Results
Growth stage, soil condition and population density had significant
effects on total biomass and almost all other traits (Table 1).
Interactions between stage and soil conditions, stage and density, soil
conditions and density were also significant for most mass traits, stage
and soil condition interaction were also significant for most
morphological traits. Except for root: shoot ratio and lateral root
length, plant size (or total biomass) accounted for a significant amount
of variations in all traits.
Total biomass was decreased by infertile vs. fertile soil, and by the
increase of density in both soil conditions at all stages except for
those of 30 d-growth in fertile soil (Fig. 1). At 30 d of growth,
neither soil condition nor population density affected total biomass,
but effects of density on shoot mass and root mass were significant in
fertile soil, though a large fraction of these variations were due to
effects of plant size (Table 2).
After removal of size effect, effects of stage, soil, density, and stage
and soil interactions were still significant for most traits (Table 1).
A greater number of traits responded to density in infertile vs. fertile
soil at 50 d and 70 d (Table 2). True plasticity occurred in all traits
except main root length in response to density, and responses of these
traits varied with soil conditions and/or growth stages (Table 2; Fig.
2-4). Density effect on main root length was significant in three-way
ANCOVA, but not in one-way ANCOVA under either soil conditions at any
stage, indicating apparent plasticity (Table 2).
At 30 d of growth, plant individuals in fertile soil conditions had
lower shoot mass, higher root mass and root: shoot ratio at medium and
high density, compared to at low density (p < 0.05), but not
in infertile soil (Fig. 1, 2). Under infertile soil, high density
enhanced shoot mass, reduced root mass and root: shoot ratio at 50 d (p
< 0.01); medium and high density enhanced shoot mass at 70 d
(p < 0.05); no significant response to density was found under
fertile soil at the two stages. For morphological traits, lateral root
length decreased with higher densities under both soil conditions (p
< 0.01), but main root diameter was decreased by high density
in infertile soil only (p < 0.05) at both stages (Fig. 3).
High density also reduced lateral root number, compared to that at low
density (p = 0.002), for 50 d-growth individuals in infertile soil.