The Geography of Phytochemical Races

18 2 Examples Within Continents
Fig. 2.9 Compounds 41–43 from Melaleuca alternifolia; Compounds 44–49 from Isotachis lyallii
The existence of six chemotypes was documented by the work of Homer et al.
(2000) who also established a set of formulae for comparing results obtained by steam
distillation versus static headspace gas chromatography (GC). Although several terpenes
are regularly observed in tea tree oils, three compounds were clearly the most signifi cant
and were used by the present workers to defi ne the chemotypes, terpinen-4-ol [42],
terpinolene [43], and 1,8-cineole. Chemotype 1 is defi ned by the presence of a high concentration
of terpinen-4-ol, accompanied by lesser amounts of α-pinene, α-terpinene,
p-cymene, and γ-terpinene (see Fig. 2.35 for cymene and γ-terpinene structures). Chemotype
2 is characterized by high levels of terpinolene. Chemotype 5 is marked by
high concentrations of 1,8-cineole and has the associated compounds limonene and
α-terpineol in lesser amounts. Chemotypes 3, 4, and 6 are dominated by 1,8-cineole, but
differ with regard to the relative amounts of terpinen-4-ol or terpinolene. (Chemotype
numbering is an historical artifact.) Geographical differentiation of the populations was
observed nearer the coast, with a predominance of chemotype 1 (high terpeinen-4-ol) in
the north (south of Casino, NSW) and a predominance of chemotypes 4 and 5 (higher
1,8-cineole) in the south (south of Grafton, NSW), but no clear-cut line of demarcation
appears to exist. Chemotype 2, however, which occurs in the vicinity of Stanthorpe,
Queensland (southwest of Brisbane), is separated by a distance of 40–50 km from the
coastal groups. It has also been noted that the Queensland specimens afford smaller
yields than do trees from New South Wales, and that this difference is likely due to the
drier conditions in which the former occur (Butcher et al., 1994).
Homer et al. (2000) speculated that differences in oil composition may refl ect
localized inbreeding leading “some stands or areas becoming more pure-breeding
for the different oil chemotypes.” They went on to conclude that low frequencies of
different chemotypes in a given area may “indicate low incidence of recessive gene
expression, occasional infl ow of genetic material from wider afi eld (e.g., during