The Geography of Phytochemical Races

214 4 Intercontinental Disjunctions
Fig. 4.19 Map showing occurrence of Heterotropa in Japan. Compounds bearing the methylenedioxy
group occur on Taiwan, Okinawa, and associated islands. Compounds that lack the
methylenedioxy group occur in Japan. Arrows show possible routes of colonization of the two
types
4.5.3 Rhododendron (Ericaceae)
Rhododendron is a large genus of some 850 species with a nearly worldwide distribution,
with areas of particular richness in the Himalayas, southeastern Asia, and
New Guinea. A single species is known from Australia (Mabberley, 1997, p. 614).
In addition to the large number of naturally occurring species, many horticulturally
important cultivars have been developed owing to the attractive evergreen foliage
and large range of fl ower size and color.
A study of pigment profi les within Rhododendron sect. Virerya, a reputedly monophyletic
group of 300 species from the Malesian-Australian region, showed the presence
of a comparatively simple array of aglycones (Harborne, 1986) with quercetin
derivatives present in most species, kaempferol derivatives in several, and myricetin in
a scattering of species. Present in lower frequencies than in other comparable groups of
Rhododendron species were azaleatin [quercetin 5-methyl ether, structure 348] and
caryatin [quercetin 3,5-dimethyl ether, structure 349]; gossypetin [8- hydroxyquercetin,
structure 319] was not observed in species from Malesian-Australian region and
North America. Harborne (1986) summarized frequencies of fl avonoid aglycone
occurrences in the genus as a whole; his summary is presented in Table 4.8.
Harborne (1986) noted that the absence of gossypetin from leaves of sect. Vireya
refl ects its absence in the fl owers and that such absence may be correlated with