Abstract: Most invasive plants experience highly variable environments during their invasion,spread and establishment,and therefore,it is crucial to understand the factors controlling the success of invasive species.However,few studies have evaluated the interactive effect of both light intensity and soil nitrogen levels on functional response in phenotypic plasticity of invasive plants.The present study examined the main effects of light intensity and soil nitrogen level and their interactions on morphological traits and biomass allocation performance of an alligator weed Alternanthera philoxeroides,a widespread invasive plant in China,by outdoor experiments.Approximately identical size of ramets from one genotype of population were planted separately in plastic pots with combined compost and sand,and grown under the combination of two light intensities(full sunlight and 16.5% of full sunlight) and three soil nitrogen levels(low,midterm and high) treatments.The result indicated that 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譶itrogen 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.In addition,the light effect on stem biomass ratio depended on nitrogen concentration greatly.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.Our 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.