The Geography of Phytochemical Races

84 2 Examples Within Continents
Fig. 2.52 Compounds
183–189, fl avonoids, from
Chenopodium fremontii
However, the position of O-methylation, the hydroxyl group at C-7 and C-3′ in the
latter, clearly sets these races apart. Detailed studies of fl avonoid O-methylation by
Ragai Ibrahim and his colleagues at Concordia University in Montreal (see Seguin
et al., 1998 for leading references) have shown convincingly that these enzymes
have high levels of position specifi city. In the present case, then, it can be argued
that the O-methyltransferases that catalyzes methylation of the 3′-hydroxyl group
of quercetin (to form isorhamnetin) is different from the one that catalyzes methylation
of the 7-hydroxyl group. Thus, in the case of the differences among the fl avonoid
races of C. fremontii, it is possible to infer specifi c enzyme, and therefore
genetic, differences among the respective populations. Other differences can be
seen: arabinosides are restricted to plants from populations in California (Race 1);
rutinosides [186] were observed only in plants from Arizona, New Mexico, and
southern Colorado (Race 2); robinobiosides [187] were not observed in plants
from Nebraska (Race 4); and kaempferol 3-O-(2-O-α-l-rhamnosyl)-d-galactoside
[188] was seen only in plants from the southern populations (Races 1 and 2). It is
also useful noting that in only two cases did plants from both southern and northern
races share any of the less common compounds: (1) kaempferol robinobioside was
detected in Races 1, 2, and 3, whereas it was not seen in plants from Nebraska; and
(2) quercetin sophoroside [189] was seen in plants from California (Race 1) and
Nebraska (Race 4).
Flavonoid variation in C. fremontii is based upon several defi nable enzymatic
steps involving O-methylation, O-monoglycosylation with different sugars, and
elaboration of a series of O-diglycosides involving different (outer) sugars as well
as different positions of substitution on the “inner” sugar (C-2″ vs. C-6″). That
these chemical differences refl ect real genetic differences of the species seems
an entirely reasonable position. What is needed to put this on more solid ground,
of course, is a detailed analysis of the enzymology of the reactions involved and,
following that, a study of the genes controlling the various reactions. Very little
work has been done in the fi eld of fl avonoid biochemistry in following this line of
inquiry. This would seem to be a logical next step in understanding the source(s)
of variation in fl avonoid profi les, and C. fremontii might offer a good system to