Studies On Reproductive Biology And Ecological Adaptability Of Invasive Plant Parthenium Hysterophorus L.

Biological invasions destroy the structure and function of native ecosystems, make the loss of the biodiversity, and cause huge economic losses, as well as threats health of human being. It is an important element in global changes and has received considerable attention during the last decades. However, the mechanisms that underlie invasiveness are still not well explained. Researches, which identify the factors that influence invasions by alien plants species are needed to be carried out. This will help us improve our ability to predict and control potentially invasive species.Parthenium hysterophorus (Asteraceae) is an invasive weed native to southern North America, Central America, the West Indies, and central South America. It is a famous noxious weed in the world. This weed has posed serious threats to crop production, natural biodiversity and human health, because of its prolific growth, rapid spreading, and production of toxic allelochemicals. It has a wide range of ecological adaptation and great reproductive potential. The populations of this invasive species have spread rapidly during recent years in China.In this paper, we studied the phenology and quantitative characters of flowering and seed-set, biomass allocation as well as reproductive process of P. hysterophorus population in different environments by field investigation and control trial. We compared the reproductive potential of this weed in different environments and in different seasons according to above research. We also studied its ecological adaptability under different nitrogen and phosphorus levels and light levels. The objective of this research was to explore the reproductive capacity and mechanism of invasion of Parthenium weed, and provide some basic information for preventing and controlling this weed. The results are as follows:(1) Parthenium weed has long reproductive period, which made this weed continuously produce abundant seeds and seedlings. It grew from March to October and propagated from early May to October in grassland, sparse forest, and roadside. It propagated until to December in farmland.(2) The quantitative characters of flowering and seed set were different under different environments and in different seasons. Therefore, the reproductive capacity was also different. In grassland, sparse forest, and roadside, flowering plants, branch numbers and capitulum's number as well as seed production were more in autumn than in summer and in spring. The reproductive capacity of the weed in the three habitats was the greatest in autumn. In farmland, branch number and capitulum's number were the most in winter. The reproductive capacity was also the greatest.In autumn, the sequence of reproductive capacity in four habitats was farmland > grassland > roadside > sparse forest. In farmland, the population density, branch number and capitulum's number were less, but the seed production was the most and the reproductive capacity was the greatest because of high seed set and 1000-seed weigh. In grassland, the population density, branch number and capitulum's number were more, the seed production was high, and the productive capacity was great. In roadside, the flowering plants, branch numbers and capitulum's number were more than those in former habitats, but the seed set and1000-seed weigh were less, so the seed production was less than that in grassland. In sparse forest, the population density, branch number, capitulum's number, and seed production were less than those in former habitats were, but the quality of seed was high. In a word, the reproductive capacity was very great in the studied sites.The seed set and 1000-seed weigh in 2008 had no significant difference with those in 2009 in the same habitat. In sparse forest and roadside, the capitulum's number and seed production in 2008 had no significant difference with those in 2009. The reproductive capacity had no much change in different years in the two habitats because of fewer disturbances. In grassland and farmland, the capitulum's number and seed production in 2008 had significant difference with that in 2009. The reproductive capacity was various because of the interferences by grazing and farming.(3) Parthenium weed adapted to different environments by adjusting each organ biomass ratio. In grassland and roadside, it competed underground resource by improving the root biomass ratio; in sparse forest, it captured more light by improving the shoot and leave biomass ratio. In farmland, it increased root biomass ratio to compete underground and improved leaf biomass ratio to capture light. The biomass allocations of Parthenium weed closely related with environments during the reproductive period, and were restricted by plant height and density. It had high plasticity to environment. The high plasticity under different environments in different seasons showed the powerful ability to invasion.(4) Parthenium weed was fast-maturated weed, which propagated by seeds. It took only 22 days from the beginning of flower bud to seed maturity in June. Pollen grain with spine prominence was easy to attach the body of pollinators. Pollen-ovule (P/O) ratio was11556.6±323.2. Pollen was starchy and shed from 9:00 to 13:00. Pollen viability was high during this period. Stigma acceptability lasted 5 days. The optimal time for accepting pollen is the second and third day after stigma protruded. There was no apomixes. The breeding system of this weed was xenogamy in China and dependent on pollen vector. The pollinators were local general insects, such as Apis cerana, A. frorea, A. andreiformis, Musca hervei, Lucillia sericata. The peak for visiting flower was 10:00-13:00.(5) Parthenium weed exhibited considerable nutrition-acclimation abilities. In a certain nitrogen range, root mass ratio (RMR) reduced with increasing nitrogen levels; total biomass, leaf mass ratio relative (LMR), flower and fruit mass ratio (FMR), leaf area index (LAI), total leaf area (TLA) and leaf area ratio (LAR), relative growth rates (RGR), branch number, as well as capitulum's number increased with increasing nitrogen levels. To phosphorus nutrient, LMR, LAR, TLA, RGR increased with rising phosphorus levels. Total biomass, LAI, branch number, capitulum's number had no difference among different phosphorus levels. Parthenium weed improved seed production with the increase of nitrogen, while improve seed quality with the increase of phosphorus. It could adjust the reproductive traits by environment nutrient which promote its invasiveness in rich nitrogen and phosphorus environments.(6) Parthenium weed could not survive under low light condictions (5% of light intensity). However, it was able to acclimate to different light condiction (22-100% of light intensity) by adjusting total biomass, RMR, LMR, FMR, TLA, LAR.The above research results show that the productive capacity of Parthenium weed,which exhibited various under different environments in different seasons, was very great. It has a broad ecological adaption under different nutrient levels and light levels. We should pay attention to the invasion of Parthenium weed under different environments in different seasons and take suitable controlling measures.