3,4,8), presumably as a result of interspecific hybridization. The relative easy hybridization, vigor and fertility of the progenies could be proved in crossing experiments. In consequence, the division of the Millefolium group into "small species" does not yet seem to be scientifically solved or accepted despite to intensive efforts (6). Table I. The Achillea species mentioned as members of the Millefolium group (section) Determination of the correct species is also difficult even using morphological traits because hardly any proved to be stable enough. As a result of hybridization, the morphological traits may show a continuous line. The spontaneous origin of autopolyploids has also been shown (9). Furthermore, the species of this genus may exhibit phenocopies as result of an adaptation process to diverse environmental conditions. Several Achillea taxa show a high morphological variability as a consequence of ecological effects. The unramnified forms of A. millefolium L. can easily be mistaken for A. collina Becker (10) under arid conditions. According to Gurevitch (11), the leaf dissection of populations belonging to the A. millefolium complex differs dramatically along an altitudinal gradient in the Sierra Nevada. This phenotypic variation consists of both genetic and other components. Biste (7) described considerable variations in height, leaf width, shoot number, branching and stomata length in populations of different origin of the same species. Even the characterization of the same plant individual after re-shooting in autumn may basically differ from that of the main growth period (12). Rauchensteiner et al. (5) recently declared that the morphology of the leaflets and rayflorets were the most suitable traits for characterization of the species. Since the classification of individual species of genus Achillea is complicated, the names and descriptors used in this review are those used by the authors of the original papers. Characteristics of Essential Oil Composition of the Genus Until the last decade, chamazulene used to be considered as the most important component of the essential oil and references almost exclusively dealt with characterization of its detection. In recent studies, in parallel with the development of analytical methods, we now find a more detailed analysis which results in a more comprehensive description of the total essential oil of Achillea species. According to studies from the last 15 years, a mean of 54 compounds have been identified in samples of different species (Table II). Among them, the largest number of components (149 compounds) was found in oils of A. millefolium, A. pannonica and A. collina (13). By evaluating the published results, it can be concluded that species belonging to this genus show several similarities concerning their oil composition. 1,8-Cineole exhibits the most frequent appearance among the monoterpenes. It has been described in about one-third of the species at least in one case as main component (Table II). Besides results summarized in the table, 1,8-cineole was detected in the oils of some further species, such as A. oligochala (14), A. teretifolia (15) and A. compacta (16). According to the published data, it can be established that besides 1,8-cineole, compounds of bornane skeleton such as camphor and borneol are among the second and third most frequently characterized components of yarrow oil (Table II). Camphor was described eight times and borneol three times as the main compound of an Achillea oil. Combinations of these monoterpenes as major components have also been frequently detected: camphor, borneol and 1,8-cineole were the main compounds in A. taygetea and A. fraasi (17), camphor and 1,8-cineole in A. albicaulis C. A. Mey. (18), A. pseudoaleppica Hub.-Mor. (19), A. pachycephala Rech.f. (20), A. talagonica Boiss. and A. vermicularis Trin (21). α- and β-Pinenes are also among the five most often detected components, especially in the group Millefolium (22). Beside A. millefolium, the pinenes were described as main components in four other species (Table II). Hofmann (23) also mentions the monoterpenes belonging to the p-menthane (in 51%), thujane (in 23%) and pinane (17%) skeletons as being the most frequent components of the oils of the investigated A. millefolium populations. Further compounds have occasionally been found as main components in yarrow. Thujone has been characterized in four species (Table II), notably in 70% of the oil of A. multifida (DC) Boiss. (24). Piperitone was also found in three species, ascaridole in two cases, linalool and limonene each in one species. The irregular skeletons artemisia ketone [A. ageratum L., (25); A. ligustica All., (26), A. pseudoaleppica Hub.-Mor., 19] and artemisia acetate [A. filipendulina Lam., (27)] have been found to occur in major proportions in some species. Sesquiterpenes have been found in a considerable number of species in the genus. Chamazulene has also been the object of several studies. The most frequently identified sesquiterpenes besides chamazulene were β-caryophyllene and its oxides (main component in three species), α-bisabolol and oxides, eudesmol, furthermore farnesene (each in two species). According to Hoffmann (23), sesquiterpenes are mostly characteristic to the taxa of lower (2n-4n) chromosome number, while monoterpenes to the ones of higher ploidy level. However, many other references do not confirm this generalization. Table II. Compounds in the distilled flower-head oils of Achillea species that exceed 5% (According to references 1-110) Table II. Compounds in the distilled flower-head oils of Achillea species that exceed 5% (According to references 1-110) Kstner et al. (28) identified 13 substances (which may serve as additional tools for identification of plant material in Mittefolium group) because their proportions proved to be independent on external factors and constant to each-other. However, this could not spread out until now. According to recent investigations, the significance of the enantiomeric composition in distinguishing and characterization of species is emphasized. Orth et al. (29) showed that the enantiomeric distribution of oil components depends neither on the habitat nor on the developmental stage or the method of isolation. However, as the enantiomeric ratios of the chiral monoterpenes α-pinene, β- pinene and sabinene from several Achillea species are different (29), they seem to represent taxonomically useful markers. While checking the ratios of hybrid strains of A. millefolium agg., Steinlesberger (30) found no correlation between the morphometric and enantiomeric parameters. In the future, numerous results are necessary to make a firm conclusion about the chemotaxonomical role and practical significance of optical isomers of terpenoids in yarrow oil. Presence of Chamazulene in the Essential Oils of Yarrow Species Until now, the majority of references have been engaged in the evaluation of chamazulene