| The plant growth and maintenance,community restoration and reconstruction of submersed macrophytes are important links in the aquatic ecosystem reconstruction in eutrophic lakes,while the growth,propagation,community structure and distribution of submersed macrophytes are affected by environmental factors.Carbon and nitrogen metabolism of submersed macrophytes are the key processes for their growth,propagation and adaptability.The growth,overwintering and germination of submersed macrophytes are influenced by the variation of their carbon and nitrogen metabolism caused by the environmental changing.In this research,clonal growth,propagule forming,overwintering and germination of submersed macrophytes were studied under different environmental conditions in controlled experiments and field investigation by measuring plant morphological and phylogical traits,e.g.,biomass and its allocation,carbon and nitrogen metabolite contents.The main results were showed as following:1.The typical clonal submersed macrophytes Vallisneria natans was selected to study the clonal growth and carbon and nitrogen metabolism in response to three levels of light(Low,moderate and high,i.e.,4%,15% and 30% natural light,respectively),three levels of nutrient(Low: 10 g,moderate: 25 g,high: 50 g controlled release fertilizer)and two severance treatments(intact or severed stolon).The plants were separated into three groups(large-,moderate-and small-sized ramet)according to the ramet biomass,and the variations of the growth traits and carbon and nitrogen metabolism for each group were analyzed after the 8-week experiment.Light affected the biomass and its allocation of V.natans significantly,explaining 80.1% of the total variation of the total biomass.Compared with high light treatments,the total biomass and the number of large-sized ramet of V.natans were significantly decreased under low light,while there was no significant difference in total ramet number.The total biomass and aboveground biomass of V.natans were significantly affected by nutrition levels in water column.Clonal fragmentation affected the total ramet number and belowground biomass significantly,explaining 61.8%and 20.7% of the total variation,respectively.The total biomass of V.natans was significantly and positively correlated with its contents of total carbon(TC),carbon to nitrogen ratio(C/N ratio),contents of soluble carbohydrate(SC)and starch in the below-ground tissues,and the ramet number was significantly and negatively correlated with the C/N ratio and the contents of SC and starch in the aboveground tissues,this indicated that biomass accumulation for V.natans was a assimilating metabolism process while clonal propagation was a carbon consumption process.High water nutrient significantly reduced the SC content of the aboveground tissue for V.natans in three light intensities,and significantly increased the TN and free amino acid(FAA)contents of the belowground tissue in moderate and high light intensities,which may because V.natans absorbed the nutrient from water and converted it into its own nitrogen by consuming the plant carbon when the water nutrient level was high.Moreover,clonal fragmentation significantly reduced the starch content in belowground tissue and increased the starch and SC contents in overground tissue of moderate-and large-sized V.natans ramets in high light with moderate and high nutrient.2.Three typical submersed macrophytes of different propagation types,Potamogeton maackianus,Myriophyllum spicatum and Hydrilla verticillata,were selected to study the growth,overwintering propagation and carbon and nitrogen metabolism in response to three levels of light(Low: 5%,moderate: 14%,high: 29% natural light)and nutrient(Low:5 g,moderate: 25 g,high: 50 g controlled release fertilizer).Variations of plant growth and carbon and nitrogen metabolism were evaluated seasonally during the 15-mounth experiment.The plant growth,propagation and overwintering in response to light and water nutrition levels were species specific,and the effects of light and water nutrition on the growth of the three submerged plants was ordered as follows: M.spicatum > P.maackianus> H.verticillata.No significant effect of light,water nutrition and their interactions were found on the growth indexes of H.verticillata in the experiment,while the biomass of M.spicatum decreased significantly at low light and high water nutrition,and light was the primary impact factor to the growth of M.spicatum and P.maackianus.The three submersed macrophytes had obvious physiological responses to low light,showing higher total nitrogen content and lower C/N ratio.In moderate and high nutrition water,the content of SC and starch of P.maackianus in autumn and winter were increased while the TN and FAA were decreased at high light.This indicates that plant consumed its nitrogen for carbon storage in autumn and winter,which maybe benefit to its successful overwintering and germination in the following year.In autumn and winter,the C/N ratio of H.verticillata and its turion number increased with increasing light intensity,and high light was beneficial to the carbon accumulation for H.verticillata.The SC content of H.verticillata affected its turion forming.Turion number of H.verticillata was markedly and positively correlated with the SC content,and the starch content in the turion decreased with increasing turion number for H.verticillata,which may potentiallyaffect the turion germination and plant growth in the next year.3.The turion of H.verticillata was selected to study the overwintering,germination,growth and carbon and nitrogen metabolism in response to three initial turion biomass(large,moderate and small turion),three sediment types(clay,mud and sand)and two depths of planting(sediment surface and 5 cm depth below the surface).Variations of the carbon and nitrogen metabolism,germination rate and seedling growth were analyzed during the experiment.The TC content of the turion decreased during the winter,the FAA content increased by deep planting.The TN content of small turion was significantly higher in the clay and mud sediment in 5 cm depth than on the surface.,Seedling formed from small turion in 5 cm depth showed lower starch content than formed from moderate and large turion,suggesting that germination of the turion is a process of carbon consumption.Seedling height was higher when formed from turion in 5 cm depth than sediment surface.The initial biomass of turion affected the seedling biomass significantly,explaining 93.6%of the total variation of the seedling biomass,with higher biomass for plant germinated from larger turion.The planting depth affected the germination rate,seedling height and SC content significantly,explaining 66.9%,79.7% and 70.4% of the total variation,respectively,and the turion in sediment surface showed high germination rate than the turion in 5 cm depth.In conclusion,large turion with shallow burring was benefit to the germination and plant growth in the coming year for H.verticillata.4.The Youshui River and Tie River in You County of Hunan Province were selected to investigate the growth,overwintering and regrowth of submersed macrophytes in autumn 2020 and spring 2021.The species richness and construction of submersed macrophyte communities were different among the sampling sites with varying environment,and the germination and regrowth of submersed macrophytes in spring also differed with the environment changing.Starch content in nutrient storage parts of submersed macrophytes was higher in April 2021 than in November 2020,which suggested that submersed macrophytes transported a large number of carbohydrates to propagules and then stored as starch in autumn and winter,which was consistent with our former experiments.To summarize,the present studies on the plant growth,overwintering and germination of submersed macrophytes,based on evaluations of growth traits and carbon and nitrogen metabolism,highlight the morphological and physiological strategies for submersed macrophytes in response to environmental heterogeneity,providing empirical values for the restoration and management of submersed macrophytes community. |
