Responses Of Coexisted Floating-leaved And Submerged Plants To Sediment And Water Level Rhythm And Differences In Genetic Structure Between The Species

Five aquatic species including Nymphoides peltatum(Gmel.)O.Kuntze,Myriophyllum spicatum L.,Cerphyllum demersum L.,Hydrocharis dubia(B1.)Backer and Trapa quadrispinosa Roxb.,frequently coexisting in common communities,play important roles in lake ecosystems.Human disturbance,sediment and the change of water level not only have effects on the growth of those aquatic plants,but also affect their genetic diversity.Therefore,in this study,we simulated the conditions of sediment and water level to explore(1)the response of two submerged plants to shading of floating plants and(2)the adaptation of aquatic plants to different sediments and the change of water level in terms of their morphological traits and biomass allocation.In addtion,we also analyzed the genetic diversity,genetic structure and sampling strategy of N.peltatum and M spicatum co-occurring in the same natural communities by SSR molecular marker technique.The results were as follows:1.When submerged C.demersum and M spicatum and floating-leaved H.dubia co-cultivated in different sediments,H.dubia grew more vigorously in silt and the mixed sediment of sand and silt,which contained higher level of nutrients than the sand sediment.The floating H.dubia did not suppress the growth of the two submerged plants.In order to compete with H.dubia for light and nutrient in water,rootless C.demersum tended to increase branch number,stem length and leaf mass ratio,so then C.demersum accumulated more biomass in presence of H.dubia.Rooted M.spicatum can absorb nutrition from water and sediment,so can weaken the competition for nutrition with H.dubia.Therefore,M.spicatum adapted to shading of H.dubia by regulating plant morphology.2.We designed two water level treatments:(1)low level in autumn and winter?high level in next spring and summer(simulating natural water level rhythm)and(2)high level in autumn and winter?low level in next spring and summer(simulating artificial water level rhythm).Submerged C.demersum and M spicatum and floating-leaved N.peltatum and T.quadrispinosa were co-cultivated under the two conditions.There were considerable differences in the performance of these species.In general,the treatment of simulating natural water level rhythm favored the growth of C.demersum and N.peltatum,compared to that of simulating artificial water level rhythm in the summer season of vigorous growth,but water rhythm had negligible influence on the growth of the species in winter.In all seasons,M spicatum grew more vigorous at the high water level than at the low level.For T.quadrispinosa,which overwintered via seeds,showed insensitive to the change of water rhythm.Therefore,the change of water rhythm may change the species composition and diversity of aquatic plant communities.3.Submerged M spicatum and floating-leaved N.peltatum coexisted in the same natural communities in Donghu and Liangzihu Lakes.We collected 244 samples of N.peltatum and 246 samples of M spicatumand analyzed their genetic structures by 12 and 10 pairs of SSR primers,respectively.The numbers of alleles detected in all samples of N.peltatum and M.spicatum were 93 and 63,respectively;the genets in all samples of N.peltatum and M.spicatum were 115 and 95,respectively.AMOVA analyses showed that the genetic variation among the subpopulation of N.peltatum was considerably high(about 46.56%),while that of M spicatum was only 6.54%.According to the genet distribution in the Donghu population,all but one genets of N.peltatum were specific to each subpopuloation.However,genotypes of genets of M spicatum shared among subpopulations frequently,and the most general genotype was shared in six subpopulations.The differences between the two species may be related to their reproduction modes.N.peltatum reproduces vegetatively mainly via rhizomes,which spread infrequently among subpopulations,thus little gene flow and notable differentiation among subpopulations.In contrast,M.spicatum always reproduces through shoot fragments,which are produced and spread readily among subpopulations,thus notable gene flow and genotype sharing among the subpopulations.According to the clustering analysis and principal component analysis,generally,for each species the samples originated from the same lakes clustered together,which were genetically distant from those from the different lakes indicating limited gene flow between lakes.Exceptionally,D2 subpopulation of N.peltatum in the Donghu Lake was genetically distant from and had higher clonal diversity than all the other subpopulations in the Donghu and Liangzihu Lakes.