Introduction
Phenotypic plasticity, defined as the ability to produce different
phenotypes in response to different environmental conditions
(Bradshaw, 1965;
Massimo Pigliucci, 2005). The expression
of a phenotype under a certain environment is the result of the
integration of local responses of many modules or traits and their
interactions (Pigliucci & Preston,
2004). Consequently, the evolution of a given trait and its plasticity
may be subject to restrictions due to its genetic correlations with
other traits (Agrawal & Stinchcombe,
2009; Gianoli & Palacio-López, 2009).
Not all species or traits are plastic, probably because the extent of
phenotypic plasticity is limited by both intrinsic and extrinsic factors
(DeWitt et al. , 1998;
Givnish, 2002;
Valladares et al ., 2007).
A process that may limit phenotypic plasticity is developmental
canalization, which is generally assessed by coefficient of variation
(CV) in traits (Woods et al. ,
1999). Developmental canalization, or robustness, is the property of an
organism that buffers development against environmental and genetic
perturbations to produce a consistent phenotype
(Waddington, 1957), including
environmental canalization and genetic canalization
(Wagner et al. , 1997).
Environmental canalization indicates the insensitivity of traits to
environmental perturbations in variable environments
(Debat & David, 2001;
S.C. Stearns et al. , 1995;
Wagner et al. , 1997), like a
process in opposition to phenotypic plasticity
(Stearns et al. , 1995;
Wilkins, 1997). But both phenotypic
plasticity and environmental canalization may reflect the ability of an
organism to adjust phenotypic expression appropriately in dealing with
environmental changes, at individual and population levels respectively
(Reed et al. , 2010;
Schlichting & Pigliucci, 1998). Genetic
canalization, refers to the buffering of the effects of genetic
variation (Wagner et al. , 1997).
The developmental canalization here may include both genetic and
environmental canalization. It is widely appreciated that stress
conditions increase phenotypic variability in traits
(Woods et al. , 1999). If both
plasticity and instability of traits increase with environmental stress,
it is reasonable to infer some common mechanisms in charge of the two
kinds of variabilities (Meiklejohn &
Hartl, 2002). Wagner et al.(1997) hypothesized that selection for environmental canalization may
facilitate the evolution of genetic canalization, given the ample
evidence for genetic canalization from empirical studies
(Dun & Fraser, 1958;
Dworkin, 2005;
Polaczyk et al. , 1998; but
see Hermisson & Wagner, 2004). However,
relevant studies did not find changes of environmental canalization
affect the level of genetic canalization
(Debat et al. , 2009;
Ian Dworkin, 2005;
Meiklejohn & Hartl, 2002). In the
unpredictable environments of Mediterranean ecosystem, environmental
stress favors both plasticity and high degree of genetic variations
across species in oak seedlings
(Valladares et al. , 2002),
implying negative correlations between plasticity and canalization.
Studies on the relationships between trait plasticity and canalization
are rarely seen, even less on how environments affect their
relationship, especially for plant species.
Phenotypic integration may also play a role as an internal constraint to
phenotypic plasticity (Gianoli, 2001,
2003;
Pigliucci et al. , 1995;
Schlichting,
1986, 1989;
Valladares et al. , 2007).
Phenotypic integration refers to the pattern and magnitude of character
correlations (M. Pigliucci & Preston,
2004), which results from genetic, developmental and/or functional
connections among traits (Pigliucci &
Preston, 2004; Schlichting & Pigliucci,
1998), and is often expressed in terms of the number of significant
phenotypic correlations between traits
(Pigliucci, 2002;
Pigliucci & Marlow, 2001;
Schlichting, 1989). Evidence has shown
that the degree of plasticity in response to shading or drought in a
given trait decreased with the increase of the number of its
correlations with other traits in two local species from Chile,
suggesting the restriction of phenotypic integration to the degree of
plasticity (Gianoli & Palacio-López,
2009), and the greater the constraints of genetic correlations for a
trait, the lower its ability to respond to environments. On the other
hand, however, the strength of phenotypic integration may also increase
with environmental stresses
(García-Verdugo et al. , 2009;
Gianoli, 2004;
Schlichting, 1989;
Waitt & Levin, 1993), which also induce
plastic responses in traits, implying the positive correlations between
phenotypic integration and plasticity in traits. Since most empirical
studies have only studied patterns of phenotypic integration in a single
environment (Pigliucci & Preston, 2004;
but see Liu et al. , 2007;
Pigliucci et al. , 1995), we still
know surprisingly little about how the environment influences levels of
phenotypic integration (Mallitt et
al. , 2010), even less on the relationship between phenotypic
integration and plasticity and its variation with environmental
conditions.
Population density, as one of the major natural stresses that result in
size variations in plants, its variation can result in the heterogeneity
of multiple environmental factors, inducing complex responses in traits.
Both variation among individuals and phenotypic plasticity can impart
integration among morphological traits
(Klingenberg, 2014). Thereby such
responses to density may also correlate with developmental canalization
and phenotypic integration. Correlations can vary with different abiotic
conditions and growth stages, due to their influences on trait
plasticity (Wang et al. , 2017),
integration and canalization (Damiánet al. , 2018; Goswami et
al ., 2015). We need more detailed studies on integration, canalization
and plasticity to generalize about the relationships among them
(Kavanagh, 2020), especially at different
stages of plant growth or under different abiotic conditions. As traits
may differ in the magnitude of their plastic responses to different
resources, resulting in a “hierarchy” of responses
(White, 1979), the importance of
considering a wider range of plant traits is underscored
(Ryser & Eek, 2000). Here we conducted a
field experiment on an annual herbaceous species
ofAbutilon theophrasti , to analyze phenotypic plasticity,
canalization and integration for a number of allocation and
morphological traits and their relationships in response to density for
plants under two contrasting soil conditions at two growth stages. We
aimed to test the following hypotheses: 1) trait plasticity,
canalization and integration increase with higher densities; 2) there
are positive correlations among the three processes, which intensified
with greater densities; 3) soil conditions and growth stage can
influence responses of their correlations to density.