| Biological invasion has become a global concern. In order to control biological invasion, it is essential to understand their patterns and mechanisms. Plantago virginica L. is an annual or biennial plant that is native to North Amarica. It has invaded croplands, orchards, gardens in many provinces in eastern, southern and central China since it was first found in 1951 in Nanchang City, Jiangxi Provence. Recently, it became a big threaten to urban ecosystems, especially lawn ecosystems.In this study, invasive mechanisms of P. virginica, including allelopathic potential, interspecific competition and phenotypic plasticity were explored by conducting greenhouse cultural experiments and common garden experiments as well as comparative field investigations between native and invasive habitats. It was found that allelopathic potential, interspecific competition and phenotypic plasticity all contributed to the successful invasion of P. virginica.In the experiment of allelopathic potential, P. virginica was found to have allelopathic potential on seed germination and growth of seedlings of all the four widely used lawn species, Agrostis matsumurae Hack. ex Honda, Cynodon dactylon(Linn.) Pers., Festuca elata Keng ex E. Alexeev, and Poa annua L. However, allelopathic potential varied with species and developmental stages. For example, under treatment with P. virginica extraction of concentration of 100 mg/ml, seed germination rates of A. matsumurae declined 51.4%, compared to control, but seed germination rates of C. dactylon and F. elata were unchanged. Under the highest concentration of extraction of P. virginica, seeding height of A. matsumurae and C. dactylon was decreased by 37.1% and 34.2% respectively, but those height for F. elata and P. annua did not change. While some promoted effects of extraction of P. virginica on native species existed, such as enhanced aboveground biomass in A. matsumurae under treatments than control, most effects were inhibitive, which supported the ―New Weapon Hypothesis‖.Comparisons on compounds and their content in P. virginica in their native habitats in US and in invasive habitats in China and in P. asiatica L., a native Plantago species in invaded habitats in China were conducted. Compounds and their content in roots, leaves, inflorescences, and seeds all changed after P. virginica invaded to a new habitat, indicating plasticity. The changes in compounds might be one of the causes for allelopathic potential. In the analysis of chemical composition with gas chromatography-mass spectrometry, there were 25 and 28 compounds were detected in leaves of P. virginica in invaded area and in native habitats, respectively. For P. virginica in invaded area, the compound that had the highest concentration in leaves, relative content of 6.85%, was 6, 10, 14-trimethyl-2- pentadecanone and the compound of the highest concentration in leaves of P. virginica in native habitats was 1-octadecane, with the relative content being 5.33%. The content of total flavonoids in leaves and seeds of P. virginica in invaded habitats in China was 2.56±0.11% and 7.68±0.16%, respectively, which was significantly higher than those in the leaves and seeds of P. virginica in their native habitats in US, 1.51±0.13% and 2.88±0.08%, respectively. Content of total flavonoids in seeds of P. virginica in native habitats in US was lower than both of invasive P. virginica and native P. asiatica in invaded habitats in China. There was no significant difference in content of total polysaccharide in leaves among P. virginica in native habitats and in invaded habitats, and native P. asiatica in invaded habitats, which were 0.24±0.05%, 0.17±0.04%, and 0.26±0.07%, respectively. Different from total polysaccharide in leaves, no difference in content of total polysaccharide in seeds between P. virginica in invaded habitats(6.60±0.10%) and in native habitats(6.73±0.31%), but they were both significantly higher than that of P. asiatica native habitats(3.20±0.26%). The content of the two main components of polysaccharide, xylose and arabinose, in seeds of P. virginica in invaded area was 23.49±0.71% and 19.74±1.01%, respectively, with no significant difference with those in native P. virginica(23.97±0.61% and 19.54±0.73%, respectively), but the content of xylose and arabinose in seeds of P. virginica in both invaded area and native habitat was significantly higher than those of native P. asiatica in invaded area(19.49±0.32% and 14.55±0.65%, respectively).The results from the interspecific competition experiment, in which seeds of P. virginica and the above four lawn species were mixed and sowed in pots, showed that the density of P. virginica seeds had different effects on the four lawn species during seed germination. In detail, seed germination of P. annua and A. matsumurae was more sensitive to P. virginica than the other two species, C. dactylon and F. elata. The germination rates of P. annua and A. matsumurae under highest density of P. virginica seed treatment deceased by 21.15%and 