CHEMOTYPY / CHEMOTYPE
Chemotypy is exhibited by several plants, more and more are being found all the time as several specimens of the same plants from around the World (or around the UK) are chemically analysed. Even the toxins any single species produces can vary from country to country, soil-type to soil-type.
With plant chemotypes, the only way the plants differ from each other is by the nature of the secondary metabolites that they produce. It could be the same secondary metabolites but in differing proportions. Or it could be an entirely differing set of secondary metabolites. The plants can accomplish this by slight alterations to their genes which control the production of these toxins or secondary metabolites. This need not involve differing genes, they can accomplish this feat by gene methylation, an epigenetic process whereby the genes remain intact; it is just the expression of those methylated sections which are inhibited by the creatively attached methyl groups. Plants which exhibit chemotypy are not sub-species, but rather they are plants with the same binomial name, which have learned, in one place, to produce one set of secondary metabolites and in another place or set of circumstances to produce a differing set of secondary metabolites which are more capable of defending the plant from the threats found in that particular region. They don't even qualify botanically as a differing variety.
Even under normal circumstances the threats are multitudinous and multidimensional; it could be heat-stress, water-stress, sun-stress (either too much or too little), salt-stress, stress due to contaminating heavy-metals in the soil, physical attack by rabbits, insects, fungi, bacterial, phytoplasmas, disease, or by competition from nearby plants; any form of onslaught. When under stress, the plants produce a greater range and a greater quantity of toxic secondary metabolites to try to defend against any attack. The more stress the plant is under, the more vicious are the plants defences. But it costs the plant energy to produce these toxic defence chemicals, so it only produces more when under stress. In general, plants produce more toxins early in the growing season when they are most vulnerable and have not yet produced seed by which to proliferate themselves. When they have produced seed, many plants put extra toxins in the seeds to help protect them from other predators and diseases. This is called Phenotypic Plasticity. It is not chemotypy.
But differing areas in which the same plants find they are growing may well present differing threats with which to deal. Some plants are able to modulate the relative amounts of each toxin they produce in defence. Or even to produce a completely differing sub-set of defence chemicals. These plants, although nominally the same, may have slightly differing genes and/or have slightly modified the genes that they use to synthesize secondary metabolites by a methylation process which either inactivates those regions of the genes or modulates their effectiveness. Plants which do this exhibit chemotypy; they synthesize differing ratios of the same toxins or produce entirely different secondary metabolites, or even both at the same time. [Your Author is unsure whether the current definition of chemotypy (which has varied over time) includes secondary metabolite changes induced by epigenetic changes to the genome induced by environmental changes - by methyl groups inactivating certain areas of the DNA? The latest definition of chemotypy may exclude epigenetic changes)
Many, but by no means all, plants which exhibit chemotypy are those that are either
metallophytes (heavy-metal tolerant - although it has been established that there are no plants which actually grow better in soil contaminated by heavy metals; rather that they
tolerate the heavy metals - having special processes within which can either excrete the dangerous heavy metals or secrete them out of harms way into specialised compartments. They rather tolerate the heavy metals where most other species cannot cope and therefore cannot grow there - but they themselves are only growing in heavy-metal contaminated ground because there is no competition from other plants which cannot grow there).
However, it turns out that if you transfer one plant chemotype into another area, it usually fares less well. Each chemotype has adapted itself to the environmental conditions in that area in which it is presently thriving. Many such chemotypes grow best in differing countries. Chemotypes cannot only depend upon where they are growing, but also at what time of year that you examine the secondary metabolites produced. [Your Author is still unsure if this is true for the modern definition of chemotypy]
When your Author finds several different lists of compounds in the essential oil for any one species, it is quite remarkable how much they can differ. This must be chemotypy in action.
Plants exhibiting chemotypy do not usually grow very well in areas other than their native territory. Perhaps this is one way sub-species can develop, leading on perhaps eventually to a differing species altogether. This process is called speciation, and is assumed to occur extremely slowly; hardly anyone has ever seen it in action, at least not in the case of plants. But chemotypy may well be one route for plants to eventually become differing species.
Wild Thyme is claimed by some to have nine such Chemotypes, each one based upon a pre-cursor secondary metabolite.
Each of these 9 compounds represents the secondary metabolite upon which the chemistry of each 9 differing chemotypes is based. For instance those chemotypes with Eucalyptol (aka
1,8-Cineol) produce a differing set of secondary metabolites to the chemotype based upon
Sabinene Hydrate (aka
Of all the monoterpenoids shown, Sabinene Hydrate is by far the most toxic.
The nomenclature adopted for Chemotypes is to use the name of the compound associated with a chemotype after the abbreviation 'ct.'. Thus the Wild Thyme chemotype attributed to Geraniol would be named Thymus vulgaris ct. geraniol (sweet thyme) (or Thymus vulgaris CT geraniol (sweet thyme)), although neither designation have any taxonomic standing. The name in brackets (which may be omitted) is the common name under which that chemotype is often known. The 'CT' can also appear after the name of the secondary metabolite.
