The Geography of Phytochemical Races

48 2 Examples Within Continents
was observed in a recent study of the essential oils of this plant (Pereira et al.,
2000). Two levels of chemical variation were recorded for this species, quantitative
differences of four compounds, and, perhaps more interestingly, differences
in the proportions of enantiomers of two of them, sabinene [92] (Fig. 2.24) and
α- terpineol [104] (see Fig. 2.26 for structures 104 and 105). Compositions of the
four major compounds are given in Table 2.8. The collection sites have been reordered
to refl ect their geographic locations on the island (they are also identifi ed
using the authors’ site numbers). UPGMA clustering emphasized the degree of nonconnectedness
of these populations, the “center” group (populations 1, 2, 3, and 4)
for example, having widely different concentrations of sabinene, thymol [105], and
carvacrol [89]. As the authors pointed out, morphological similarity and the lack of
climatic and elevational effects suggest that the differences are based upon either
genetic or edaphic differences. Until common garden studies are done, the effect of
edaphic differences cannot be ruled out. Considering the small size of the island,
it seems as likely that populations could have been readily established by mediumdistance
seed dispersal.
Of added interest in this study was the fi nding that the enantiomeric ratios of
α-terpineol also differ widely among populations. In southern populations and the
population from the northwestern tip of the island, the amount of (+)-α-terpineol
was shown to range from 92.3 to 97.0%, while this enantiomer made up 85.6% in the
upper-middle population, but only 66.0% in the population from the southeastern
tip of the island. It would be of interest to see if the enzymes responsible for the biosynthesis
of α-terpineol have different stereochemical requirements in these populations,
or whether some isomerization has occurred in the formation, preparation,
or analyses of these oil samples.
The next example involves a recent study of essential oil polymorphism in
Thymus praecox Opiz subsp. arcticus (E. Durand) Jalas (syn. T. drucei Ronn.) on
the British Isles (Schmidt et al., 2004). More than 700 specimens of the plant were
collected in Ireland, Scotland, and southern England and subjected to gas chromatographic
analysis (coupled with mass spectroscopy). Sixty-nine constituents
were identifi ed, the majority of which were mono- and sesquiterpenoids. Analysis
of the data revealed a highly polymorphic assemblage with 13 chemotypes in
Scotland, 11 in Ireland, and 17 in the south of England. Polymorphism seems to
Table 2.8 Percentage composition of essential oils of Thymus caespitosus on Sao Jorge, Azores
(from Pereira et al., 2000)
Terpene NWa Center Mid South SE
Sabinene 0.1 0.1 2.3 40.9 4.2 0.1 2.1 0.6 0.9 0.4
α-Terpineol 10.4 t 1.4 0.2 0.2 6.4 43.0 46.6 68.3 15.3
Thymol 44.4 57.9 2.5 4.3 44.5 19.8 7.0 8.4 3.0 1.4
Carvacrol 2.9 3.5 52.3 17.2 2.5 31.5 1.5 2.3 0.5 35.9
a Locations on Sao Jorge: NW = northwestern tip; Center = center of the island; Mid = northwest
of the central site; South = southern populations; SE = southeastern tip of the island.