Morphological plasticity in response to population density varies with soil conditions and growth stage in Abutilon theophrasti (Malvaceae)

Abstract

The complexity of plant response to density in nature has been recognized, but seldom interpreted, especially in the context of various abiotic environments or plant ontogeny. Our objective was to investigate whether and how soil conditions and plant growth stage affected plant responses to density in a variety of traits under field conditions. Abutilon theophrasti (annual species) was planted at low, medium, and high densities (13.4, 36, and 121 plants m−2, respectively) under good or poor soil conditions and harvested 30, 50, or 70 days after seedling emergence. Responses of many traits to density varied with soil conditions and growth stages. Initially, leaf size, petiole angle, and stem diameter were decreased by high density in good soil, but not in poor soil. After 50 days, high density induced extra stem elongation and stem mass increment in both soil conditions; this was at the cost of biomass allocated to other organs in good soil. In contrast, in poor soil, high density decreased more morphological traits, with less effects on allocation patterns. At 70 days, there were decreases in more traits in response to density in poor soil, whereas most traits remained stable in good soil. Results demonstrated that density–plasticity was inherently complex, including multiple responsive trends within and among traits, and revealed that the ability of the plants to adapt to variation in density via responses of various modular traits has been underestimated. Soil conditions and growth stage may affect these responses, either directly through effects on plant size and competition strength, or indirectly through effects on strength of stem response. Contrasting stage-dependent plasticity to density under two soil conditions also revealed different growth strategies of plants in dealing with biotic environmental challenges. These findings highlighted the importance of considering roles of abiotic environmental factors and growth stage in modulating plasticity to biotic environments.