Morphological canalization, integration, and plasticity in response to population density in Abutilon theophrasti: Influences of soil conditions and growth stages

Phenotypic integration and developmental canalization have been
hypothesized to constrain the degree of phenotypic plasticity, but little
evidence exists, probably due to the lack in studies on the relationships
among the three processes, especially for plants under different
environments. We conducted a field experiment by subjecting plants of
Abutilon theophrasti to three densities, under infertile and fertile soil
conditions, and analyzing correlations among canalization, integration,
and plasticity in a variety of measured morphological traits after 50 and
70 d, to investigate the relationships among the three variables in
response to density and how these responses vary with soil conditions and
growth stages. Results showed trait canalization decreased, phenotypic
integration and the degree of plasticity (absolute plasticity) in traits
increased with density. Phenotypic integration often positively correlated
with absolute plasticity; whereas correlations between trait canalization
and plasticity were insignificant in most cases, with a few positive ones
between canalization and absolute plasticity at low and medium densities.
As plants grew, these correlations intensified in infertile soil and
attenuated in fertile soil. Our findings suggested the complexity of the
relationship between canalization and plasticity: decreased canalization
is more likely to facilitate active plastic responses under more favorable
conditions; whereas increased level of integration should mainly be an
outcome of plastic responses. Soil conditions and growth stage may affect
responses of these correlations to density via modifying plant size,
competition strength and plastic responses in traits. We also predicted
that decreased canalization can be advantageous or disadvantageous, and
the lack of response to stress may demonstrate a stronger ability of
adaptation than passive response, thus should be adaptive plasticity as
active response.