The change in the chemical composition of essential oils is influenced by soil conditions , climate and the conditions of plant crops and conservation  method, it makes sense that their values differ from place to another place

Dear Prof. Mohammed

Can you please provide some examples with reference to specific aromatic crops and give any associated references?

Thank you for your response.

Good luck.

A 2% yield of essential oil obtained from Thymus vulgaris leaves (cheurfa et al., 2013), compared to that given by dob et al 2006 obtained from the stems and leaves of Thymus fontanesii (0.9%) .

Cheurfa M, Allem R, Sebaihia M, Belhireche S. (2013). Effet de l’huile essentielle de Thymus vulgaris sur les bactéries pathogènes responsables de gastroentérites. Phytothérapie, 11:154-160.

Dob T, Dahmane D, Benabdelkader T, Chelghoum C (2006) Composition and Antimicrobial Activity of the Essential Oil of Thymus fontanesii. Pharmaceutical Biology, 44:607 – 612.

and also, the ratio of different parts of plant which are  harvested  for extracting. stem, leaf, flower, etc. another factor is time of harvesting until extraction. storing time and conditions of storing. variation of genotypes and many other factors

provenance, weather and soil conditions, time of harvest and the drying technique can , over time , change the chemical profile of essential oils. Thus, the chemical composition depends on all these factors that can direct the biosynthesis for certain terpene molecules depends on other components whose synthesis can be stopped

36. Kulkarni N., Baskani K., Raresh S., and Kumar S., (1996), Intra-clonal variation for essential oil content and composition in plants derived from leaf cuttings of rose scented geranium (Pelargonium sp). Industrial crops and products, 6, 107-112.

37. Loziene K. and Venskutonis P.R., (2004), Influence of environmental and genetic factorson the stability of essential oil composition of Thymus pulegioides. Biochemical systematics and ecology, 33, 517-525.

38. Rajeswara Rao B.R., Kaul P.N, Mallavarapu G.R., and Ramesh S., (1996), Effect of seasonal climatic changes on biomass yield and terpenoid composition of rosescented geranium. Biochemical systematics and ecology, 24, 627-635.

39. Boira H. and Blanquer A., (1997), Environmental factors affecting chemical variability of essential oils in Thymus piperella L. Biochem. Systematics and Ecology, 26, 811-822.

Dear B.R. Rajeswara Rao

We proposed a hypothesis (and found statistical proof to back it up) that there is a possible link between the oil yield and the oil composition. The main volatiles of essential oil-poor species (yields less then 0.1%) are fatty acid- and carotenoid derived compounds, while essential-oil rich taxa (essential oil yields much higher then 0.1%) are generally characterized by the specific production of mono- and sesquiterpenoids, and/or phenylpropanoids. Please, if interested, take a look at the attached papers.

Best regards,

Niko

Dear Niko

It is an interesting finding. Many thanks for your papers. Do you have publications on the factors that influence the yield or composition of the essential oils such as environment (location, altitude, light, temperature, wind, rainfall etc.), genetic make up of the plant (varieties, chemotypes, morphotypes etc.), physiology of the plant (plant parts, plant age, ontogeny of plant parts etc.), agronomic management (nutrients, pests and diseases, irrigation, pesticide application, harvesting height. harvesting date etc.), post-harvest technology (drying biomass, cutting biomass into small pieces etc.), distillation (method, duration, process parameters etc.), storage (container, presence of air/water, storage period etc.), transport and other factors.

Best wishes and good luck.

The synthesis of secondary metabolites in aromatic plants is mainly controlled by genetic factors, however, numerous other factors can also significantly affect the yield and chemical composition. These factors mentioned above in detail by different researchers. Sometimes it become very difficult to distinguish these factors from each other as many are interdependent and influence one another.

In addition to various factors, diurnal variation (harvesting in day time/night: morning, noon, evening, night) is also an important factor which affects essential oil yield and chemical composition significantly. Following articles on Ocimum gratissimum and Lantana camara are good examples of diurnal variation.

Kpoviessi et al., 2012, Chemical Variation of Essential Oil Constituents of Ocimum gratissimum L. from Benin, and Impact on Antimicrobial Properties and Toxicity against Artemia salina Leach. Chemistry & Biodiversity, 9, 139.

Silva et al., 1999, Chemical variation during daytime of constituents of the essential oil ofOcimum gratissimum leaves. Fitoterapia, 70, 32-34.

Sousa et al., 2010. Effect of Collection Time on Essential Oil Composition of Lantana camara Linn (Verbenaceae) Growing in Brazil Northeastern. Rec. Nat. Prod. 4:1 (2010) 31-37.

In my opinion, exposure to stress is the major factor for production of EOs in any plant. Higher the stress higher the chances for production of volatile components to overcome environmental factors. Its not like that those plants which have been given full attention(fertilizers, pestiscides, hyper care) gives higher yield of EO because they do not have to fight with other factors of envirnoment. Its like the human life, those people who has survived from great struggle are more knowledgable and practical as compared to other people who has sophisticated life style from the birth. So struggle or stress is most important factors for high yield of esssential oils. Of course other factors like genetic, weather, soil, time of harvest, parts of harvest plays important role in yield quality and quantity. But stressed plants can produced more EOs.

Also be aware that pre and post harvest controls can provide adulterants (pesticides and mycotoxin, respectively).

Dear Phil

Can you please provide me some references on mycotoxin and pesticide contamination of commercially important essential oils?

Best wishes.

Here are a couplr - I'll look in my files for others

http://pubs.acs.org/doi/abs/10.1021/jf990331z

http://www.sciencedirect.com/science/article/pii/S0045653504002929

Also compositional differences based on method of extracfion

http://www.sciencedirect.com/science/article/pii/S030881460300534X

Thanks for the informative links.