Se. Sultan et Fa. Bazzaz, PHENOTYPIC PLASTICITY IN POLYGONUM-PERSICARIA .3. THE EVOLUTION OF ECOLOGICAL BREADTH FOR NUTRIENT ENVIRONMENT, Evolution, 47(4), 1993, pp. 1050-1071
Norms of reaction for a number of growth and reproductive characters w
ere determined for 15 randomly sampled Polygonum persicaria genotypes,
from two natural populations originating in sites with very different
nutrient availabilities. Under severely limiting nutrient conditions,
these genotypes shared not only plastic responses such as increased r
oot-to-shoot ratio, but a surprising constancy in such functionally es
sential characters as leaf area ratio, leaf nitrogen concentration, an
d propagule nitrogen content. Because functional homeostasis depends o
n flexibility in underlying characters, similar homeostatic results ca
n be achieved through different combinations of underlying plastic and
fixed responses in genetically different entities. For example, plant
s in each population maintained a relatively constant propagule nitrog
en content under extreme low-nitrogen conditions by varying either the
size or the tissue nitrogen concentration of propagules. These genoty
pes also tolerated excessive nutrient levels toxic to many plants, evi
dently by storing excess nutrients in shoots. Although development was
altered under such circumstances, reproductive fitness was maintained
. Genotypes of both populations thus were universally able to tolerate
very limited as well as excessive nutrient supplies and to exploit fa
vorable nutrient conditions. This capacity of individual genotypes to
accommodate diverse nutrient environments reflects the specific nature
of mineral resources and of plant physiology: because nutrient availa
bility can be manipulated via root-system adjustments and facultative
uptake mechanisms, and ions can be differentially allocated and transl
ocated among plant parts, nutrient supply may be to a considerable ext
ent mediated by the plant individual. The results further suggest that
the response mechanisms conferring ecological breadth for nutrient en
vironment may entail neither physiological costs nor fitness trade-off
s, conditions favoring the evolution of plasticity rather than genetic
specialization. The evolution of such plasticity also reflects the hi
ghly variable nutrient environment plants experience, because of fluct
uations not only in soil minerals but in complex interacting factors s
uch as moisture. General conclusions based on the entire, three-part s
tudy follow the discussion.