content as main component in the distilled oil. It is known to be the thermal degradation product of matricine (a proazulene) during steam distillation. As in the vast majority of literature references, there are only data on "azulene" or "chamazulene" content without mentioning and examining the type of proazulene compounds; here we are not engaged in the chemical details of the genuine guaianolides either. Numerous studies state that the presence of chamazulene seems to remain characteristics of the members of Millefolium group (Table II). Only in exceptional cases can references be found describing chamazulene in species outside of this group: in A. ageratum L. (31), A. wilsoniana Willd. (32) or A. compacta (16). The presence of azulenes is not a universal phenomenon for each species within the group Millefolium. Many contradictions can be found in the literature concerning the content of chamazulenes in the individual species. Beside the obvious differences in consequence of diverse isolation methods, the compositional differences seem to also have biological-genetic backgrounds, worth discussion. Table III. Presence of azulenes in the species of the Millefolium group according to different references Some authors declare a defined connection between the species' chromosome number and the potential for accumulation of chamazulene. Oswiecimska(33) associates the presence of chamazulenes with the tetraploid level, while she described the hexa- (A. millefolium L.) and octoploids (A. pannonica) as being azulene-free taxa. She did not, however, exclude the existence of azulene-free tetraploids, beside the azulene-containing ones which she investigated (A. collina and A. asiatica Serg.). In oils of A. asiatica, Yusubov et al. (34) and Kalinkina et al. (22) also found chamazulene. The presence of azulenes in polyploids is supposed to depend on the chemism of the original diploid species (azulene containing or azulene-free), which might be the parents of the allo- or auto- tetraploids. This was recently supported by the work of Rauchensteiner et al. (5) who described A. pratensis and A. styriaca as tetraploid species devoid of chamazulene. Beside tetraploids, Bugge (35) established that some diploids (A. asplenifolia and A. roseo-alba) may also contain high levels of chamazulene. Studies on this latter species are scarce (28). According to morphological traits, Ehrendorfer (3) supposed that its origin could be traced back to the spontaneous hybridization of A. setacea W.et K. and A. asplenifolia among which the latter has the potential for proazulene synthesis. During investigations on different populations belonging to the group Millefolium, Hofmann and Fritz (36) could not prove the correlation between ploidy level and presence of azulenes. They only established a decreasing tendency of chamazulene content with a growing number of chromosomes. Lithuanian authors reported some A. millefolium L. ssp. millefolium populations having chamazulene as main compound and some others lacking of chamazulene but containing different monoterpenes as major component (β-pinene, borneol, cineole, camphor, nerolidol) (37). Comparing the references on the chemism of yarrow, several different data can be found concerning the name of species, the number of chromosomes and presence of azulenes (Table III). The mentioned interspecific hybridization may be one of the reasons why the definition and chemical characterization of certain taxa is often contradictory. Orth et al. (38) investigated five distinct taxa of which three could be defined as A. collina Becker and containing chamazulene, while one chamazulene-free triploid and a chamazulene-rich diploid were concluded to be spontaneous hybrids. Also, Dabrowska (8) reported a tetraploid chamazulene containing taxon which might also be of hybrid origin, according to morphological traits. Earlier, Ttnyi (39) noted in his chemotaxonomical review that seven of the mentioned 13 Achillea species were both azulene-free and azulene-containing according to different references. As result of recent investigations Radusiene and Gudaityte (40) supposed that the rapid identification ofproazulene-containing plants might be solved according to their productivity. In numerous analytical reports the exact definition of the taxon seems to be missing. Presumably, the description of the taxa was carried out according to insufficient botanical-systematical examinations, as presumed also by Kastner et al. (28). These investigations have given rise to confusing results: the chamazulene content of A. millefolium oil varying between 0% and 85% (41-48). We can generally conclude that in the group Millefolium the accumulation of azulenes seems to be restricted to A. asplenifolia (2n), A. roseo-alba (2n) and A. collina (4n), while azulenes are absent in A. setacea W.et K. (2n), A. millefolium (6n) and A. pannonica (8n). Results which do not agree with these findings might be traced back to the false definition of the species focusing on morphological traits in the extremely variable group Millefolium without chromosome counting; or on the contrary, focusing on chromosome numbers and mistaken polyploids of azulene-free and azulene containing diploid taxa. Chemotaxonomic Aspects of Essential Oil Variability Today the existence of intraspecific chemical variability concerning essential oil composition seems to be a well known phenomenon which must be taken into consideration both theoretically and practically. In principle both qualitative (components present or absent) and quantitative (components in considerably different proportions) chemical races might be present within a species (39). The species of the genus Achillea may serve as examples for both. At the same time the increased sensitivity of analytical methods drastically decreases the limit of detection and as a result, several taxa have become only quantitatively distinguishable, in contrast to previous examinations where qualitative differences had been described. From the cited references it can be concluded the number of the identified main components ranges from one to three compounds in the majority of species. In most cases these different chemical races had been detected separately and mentioned by different authors which makes the comparison and evaluation of data more complicated. Of course, the number of the reported main components in the oil may to some extent also depend on the frequency of investigations of the target species. In the oil of the most often investigated taxon called "A. millefolium," 10 different main components could be found according to the different references (Table IL). Evaluation of distinct chemotypes within the same study is rarely found, but the first results were reported 40 years ago: populations of A. asplenifolia Vent. devoid of or possessing a low or high content of chamazulene had been described by Tyihk et al. (49).
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