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.