[Note: Your Author has drawn each set in such a juxtapositional arrangement whereby the resemblance is immediately apparent. But on the internet the reader will find drawings for
Sabinene Hydrate (aka trans-4-Thujanol) and Eucalyptol (aka 1,8-Cineole) where the components are drawn in a differing spatial arrangement; but if the reader studies any alternative arrangement closely he will discover that they are topologically equivalent: all the lines go to all the correct corners].
Your Author has since found a book which lists two extra chemotypes of Wild Thyme: the
para-Cymene and the
Chemotypes are named after the most abundant secondary metabolite that they produce. But each chemotype can produce secondary products which are highly variable both in quantity and in identity. Two specimens having the same chemotype can have a mix of totally differing compounds; only the main secondary metabolite is the same.
- Thymol CT (Red Thyme) is strongly antiseptic and attacks human skin, for when harvested in Autumn, the essential oil consists of 60 to 70%
Thymol. But if harvested in the spring it is a differing chemotype:
- Carvacrol CT is strongly antiseptic and similar to Oregano. Harvested in spring the essential oil has but 30% Carvacrol, but harvested in Autumn 60-80% Carvacrol
- Linalool CT Grown in low altitudes it is soothing, but grown at high altitudes it is anti-fungal, anti-parasitic and uterotonic.
- Thujanol CT 50%
trans-4-Thujanol (aka Sabinene Hydrate) irrespective of season and found only in the wild, for when cultivated does not exhibit this Chemotype.
- α-Terpineol CT has a slightly peppery taste and obtained by harvesting early in spring.
- Geraniol CT harvested in autumn has a lemony smell and is grown at high altitudes. Exhibits cardiotonic and anti-viral properties.
- Eucalyptol CT (aka 1,8-Cineole CT) 80-90% Eucalyptol. It exhibits insect repellency with analgesic, decongestive and diuretic properties.
- para-Cymene CT Analgesic when applied to the skin, useful for arthritis and rheumatism. Harvest in spring for para-Cyment CT; harvested in Autumn it becomes the Thymol CT.
- Phenol CT This chemotypy is a result of high latitude growing (the further north the higher the Phenolic compounds). Grown in Finland it is 90% phenol.
Other plants exhibiting chemotypy are:
Rosemary (Rosmarinus officinalis) [
Basil (Ocimum basilicum) [Fenchol, Eugenol, Linalool,
Lemon Balm (Melissa officinalis) [
Sage (Salvia officinalis) [Thujone,
Tarragon (Artemisia draculuncus) [Estragole, Sabinene]
Valerian (Valeriana officinalis) [
The Mint Family (Lamiaceae) is the family which produces the most plants exhibiting chemotypy. Amongst them
Clary Sage (Salvia sclaria),
Hyssop (Hyssopus officinalis),
Lavender (Lavandula angustifolia),
Marjoram (Origanum majorana),
Patchouly (Pogostemon cablin),
Pennyroyal (Mentha pulgenium),
Peppermint (Mentha piperita), Spear Mint (Mentha spicata),
Spike Lavender (Lavendula latifolia) as well as
Basil, Wild Thyme, Sage, Lemon Balm and Rosemary already mentioned above.
But another source says that much of what has been written in books and on the internet on chemotypy is just plain wrong. It says that, although chemotypes are not a sub-species or in any other taxonomic category, a chemotype is where a plant produces a distinctly differing set of secondary metabolites which can be grouped on a 2-dimensional plot such that differing chemotypes occupy differing areas on that 2-axis plot (it doesn't say what the 2 axes represent though! - and why not a 3-axes graph or multidimensional plot). But also, and this is the crucial difference to some of the other tables of 'chemotypes' shown above, these different chemotypes must remain stable in changing environments and they must also be heritable (which may rule out environmental epigenetic changes to the genome by methylation - but some epigenetic changes are heritable [but perhaps not permanently so]).
That is, the chemotypes must reflect differences in their genes, rather than
That last requirement puts some of the members in the above nine-membered table at distinct odds with this definition. In particular with the
para-Cymene CT and the
Thymol CT - in true chemotypy your Author thinks that it is forbidden to change from
para-Cymol CT to
Thymol CT as a result of looking at it later in the season! Your Author thinks that the
Thujanol CT also seems to break this stringent definition of chemotypy: for in a cultivated setting it does not exhibit
Thujanol CT. This seeming breaking of the rules in at least three places puts the whole above Wild Thyme table into question.
So, your Author leaves it to the reader to disentangle the true definition of chemotypy. But your Author would point out that since chemotypy (of one sort or another) was discovered, DNA profiling was instigated and has become much more extensive and accurate, so it is little wonder that the definition of chemotypy has wandered around and evolved over that time. Perhaps it is just as well that chemotypy is not (yet) a part of taxonomy...
PHENOTYPY / PHENOTYPE
Here is Wikipedias take on : /Phenotype