Phenotypic Plasticity Of An Invasive Species Alternanthera Philoxeroides In Response To Heterogeneous Habitats

Most invasive plants experience highly variable environments during their invasion, spread and establishment, therefore, it is crucial to understand the factors controlling the success of invasive species. Few studies have evaluated the phenotypic plasticity of invasive plants in response to heterogeneous habitats. This study reports on the effects of light intensity, soil nitrogen concentration, soil temperature and soil water on morphological traits and biomass allocation performance of Alternanthera philoxeroides, both interactive and main effects of independent variables were analyzed.Controlled experiments were carried out to examine the effects of light intensity, soil nitrogen concentration, soil temperature and soil water on A. philoxeroides. Approximately identical size of ramets from one genotype of population were planted separately in plastic pots and then grown under different treatments. The results are mainly as follows:1. Full sunlight enhanced the production accumulation of A.philoxeroides and the plant could allocate more resources to leaf and above-ground stem and less to below-ground root under shade as a response to low light condition. Soil nitrogen level also had a significant effect on plant growth, showing an increase of total biomass, plant height, total leaf area, stem biomass ratio and leaf biomass ratio with the increasing soil nitrogen concentration. Additionally, there was a significant light×nitrogen interaction on total biomass, root biomass ratio, leaf biomass ratio and stem biomass ratio. Differences in leaf area, total biomass and leaf biomass ratio between two light intensities reduced with the increasing soil nitrogen concentration, whereas those in plant height and leaf biomass ratio showed an opposite trend. It was striking that the light effect on stem biomass ratio was greatly dependent on nitrogen concentration. Under low nitrogen condition, stem biomass ratio was significantly higher in full sunlight treatment compared with that in shade. But it was significantly lower under middle nitrogen condition, and the tendency became even more obvious under high nitrogen condition. Our data further implied that the negative effect caused by shade on plant growth of A. philoxeroides could be altered by phenotypic plasticity in morphology and biomass allocation performance in a response to high nitrogen concentration, which in turn adapts the plant to low light condition. This suggested that the great success of A. philoxeroides over a variety of heterogeneous habitats is probably due to both a high degree of phenotypic plasticity of individuals and compensation of one resource for another.2. The number of fibre and the mean diameter of taproot of A. philoxeroides increased dramatically with soil temperature increased. Plant invested more resources to underground and less to aboveground under higher soil temperature. There was a positive effect of soil temperature on the morphological traits of A. philoxeroides. As soil temperature increased, the stem diameter and total biomass of plant increased, but height, leaf areas and biomass allocation of plant exhibit no obvious change. It showed that the higher total biomass production of A. philoxeroides may result from the increase of stem diameter, the number of fibre and the mean taproot diameter as well in high soil temperature. Under the soil temperature treatment, plant can increase the stem diameter and leaf areas in order to capture the light resource efficiently, and thus, enhance its growth potentiality. There is no significant effect of soil temperature on biomass allocation of A. philoxeroide. The explanation for this may be that plants are predominately distributed in both subtropic and tropic regions. On the other hand, plant can increase the number of fibre and the mean taproot diameter in response to the reduction of soil water resulted from the increase of soil temperature.3. Plant height, total biomass, stem diameter, and leaf areas of A. philoxeroides decreased as soil water decreased. It showed that plant can enhance the ability to capture light by increasing plant height and stem diameter in aquatic habitats. In drought habitat, plant invests more sources to underground (root) and less to aboveground (stem and leaf), reducing the limitation of soil water to plant growth. The results further imply that there is an intergrative response of plant to soil water, and the different reaction exists in different plant organs.Our data showed that environment factors such as light intensity, soil nitrogen, soil temperature and soil water are important factors influencing the success of A. philoxeroide growth. Plant can enhance its adaptive ability by phenotypic plasticity to various environmental conditions. The study also confirmed the importance of nitrogen in affecting the establishment of invasive A. philoxeroides populations, and therefore, control of A. philoxeroides may be aided by nutrient management.