| Biological invasion has become one of the top five global environmental issues of the21st century. It not only does cause serious harm to the environment, but also caused incalculable damage to human health and economy. Scientists have put a lot of effort to the study of biological invasion mechanisms and prevention methods and found that there is a close link between microbial and plant invasion. Arbuscular mycorrhizal fungi (AMF) is a widely distributed fungi in farmland, grassland, forests and other terrestrial ecosystems, which play an important role in the regulation of nutrient, maintaining ecosystem stability and improving plant resistance. As AMF could forming a symbiotic relationship with more than90%plants, which including most of the exotic invasive plants, thus the relationship between invasive plants and AMF has become a hot research. Several studies have been conducted with common garden experiments or field experiments, however it is one-side and incomplete just to use one of the two experiments. We developed an experimental system which including field experiment, common garden experiment and in vitro experiment to avoid these problems. According to current reports, this experiment is a comprehensive and systematic research methods to study the relationship of AMF and invasive plants.Wedelia trilobata, an typical invasive plant caused serious ecological harm in south China, was studied in field experiment, common garden experiment and in vitro experiment in this paper. The the effects of W. trilobata invasions on rhizosphere microbial communities, the effects of AMF on W. trilobata phenotypic plasticity and allometry, the mycorrhizal dependency, the effect of extracts and root exudates of W. trilobata on AMF spore germination and mycelial growth was invested in this study, with the aim to provide a complete relationship between AMF and plant invasions. The main results were as follows:1. Rhizospheric soils of Wedelia trilobata with different degrees of invasion (uninvaded, low-degree, and high-degree using its coverage in the invaded ecosystems) were collected from five discrete areas in Hainan Province. Soil physicochemical properties and the community structure of the soil microorganisms were assessed. Low degrees of W. trilobata invasion significantly increased soil pH values whereas high degrees of invasion did not significantly affected soil pH values. Moreover, the degree of W. trilobata invasion exerted significant effects on soil Ca concentration but did not significantly change other indices of soil physicochemical properties. Low and high degrees of W. trilobata invasion increased the richness of the soil fungal community but did not pose obvious effects on the soil bacterial community. W. trilobata invasion also exerted obvious effects on the community structure of soil microorganisms that take part in soil nitrogen cycling. These changes in soil physicochemical properties and community structure of soil microbial communities mediated by different degrees of W. trilobata invasion may present significant functions in further facilitating the invasion process.2. An experiment was conducted in Hainan Island, to verify the effects of density and AMF on the typical invasive clone plant W. trilobata. Two AMF levels were obtained by applying a fungicide that suppresses the AMF naturally present in soil. Plant sowing density was set to25or50seedlings m-2. Samples were collected at30,60,90, and120d after planting. The colonization rate of W. trilobata was promoted by high-density and high-AMF treatments. Both density and AMF treatments affected the phenotypic plasticity and allometry of W. trilobata, and the effect of AMF on plant phenotypic plasticity and allometry varied at different densities. Low-density and high-AMF treatments increased plant total biomass, whereas high AMF treatment reduced belowground biomass allocation. To the best of our knowledge, this study is the first to test the effects of AMF and density on the phenotypic plasticity and allometry of invasive clonal plants. The results of this study could serve as reference to elucidate the mechanism of clonal plant invasion.3. A common garden experiment, which with two soil nutrition treatments and five AMF treatments, was conducted in Jiangsu University to study the mycorrhizal dependency of W. trilobata. Samples were harvested at120d after planting. W. trilobata could symbiosis with many AMF, and high infection rate was found with high and low soil nutrition treatments. Inoculated independently or together with Glomus etunicatum, Glomus mosseae, Glomus intraradices all increased the shoot biomass and significant changed the proportion of rate of root/shoot. Low nutrient and AMF inoculation reduced the length of plant roots. W. trilobata showed a low mycorrhizal dependency of120%-170%in this study. Relative to the low nutrient treatments, AMF grows more spore in the high nutrient treatments.4. An in vitro system using Ri T-DNA transformed roots and AMF was used to obtain the spore of G.i. first. Then the effects of root exudates and plant extractives of W. trilobata on spore germination and hyphae growth of G.i. were examined. Root exudates aqueous extracts did not affect the spore germination and hyphae growth, while the root exudates organic phase extracts inhibited germination rate and hyphae growth. Isobutyl phthalate inhibited spore germination and hyphal growth. Iso-octyl phthalate did not affect the spore germination and hyphae growth. Spore germination and hyphae growth were not affected by the10#purified composition of plant extractives, while were inhibited by3#purified composition of plant extractives.5. In the field experiment, we found that W. trilobata invasion increased the richness of the soil fungal community. The study results suggest that W. trilobata invasion may have close contact with soil fungi.Ini the common garden experiment, the use of fungal inhibitors affected the growth of W. trilobata. The following mycorrhizal dependency experiment, W. trilobata showed a low mycorrhizal dependency. And some root exudates and plant extractives of W. trilobata inhibited pure cultured AMF spore germination and hyphae growth.In summary, W. trilobata invade new environments may be not only dependent on AMF symbiosis but also associated with other soil fungi and environment factors. |
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