Se. Sultan et Fa. Bazzaz, PHENOTYPIC PLASTICITY IN POLYGONUM-PERSICARIA .2. NORMS OF REACTION TO SOIL-MOISTURE AND THE MAINTENANCE OF GENETIC DIVERSITY, Evolution, 47(4), 1993, pp. 1032-1049
Adaptive phenotypic plasticity is the predicted evolutionary response
to fine-grained fluctuation in major environmental factors, such as so
il moisture in plant habitats. This study examines genotypes from two
natural populations of Polygonum persicaria, one from a relatively hom
ogeneous, moderately moist site, and one from a site in which severe d
rought and root flooding occur within single growth seasons. Norms of
reaction (phenotypic response curves) were determined for a random sam
ple of eight and ten cloned genotypes, respectively, from each of the
populations over a controlled moisture gradient ranging from drought t
o flooding. Genotypes of both populations exhibited marked allocationa
l and morphological plasticity in characters relating to plant functio
n at both low moisture availability and flooded soil, including root c
haracters that directly affect plant water status. Associated with thi
s plasticity was the ability of all genotypes to survive and reproduce
across the entire range of moisture conditions: individual genotypes
possess a remarkable degree of ecological tolerance. This tolerance wa
s equally broad in genotypes from both populations, although only one
population encounters soil flooding in its native site. Among the Poly
gonum genotypes studied, certain individuals exhibited relatively poor
growth and reproductive output at several moisture levels. Although t
hese genotypes thus showed significantly low average fitness, the magn
itude and/or direction of the fitness differentials varied from one mo
isture treatment to another. The precise arrays of norm of reaction di
versity among these genotypes, together with patterns of moisture vari
ability at the field sites, indicate how genotype by environment inter
action and environmental variability may lead to the maintenance of ge
netic diversity.