Results available regarding the influence of phenology on essential oil chemical composition are often dissenting and even with conflicting results and are highly dependent on the species (Table 4). The phenological variation has been reported as highly noticeable both in wild collected plants and in cultivated plant material. Nonetheless, cultivation provides more clarity and easier enforcement to understand the behaviors and patterns. Some aspects must be taken into account when referring to phenological changes in the essential oil components isolated from distillation of aerial parts. First of all, different developmental stage involve different plant parts collected and with different harvesting ratios. Flowers, new leaves, growing steams and fruits can be collected depending on the phenology. The irregular allocation of essential oil compounds has been suggested in several studies (Bertoli et al., 2004; Bettaieb et al., 2010; Tounsi et al., 2011). Flowers of numerous species have been reported with remarkable different composition compared with leaves (Table 6). The level and activity of various terpene synthases involved in the production of essential oils is presented differently between plant parts and also between species considering the same
organ. The methyl-erythritol phosphate (MEP) pathway which gives rise to the formation of monoterpenes and diterpenes and the cytosolic mevalonate (MVA) pathway from which sesquiterpenes result may be boosted differentially between different plant organs (Degenhardt et al., 2009). The content of sesquiterpenes was reported in a remarkable higher content in flowers of Rosmarinus officinalis and Psoralea bituminosa with a large presence of β-caryophyllene, ß- farnesene, and α-humulene, reaching up to 45% and 80% of total identified compounds, respectively (Bertoli et al., 2004; Yosr et al., 2013). Similarly, fragrance from Cananga odorata due to its flowers is mainly produced by the predominance of sesquiterpenes in its chromatographic profile mainly by the involvement of sesquiterpene synthases (Jin et al., 2015). On the other hand, flowers of Eucalyptus oleosa and Salvia officinalis showed high percentage of 1,8-cineole as major oxygenated monoterpene (Perry et al., 1999; Ben Marzoug et al., 2011). In flowers of Pelargonium graveolens oxygenated monotepenes, represented 70.3% of the total essential oils, β-citronellol, geraniol and δ-selinene being the dominant components (Boukhris et al., 2013). The chemical composition of the essential oil from flowers of Salvia lavandulifolia (Spanish sage) was remarkably different to that described for leaves and stems and in turn totally different compared with its close species S. officinalis. This essential oil mostly contains linalool (36.9%) and linalyl acetate (17.9%) which were only described as traces in leaves and stems being in turn characterized by 1,8-cineole, camphor, α-pinene, β-pinene and limonene throughout the whole phenology (Usano-Alemany 2012). These two main compounds of flowers are also the most representative ones in essential oils of other species as lavender (Lavandula angustifolia) and spike lavender (Lavandula latifolia) (only linalool), species that share habitat with Spanish sage and whose essential oil are basically obtained from their inflorescences. Hence a distinct ecological and functional character of the volatiles is suggested in that case. Another interesting case of a very polarized chemical composition between plant parts is given by Eryngium species. Some species like E. corniculatum, E.
campestre or E. planum provide high content of valuable compounds such as phyllocladenes and falcarinol in their essential oils isolated only from roots (Thiem et al., 2001; Palá-Paúl et al., 2008, 2010).
In the above mentioned case of Lavender species and many others, in which the isolation of essential oils is undertaken by distillation-extraction of concrete parts, the chemical composition is given by the content in this organ. Nonetheless, species in which whole aerial parts are subjected to distillation, the relation between leaves, stems and flowers is an important fact to consider when phenology is evaluated. Volatile organic compounds are important components of plant´s chemical phenotype and their ecological relevance has been raised for decades (Maffei et al., 2011). Leaves, flowers, fruits and roots produce them as pollinator-atractors and seed dispersers, defense against herbivores and pathogens and for signaling involvement in plant-plant communication. These ecological roles determine their different distribution within plant organs and eventually, may have major implications when aerial parts are subjected to distillation. It is therefore very important for the evaluation and reporting of phenological studies to clarify this relationship between plant organs when aerial parts are collected for distillation.
Table 6. Essential oil main compounds reported from different organs in representative wild and cultivated aromatic and medicinal plants
Species
(a) Main essential oil components Plant part* Strength of
differences
Leaves Steams Flowers Fruits
1. 2-Nonanol/Trans-Linanool oxide
2. p-Cymene/γ-Terpinene/Carvacrol 6.3/5.4/54.1 - 3.5/6.3/77.6 5.4/5.4/76.7 Strong
3. α-Pinene/1,8-Cineole/Camphor
5. β-Pinene/Bornyl Acetate/γ-Terpinene 0.51/5.27/1.62 13.6 /23.8 /- 2.0/0.2/51.7 - Strong
6. β-Pinene/Pinocarvone
(a) Species correspond to those reported in Table 3. *All data presented are referred to mean values published as %.
Figure 2. Temporal patterns of the essential oil chemical composition of Salvia lavandulifolia throughout four years of cultivation. Three groups of correlated compounds are presented. Group 1 comprises α-pinene, the sum of β-pinene + myrcene, group 2 includes camphene, limonene, camphor, (E)-β-caryophyllene and caryophyllene oxide and group 3 corresponding to 1,8-cineole. LF: leaves formation, FB: Full bloom, SM: seeds maturation.
Another important factor is the distribution and the total number of glandular trichomes.
Once the essential oil is formed and accumulated, it is stored in the oil glands for a long time.
An analysis comprising a mixture of leaves of different age would not allow us to estimate the influence of environmental conditions (Grausgruber-Gröger et al., 2012) and might be behind some of the chemical differences found according to the phenology. Essential oils composition is highly related to the glandular trichomes distribution, morphology and anatomy. That is the case of Mentha × piperita in which there was a strong correlation with differences in monoterpenoids (Rios-Estepa et al., 2010). The outcome of monoterpene and sesquiterpene synthases activity is stored in these epidermial glands and hence their location and developmental stage is crucial for the subsequent quality that can be obtained.