The Geography of Phytochemical Races

2.7 North and Central America 91
C-type pigmentation were observed in the northern part of the range of L. californica.
Populations exhibiting (predominantly) A-type fl avonoid chemistry occurred in
Arizona and central and southern California. The study area at Jasper Ridge belongs
to the latter group.
Other differences between the two fl avonoid races emerged from a biosystematic
study of L. californica collected from its entire range. Differences in allozyme banding
patterns and pappus structure correlated extremely well with the fl avonoid types
described above (Desrochers, 1992; Desrochers and Bohm, 1995). A more recent
examination of achenes from the Jasper Ridge transect has shown that the two pigment
races can be differentiated on the basis of achene dimensions and weight as well
(Rajakaruna et al., 2003 and citations therein). A detailed study of soil chemistry has
shown a series of trends along the original short transects involving pH, soil moisture,
clay content, cation exchange capacity, soil conductivity, and concentrations of several
elements (Rajakaruna et al., 2003). There was no abrupt change in any of those factors
at the 45 m point, where we have seen the transition between race C plants (upper part
of slope) and race A plants (lower part of slope). That there is a difference between the
soils above and below the transition point is clear from the results, but what factor or
combination of factors is/are responsible for the change in race within the transition
zone is not known. A series of experiments involving germination of achenes, growth
of seedlings, and maturation of plants (seed set) add to the view that strong selection
is operating in this system. Achenes from both races germinated and grew in soil from
either the upper part of the transect (type C soil) or from the lower part (type A soil),
although not equally well. (Both races performed better in potting soil.) The most
striking fi nding of this study was that race C plants never reached maturity—they set
no seed—when grown in soil from the lower part of the slope. A parallel set of experiments
was run using an aqueous extract of upper and lower soils to irrigate achenes of
both races; the results were identical to those from the soil-growth experiments.
Two additional sets of observations provided further evidence that two unique biological
“entities” (thus avoiding any taxonomic decision, at least at this time!) along
the transect at Jasper Ridge. Field observations suggested that the fl owering times
of race A and race C plants differ by as much as 10 days, race C being the earlier to
reach reproductive maturity. Greenhouse studies supported this with the difference in
fl owering times ranging from 7 to 10 days. Earlier attempts to determine the breeding
behavior of these races were inconclusive owing to small sample numbers, but
recent work (Rajakaruna and Whitton, 2004) has shown that A × A and C × C crosses
(all done reciprocally) produced approximately 80% seed set, whereas A × C crosses
(reciprocal) gave lower percentages of seed set (ca. 20%) with the lowest level seen
in crosses of A and C plants originating from Jasper Ridge (ca. 5%).
An interesting case of variation among populations over comparably short distances
emerged from studies of fl avonoids in L. burkei (Greene) Greene (Bohm and Banek,
1987). This species occurs in a comparatively small area in the north coast ranges of
California, with populations in Lake, Mendocino, and Sonoma counties. Five populations
were sampled, populations A and B from Sonoma County, and populations C,
D1, and D2 from Lake County. Populations A and B were located between Santa
Rosa and Healdsburg and are separated by about 10 km. Population C was found in