Weed evolution and community structure - Ecological Management of Agricultural Weeds - page 469

Table 10.5. Phenotypic plasticity in Echinochloa crus-galli var. crus-galli a

Character Fertilized b Density stressed c

Above-ground biomass (g) 219 0.026

Time to flowering (days) 54 95

Tiller production 17.6 1.0

Seed production 17900 1.9

Harvest index (%) 13.4 10.4

Notes.
a Values are the means ofplants in 10 pots.
b Plants in the fertilized treatment were grown singly in large (31860 cm 3 ) pots and fertilized
with N-P-K.
c Plants in the density stressed treatment were grown in small (88 cm 3 ) pots at a starting density
of100 plants.
Source : Barrett (1992).

to what extent weed races have increased or decreased phenotypic plasticity
relative to their wild progenitors could yield insight into the evolutionary
processes that create weeds.
Theory indicates that the evolution of plasticity for a character should be
favored over genetic polymorphism in situations where environmental condi-
tions vary greatly in short time spans or within the space occupied by an indi-
vidual plant (Levins, 1963, 1968, pp. 10-22, 66-72). For example, if the light
environment varies between high light and shade during the day or over the
course of the growing season, then plasticity in response to light level should
be favored rather than differentiation of sun- and shade-adapted genotypes
(Gross, 1984; Sultan & Bazzaz, 1993b). Even if genetic variability for a charac-
ter exists in a population, plasticity may prevent evolutionary change by
allowing the various genotypes to assume similar phenotypes (Bradshaw,
1965; Sultan, 1987).
Plasticity and adaptation are largely complementary processes. High levels
of plasticity in many weed characteristics allow weeds to avoid “dead-end”
adaptation to temporary or local conditions. However, even highly plastic
characteristics may shift under selection if the environment favors forms that
are beyond the range of the plasticity response or, as in the case of Camelina
sativa var. linicola cited above, the plasticity response itself is selectively
penalized.

Ecotype formation in weeds
Well-differentiated ecotypes, races, and subspecies are the norm in
weed species. Colonization with small founding populations and episodic