15.38%, respectively, in contrast, those for C. dactylon and F. elata only deceased by 9.52%and 9.23%, respectively. At the later stage when seedlings grew, density of P. virginica seedlings had significant effects on biomass clipped and soluble sugar of the four lawn species. For example, P. virginica had less effects on soluble sugar in leaves of F. elata and P. annua than those of C. dactylon and A. matsumurae. Under highest density of P. virginica, soluble sugar in leaves of F. elata and P. annua declined by 31.72% and 33.43%, respectively, but those values for C. dactylon and A. matsumurae were 66.38% and 48.96%, respectively, indicating more resistant in F. elata and P. annua than in C. dactylon and A. matsumurae. The mechanisms for these effects were complicated, including not only interspecific competition, but also alsoallelopathic potential.The phenotypic plasticity studies by investigations on population ecology and comparison of physiological and reproductive characteristics of P. virginica between native and invasive habitats revealed that after invasion, P. virginica exhibited phenotypic plasticity, to different extent, in morphology, physiological and reproductive characteristics under different population density. For example, plant height of P. virginica in the middle-late growing stage was 15.89±1.62 cm in native habitat, and was 25.87±5.59 cm, 15.66±3.22 cm, and 8.69±1.30 cm for low, medium and high density populations in invaded regions, respectively. The final purpose of phenotypic plasticity was to produce more reproductive organs in order to colonize, grow, and spread. Infructescence biomass of P. virginica in native habitat and in invaded habitat was 48.77±16.65 g/m-2 and 158.16±25.66 g/m-2, respectively. Invasion by P. virginica significantly reduced the biodiversity of invaded ecosystems.In the experiment to study the effects of P. virginica invasion on soil physical and chemical properties and microbial populations and structures, significant changes in soil properties were detected after invasion. Among those changes, the most significant change was content of phosphorus. Content of total phosphorus was much lower in invasive soils(0.09±0.04%) than that in non-invaded soils(0.58±0.03%). This indicated that P. virginica consumed substantial phosphorus when they grew, which was favorable to the environment, on the other hand. In addition, invasion by P. virginica increased content of soil organic carbon and soil organic matter. Soil organic carbon content in invaded soils and non-invaded soils was 2.01±0.08% and 1.34±0.17%, respectively and soil organic matter content in invaded soils and non-invaded soils was 3.47±0.14% and 2.32±0.28%, respectively. Abundance and diversity of bacteria in soils were higher than those of fungi and metazoa. Moreover, bacterial abundance was higher in invaded soils than that of non-invaded soils at phylum and genus level. Opposite to abundance, bacterial diversity was higher in non-invaded soils. However, the difference in both abundance and diversity of bacteria was not statistically significant between invaded and non-invaded soils. Similar patterns were found for biodiversity of fungi and metazoan. Therefore, at phylum and genus level, bacteria, fungi and metazoan all exhibited some difference between invaded and non-invaded soils, indicating plasticity in microbial species composition and community structure. Difference of biodiversity of bacteria between invaded and non-invaded soils was even more obvious at genus level. Compared to bacteria, changes of fungi were bigger. 77.3% of total fungal genus in invaded soils and 65.2% of total fungal genus in non-invaded soils showed different between the two types of soils, but for bacteria, the two values were only 17. 1% and 13.5%, respectively. These results implied more critical roles by fungi than bacteria in the processes of P. virginica invasion. These changes were as a result of adaption of P. virginica to the new habitats and at the same time, were one of its impacts on the invaded environment.Overall, the mechanisms and strategies of successful invasion of P. virginica were explored in this study. We concluded that allelopathic potential, interspecific competition and phenotypic plasticity all contributed to the success of invasion and spread of P. virginica. Changes in compounds of P. virginica from native and invaded habitats were also examined. Moreover, differences in physical and chemical properties of soils as well as microbial populations and community structure between invaded and non-invaded habitats were investigated. The relationship between changes in compounds, soil physical and chemical properties and microbial populations and community structure and successful invasion was fully discussed. This study revealed the mechanisms of successful invasion of P. virginica and its impacts on the local environment and therefore, would provide important evidences for controlling the invasion by P. virginica. |
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