Asexual Propagation Of Aquatic Plants Response To Sediment Nutrients

Asexual reproduction, one of the most important reproduction means of plants, has emerged as a focal point of ecological research. Meanwhile, it has been studied by numerousforeign and domestic researchers in many aspects of plant ecology. Aquatic plantis one of the most impontent elements of aquatic ecosystem.Many aquatic plants reproduce asexually and asexual reproduction is more frequent in aquatic plant than in terrestrial plant. However,thebasic theoretical studyof asexual reproductionin aquatic plant is still limited in China, especiallywith the combination of main environmental factors. In this thesis, asexual reproduction of 9 aquatic plants and the effects of sediment nutrient on invasion of exotic aquatic plants under different levels of sediment nutrientwere studied using greenhouse experiments and ecophysiology methods. Main contents and conclusions are as follows:(1) To determine how the size related auto-fragmentation in the submersed macrophyte Myriophyllum spicatum in response to the sediment nutrients. An experiment was carried out in two nutrient-level sediments, and three harvests were conducted at three-week intervals. Our results revealed that sediment nutrient significantly increased total plant biomass, auto-fragment biomass and amount of total non-structure carbohydrate (TNC) in auto-fragment.These traits also increased significantly over time. The linear regression model was used and found that the auto-fragments biomass and the amount of TNC in auto-fragment increased proportionally with total plant biomass (size) across the three harvest weeks. The sediment nutrients significantly raised the slopes of linear regressions in auto-fragments biomass and TNC storage.But, its intercepts were not changed. These results confirmed that the auto-fragmentation of M. spicatum is strongly dependent on individual size. Meanwhile, the changes in linear coefficients among treatments indicated sediment nutrients positively rather than plant size affects the auto-fragmentation, which suggested the size-independent effect. Furthermore, the changing of plant biomass partitioning may explain the size-independent auto-fragmentation under different nutrient-level sediments.(2) Nutrient availability and plant density may determine the asexual reproduction of plants. It remains unclear that whether the auto-fragmentation ofM. spicatumvariations in response to environmental factors are driven only by a size dependency effect or if the plasticityof plant may also shift production of auto-fragments. We conductedthis experiment by planting apical shoots of M. spicatumunder two different nutrient-level sedimentsand two different plant density levels, respectively.After five month growth, sediment nutrients and plant density brought about a strong dependency of auto-fragment production and the amount of TNC storage in auto-fragments on individual plant size (total plant biomass). However, these two factors acted differently on size dependency. Sediment nutrients positively affected auto-fragment production and the amount of TNC in auto-fragments of M. spicatum. High concentrations of sediment nutrients significantly increased these two traits in absolute value and the value relative to plant size.Although the auto-fragment biomass and the amount of TNC in auto-fragments did not differ between density treatments when plant size was considered, the absolute values of these two traits were much larger in the low plant density treatment than in the high plant density treatment, which suggested an indirect negativeeffect of plant density on the auto-fragmentation of M.spicatum. In addition, higher percentages of large auto-fragments (>100mg) were produced by plants that grew in low-level sediment nutrients and low-level plant density environment than plants in high-level sediment nutrients and high-level plant density environment. These results highlight a size-independent effect of auto-fragment production and the amount of TNC in auto-fragments of M. spicatum. Furthermore, such size-independent effects can be explained by the significant biomass partitioning differences and the similar TNC-concentrationsin auto-fragmentsunder different environmental conditions.(3) Unlike other aquatic macrophytes, turion of Potamogetoncrispus is produced in the early summer and helps this species tolerance the high summer temperature. Out-door experiment wasconducted to study the effects of sediment nutrients on turion production. After 10 weeks growth, plant grew under low nutrient sediment were tend to produce turions, and the turion size was bigger. Turions formatting under low nutrient sediment stored more total amount of non-structural carbohydrate than turions under high nutrient sediment. However, the total amount of nitrogen and total amount of phosphorus was greater under high nutrient sediment.Path analysis revealed that the total plant biomass has a direct effect on turion production, and an indirect effect on the size-number trade-off of turions. Meanwhile, sediment nutrients mediated the turion production and the size-number trade-off by affecting the vegetative traits (e.g., leaf and stem production, ramets number).This experiment implied that P. crispus produced lager size of turion with weak spread abilities to increase the local adaption under low nutrient sediment, which exhibited the resistance strategy; whereas P. crispus produced smaller turions to increase spread, because the plant density was high and the survival may lowunder high nutrient sediment, which exhibited the escapt strategy. Such flexible reproduction strategy maximum the turion survival under heterogeneous environment, and could buffer the negative impact of environmental selection.(4) Few studies have focused on the asexual propagule pressure and the interaction of propagule pressure with other environmental factors that regulate the success of invasive speciesin aquatic habitats. This study tested the hypothesis that the differences in asexual propagule production and the establishment between exotic and native species may be driven by the different sediment nutrient conditions, and these differences may ultimately influence the success of invasion in aquatic habitats. Three pairs of aquatic exotic macrophytesand their native counterparts from Hydrocharitaceae(Elodea nuttallii vs. H. verticillata and Egeria densa vs. H. Verticillata)Haloragaceae (M. propinquum vs. M.ussuriense) were used to comparethe growth and rooting efficiency differences in nutrient-poor and nutrient-rich sediments, respectively. After 8 weeks growth, exotic species tend to produce more branch biomass and apical shoots (potential propagule) than native species in nutrient-rich sediment. The rooting efficiency and root growth of exotic fragments were higher than those of native species in nutrient-rich sediment. The survival percentages of these species were not significant between high and low nutrient sediments. Our experiments provide evidence that sediment nutrient enrichment not only increases the asexual propagule production of exotic macrophytes but also enhances the likelihood that exotic asexual propagules with establish themselves in a new environment, and our results suggest that sediment nutrientsare important factors that drive the propagule pressure in aquatic habitats.(5)Resouces availability and propagule pressure would influence the success of plant invasion. To investigate the interaction between local environment and propagule number, we compared the fragments growth between M. aquaticum and M. oguraensein different local environments. The results revealed that fragments of M. aquaticum had greater rooting efficiency than fragments of M. oguraense. After four months experiment, biomass of both plants was increased with the increasing of sediment nutrients, butM. aquaticum did not response to the light availability. The abundance of total nitrogen and total phosphorusfacilitated the biomass accumulation of M. aquaticum, and a negative relationship between M. aquaticum biomass and M. oguraense biomass was also observed. By analyzing the content of total nitrogen and total phosphorus in M. aquaticum, in LiangziLake, the total phosphorus limited its growth. Meanwhile, the small wetland plants which exist after the removing the above-ground of Carex species may resist the invasion of M. aquaticum in wetland environment.Sediment nutrient was an important environmental factor to determine the asexual reproduction of aquatic plants. Different reproduction strategies were exhibited under different nutrient-level sediments. Changes of these strategies were not only regulated by plant size but also affected by the micro-environments where plants growing. Both asexual propagule production and storage in propagules were affected by the different sediment nutrients. To maximum the survival of asexual propagule, the optimal allocation of storage was conducted by plants under heterogeneous environment. Furthermore, sediment nutrient enrichment may increase the propagule pressure of exotic aquatic plants and facilitated the invasion success of exotic aquatic plants.