How plants respond to germination timing via morphological plasticity is not well documented, in spite of its importance for understanding plant strategies in dealing with natural environmental challenges of complicacy, especially in the perspective of the entire life cycle of plants. To address this issue, we conducted a field experiment with Abutilon theophrasti by growing plants in four periods, as four germination treatments (GT1~GT4), before measuring a number of mass and morphological traits on them at three (or four) growth stages (EX, I~III). Results the optimal germination time for A. theophrasti was late spring, as plants that germinated in this period achieved the maximum total mass, with the highest stem and reproductive allocation and the lowest leaf allocation, among plants of all germination treatments. Plants that germinated earlier in spring used a longer time for vegetative growth and did not outperform late-spring germinants, probably due to exposure to spring drought and response to competition. Delaying germination into summer led to a faster growth, increased leaf allocation, decreased stem allocation, advanced reproduction and shorter life cycle, but further delay of germination into late summer led to insufficient reproduction and incomplete life cycle due to extremely short growth period. Results suggested plants that germinated within the optimal period can maximize their growth potential in relatively favorable conditions. In spite of conspicuous disadvantages, plants with advanced and delayed germinated were still able to use different strategies to better adapt to subsequent environments, via plasticity in a number of allocation and morphological traits. Root plasticity may play a predominant or fundamental role in plant response to environments, or it is crucial to maintain root allocation stable, while stem or leaf allocation can often be sacrificed depending on specific situations.

Most studies on animals have conducted comparative studies to deduce the possible relationships among developmental stability, canalization and phenotypic plasticity, there is a lack of direct evidence in plants, which should be better study materials. To investigate the correlations among developmental stability, canalization and plasticity in plants, we conducted a field experiment with Abutilon theophrasti, by subjected plants to three densities under infertile vs. fertile soil conditions, and measured leaf size, leaf fluctuating asymmetry (FA), and calculated coefficient of variation among leaves within individuals (CVleaf) and among individuals (CVin) and relative plasticity (PIrel) and its degree in leaf size at three growth stages, to analyze the responses of their correlations to density and how they may vary with soil conditions or growth stages. Results showed a decrease of FA, CVleaf and PIrel and an increase of CVin in leaf size, with increased density. In most cases, there were no correlations among these variables, but negative correlations between CVin and PIrel, positive correlations between FA and PIrel at high density and/or in fertile soil, in infertile soil. It suggested that higher FA may indicate the state of faster growth rather than an indicator of environmental stresses; there are correlations among developmental stability, canalization and plasticity, which may be complex, affected by other factors. The loss of developmental stability may be beneficial for plant response to environmental stresses, while decreased canalization can be either disadvantageous or advantageous, depending on that the size variation results from an increase or decrease of smaller individuals, and whether its correlations with other variables reflect beneficial or adverse environmental effects.

Phenotypic integration and developmental canalization have been hypothesized to constrain the degree of phenotypic plasticity, but there is little evidence for the relationships among the three processes in different environments, especially for plants under natural conditions. To address this issue, we conducted a field experiment by subjecting plants of Abutilon theophrasti to low, medium and high densities, under infertile and fertile soil conditions, measured a variety of traits and analyzed canalization (coefficient of variation [CV]), integration (coefficient of integration [CI] and the number of significant correlations of a trait with other traits [NC]), and plasticity (REL RDPIs and ABS RDPIs) in these traits and their relationships at two stages of plant growth. Our results showed an increase in mean CV, NC and ABS RDPIs of traits with density, and the positive correlations between trait NC and ABS RDPIs became stronger with higher densities but weaker over time in fertile soil, while correlations among trait CV, NC and ABS RDPIs became stronger over time in infertile soil. Results suggested shared or cooperation mechanisms among phenotypic integration, canalization and plasticity. Soil conditions and growth stage may affect responses of these correlations to density via modifying plant size and competition strength. The attenuated canalization and enhanced integration may be helpful for the production of plasticity, especially under intense competition.

How plants cope with the increase of population density via root plasticity is not well documented. Abiotic environments and plant ontogeny may play an important role in determining plant response to density and thus contribute to understanding this issue. We aimed to investigate root plasticity in response to density under contrasting soil conditions at three stages of plant growth in an annual herbaceous species Abutilon theophrasti. We conducted a field experiment by subjecting plant individuals to low, medium and high densities (13.4, 36.0 and 121.0 plants m-2, respectively) under fertile and infertile soil conditions, and a series of root traits were measured at three harvests when they had grown for 30, 50 and 70 d. Results revealed the complexity of root response to density, which may increase, decrease or canalize, depending on the strength of above- and below-ground interactions, which varied with soil conditions or growth stage. The intensity of above- and/or below-ground interactions increased with decreased soil resources, but first increased then decreased with growth stage. Facilitation is more likely to occur at low to moderate below-ground interaction, when above-ground interaction is negligible, and resources are abundant and at early stage of plant growth. Plants may prefer to adjust biomass allocation to maintain total mass stable initially, before suffering decreased total mass, in response to intraspecific interactions.