Physio-ecological Response Of Submerged Macrophytes In Poyang Lake Wetland To Environmental Changes

In recent years,the degradation of submerged vegetation in Poyang Lake wetland is obvious,and the biodiversity is reduced.It is important to explore the important environmental factors affecting the growth,reproduction and distribution of submerged macrophytes and their mechanism,which is of great significance for maintaining the ecological balance of Poyang Lake wetland.In this paper,combined with the determination of physicochemical indexes such as macrophytes and sedim-ents,the physiological and ecological responses of typical submerged macrophytes in Poyang Lake under the combined action of different water depths and nutrients were studied by using chlorophyll fluorescence kinetics and mesoscale experiments.The internal mechanism of photosynthetic system under heavy metals stress in submerged macrophytes was discussed.The results are as follows:?1?The three submerged macrophytes?Vallisneria natans,Hydrilla verticillata,Potamogeton Malaianus?were adapted to the shallow water area with sufficient light source by lateral growth?increasing branches or ramets?,and the deep water area by vertical growth?increasing plant height or pitch spacing?to obtain more light source.The ability of V.natans adapting to high light intensity was weaker than that of H.verticillata and P.Malaianus.The ability adapting to low light intensity was stronger than them,so it was more suitable for growth in the lower water layer.P.Malaianus has the smallest range of nutrient tolerance.Under the same water depth and nutrients,the photon efficiency of H.verticillata is higher than that of P.Malaianus and V.natans.The widest range of nutrient tolerance of H.verticillata,and its rapid reproduction,which are the main reasons for its increasing distribution in Poyang Lake.?2?In shallow water?<1.0 m?or deep water?>2.0 m?,the photosynthesis of V.natans was inhibited.Fv/Fm,Y?II?,ETRmax,soluble protein content and biomass decreased,but MDA content increased.The interaction between water depth and nutrients only had significant effect on plant height,biomass,Fv/Fm,CAT and MDA.In moderate eutrophication,the biomass,Fv/Fm,Y?II?,and ETRmax of V.natans were the biggest.V.natans was most suitable for growth in a moderately eutrophic environment in which the most suitable water depth is 1.0-2.0 m.?3?The effect of nutrients on the morphological index,photosynthetic fluores-scence characteristics and physiological indexes of P.Malaianus were greater than that caused by water depth.The interaction between water depth and nutrients only had significant effect on morphological index?plant height,ramet mubber,biomass?,Fv/Fm,Y?NPQ?,ETRmax and physiological index.In medium nutrition,the biomass,Fv/Fm,Y?II?and ETRmax of P.Malaianus were the largest.With the increase of nutrients and water depth,the MDA content increased rapidly and the electron transport rate decreased,the plants gradually withered and died.P.Malaianus was most suitable for growth in medium nutrient environment in which the most suitable water depth was 1.0-1.5 m.?4?The effect of water depth on morphological indicators?except branch number?,photosynthetic fluorescence characteristics?except Y?NPQ??and physiolo-gical index?except soluble sugar content?of H.verticillata was greater than that of nutrients.The interactions between two factors had had significant effect on the biomass,nodes,soluble protein,and soluble sugar.The differences of biomass,Fv/Fm,Y?II?and ETRmax were not significant between moderate and severe eutro-phication.H.verticillata was suitable for growth in these two environments in which the most suitable water depth is 1.0-1.5 m.?5?With the increase of Cu²⁺,Cd²⁺and combined concentration in water,the plant height and biomass of V.natans and P.Malaianus were significantly decreased;the content of photosynthetic pigments decreased gradually,and chlorophyll a decreased significantly compared with chlorophyll b.The toxic effect of Cu²⁺on chloroplasts was greater than that of Cd²⁺.V.natans and P.Malaianus were super-enriched macrophytes of copper and cadmium,and both had stronger copper enrichment ability than cadmium.The ability tolerating copper and cadmium of P.Malaianus was stronger than that of V.natans.P.Malaianus could be used as a repairing species with low concentrations of Cu²⁺and Cd²⁺contaminated waters.?6?V.natans had normal photosynthetic activity when Cu²⁺concentration was lower than 0.5 mg/L or Cd²⁺concentration was lower than 1.0 mg/L in water body.V.natans could not survive for a long time when Cu²⁺concentration exceeded 2.0 mg/L.V.natans had not the self-regulation ability when the combined concentration of Cu²⁺and Cd²⁺exceeded 3.0 mg/L.With the increase of heavy metal concentration,the efficiency of utilizing light energy of leaves was decreased,and the electron transfer of PSII reaction center was obviously inhibited.V.natans consumed excess light energy in the form of heat by self-regulation,to reduce the damage of the PSII reaction center.?7?P.Malaianus had strong photosynthesis activity in the environment when Cu²⁺concentration was lower than 1.0 mg/L or Cd²⁺concentration was lower than2.0 mg/L.P.Malaianus could not survive for a long time when Cu²⁺concentration exceeded 3.0 mg/L.P.Malaianus lost their ability to self-regulation ability when the concentration of Cu²⁺+Cd²⁺exceeded 4.0 mg/L.It was severely damaged.The fluorescence characteristics could not be measured.?8?The damage began from the edge of the leaves,and sequenced to the vicinity of the veins.The degree of damage to the leaves was expressed as:Cu²⁺>Cu²⁺+Cd²⁺>Cd²⁺.The reduction of chlorophyll content was less than that of photosynthetic rate.Fv/Fm,Y?II?and ETRmax were the most sensitive to the stress,and could be used as biomarkers for damage of V.natans and P.Malaianus.