Every wondered what a chemotype was?

B: When referring to essential oils, chemotypes or chemo-forms it means that morphologically identical single species or variety (taxon) can be found possessing different essential oil compositions. For example In North America there are four chemotypes of the wild growing Mentha canadensis L. (2n=96). They are Type 1, oils rich in pulegone and menthone, Type 2, oils rich in cis-and trans-isopulegones, Type 3, oils rich in linalool and Type 4, oils rich in 1,8-cineole and acyclic monoterpenes. However, the compositions of oils of each chemotype are rarely the same. For example Type 1 oils contain varied amounts of pulegone and menthone , sometimes pulegone is the major component and sometimes menthone is the major component;nevertheless, this chemotype relates to the biosynthetic pathway responsible for creating the essential oil composition. The same is true for the other three chemotypes and their representative pathways.
By the way this is a simple group of chemotypes found with M. canadensis. Many other members of the Lamiaceae family can possess more than four chemotypes. Also chemotypes abound in the Asteraceae family 


C:  here is a great explanation of chemotype on wiki: http://en.m.wikipedia.org/wiki/Chemotype . Most of it, along with B's great explanation, is over my head, but the following passage from it makes good commonsense to me:

"Because chemotypes are defined only by the most abundant secondary metabolite, they may have little practical meaning as a group of organisms sharing the same trait. Individuals of one chemotype may have vastly different chemical profiles, varying in the abundance of kind of the next most abundant chemical. This means two individuals of the same chemotype could have different impacts on herbivores, pollinators, or resistance to pests. A study by Ken Keefover-Ring and colleagues in 2008 cautioned that, "...this can be a very qualitative assessment of an individual's chemical profile, under which may be hiding significant chemical diversity."[1]"


B:  In the literature a number of reports can be found that refer to the existence of many chemotypes based on the existence of a major component independent of its functional group or its interrelationship with the other components in the oil. This is unfortunate because it causes incorrect interpretation of chemotypic form existence. Remember chemotypes developed strictly for the taxon's survival whether it be to protect it from predators,including herbivores, insects, bacteria and fungi or to act as an attractant for pollinators or fruit eaters to disperse its seeds(not for herbaceous plants but shrubs and trees). The determining factor for characterization of a chemotype is the closeness of the biosynthetic pathway. Another example of the existence of a single chemotype is Hyssopus officinalis which is rich in pinocarvone, pinocamphone and isopinocamphone all of which are closely related from a biosynthetic standpoint. Some oils possess the major components in the following order: isopinocamphone>pinocamphone>pinocarvone while other oils are found suchas pinocamphone>isopinocamphone>pinocarvone or Pinocarvone >pinocamphone >isopinocamphone etc. These are all actually a single chemotype with the expected variance amongst biosynthetically related components.
The existence of a single set of compounds such as alcohols, ketones, aldehydes etc. is not a point of differentiation in an oil to designate a chemotype

 B: Chris, yes it is an over simplification of the chemotype term. All Lavandula angustifolia cultivars are of the same chemotype irrespective of which cultivar, because to the best of my knowledge there have not been any chemoypic forms of L. angustifolia or L .x intermedia found.The formation of linalyl acetate from linalool is literally one biosynthetic step from linalool which is the tertiary alcohol produced as the first characterizing step in the pathway from the diphosphate precursor in the Lavandula genus.However, chemotypes have been found in other Lavandula species but not in lavender or the hybrid lavandin. 

M:  Chris - Did you know the concept of chemotypes was and Aussie idea?

Baker & Smith back in the early 1900's discovered it while working on some of our Eucalyptus species. There are lots of examples within the Family Myrtaceae:

Euc. dives CT cineole & CT piperitone
Melaleuca quinquinervia CT cineole (Niaouli), CT Nerolidol/Linalool (Nerolina) & CT Viridiflorol (MQV)
Melaleuca teritifolia CT cineole, CT citral (a beautiful lemon tea tree)