Simulation Study On Driving Mechanism Of Typical Submerged Macrophyte

Lake eutrophication has been considered as the important force to drive the heavy decline of submersed macorphytes vegetation as the key functional groups of the lake.However,the underlying mechanisms of decline have been limitedly documented.In the aim to explore and uncover the decline of submerged vegetation in eutrophic shallow lakes,and to understand the tolerance response to environmental factors in a typical submerged plant — Vallisneria natans,the complex hydro-ecological studies were conducted on the driving effect of multiple stressors to the decline.It will provide a certain theoretical basis for the restoration and reconstruction of submerged macrophytes in eutrophic lakes and the reparation of healthy ecosystem functions.In this study,we start from a phytotoxic compound sulphide,analyzing the effects of different concentrations of sulphide on the growth,photosynthesis and C-N physiological metabolism of V.natans.And the other two stress factors,high ammonia and low light,which are most closely related to the growth of submerged plant,we were taken into the further studies.Effects of three environmental factors and their interactions on the acute and sub-acute responses of its phenotypic growth and C-N physiological metabolism were studied through physio-ecological methods and outdoor simulation experiments with orthogonal design methods.The main conclusions were as follows:1.When the concentration of sulphide is inferior to 0.1 m M,it is beneficial to accumulate biomass,synthetic of resource-based carbohydrates such as soluble carbohydrates(SC),and accelerate the progress of photosynthesis of V.natans.When the concentration of sulphide is over 0.1 m M,it would have serious stress effects on fresh weight,affect the plant growth and development;obstructing the process of photosynthesis by blocking the synthesis of chlorophyll(a+b);disordering C-N physiological metabolism resulting a sharp decrease in both free amino acids(FAA)and SC content.In addition,there were a series of symptoms of sulfur poisoning such as black rot in the roots,chlorosis of plant leaves,and even death of V.natans in the high-sulfide treatment group.These responses to sulfide stress were similar to the symptoms found in the failure of ecological restoration projects in eutrophication lakes.2.In the acute stress experimental study,low light,high ammonia and sulfide and their interactions can exert stress effects on the growth,photosynthesis and C-N metabolism of V.natans,and sulfide is the principal acute effect factor.2 mg/L high ammonia treatment could significantly increase the concent of FAA and decreased the SC,but the effect of acute stress on plant height,fresh weight and photosynthesis was not significant.In the unshaded treatment group,V.natans showed a certain degree of acute inhibition,3% of low light also inhibited the synthesis of photosynthetic pigments,but increased the plant height to obtain maximum photosynthesis,increased the decomposition of starch and significantly increase the amount of SC synthesis,resulting in an increasement of FAA.1?5 m M concentration of sulfide can strongly inhibit the photosynthesis in a short period of time and disturb C-N metabolism even be poisoned to death.After 4d exposure of V.natans under 1 m M sulphide concentration,45% light intensity,and 2 mg/L ammonium nitrogen concentration for 4 d,the SC content was the lowest and only 2.39% of the initial value.V.natans is difficult to survive because of the depletion of sugar stocks and lack supply for a short period of time under this adverse condition.In addition,high ammonia can promote the inhibition effect of high concentrations of sulfide on V.natans,and low light will reduce the toxic effects of sulfides and high ammonium.3.Sub-acute stress experimental study of 32 d showed that the growth and C-N metabolism of V.natans were affected not only by high ammonium nitrogen concentration and low light,but also significantly by the high concentration of sulfides.Various factors also showed different degrees interactions.Sulfide had a significant effect on the fresh weight of leaves and root,length of roots of V.natans,followed by low light and high ammonia nitrogen.The length of leaves was significantly affected by the interaction between light intensity and ammonium nitrogen concentration,and the degree of interaction was higher than the interaction between light intensity and sulfide.Sulfide had the most significant influence on chlorophyll(a+b)concentration,followed by light intensity and ammonium nitrogen concentration.The interaction among the three factors had little effect on photosynthesis of V.natans.The effects of sulfides on SC and starch content and FAA content in roots and leaves of V.natans were higher than those under low light and ammonium nitrogen.For the FAA content of leaves,ammonium nitrogen had the strongest effect.The contents of FAA and SC in leaves and roots of V.natans were significantly affected by three factors and their interactions.The starch content of leaves was extremely significant affected by sulfide,the interaction of light intensity and ammonium nitrogen.V.natans growing best under the condition of 0.1 m M sulfide concentration,3% light intensity,and 0.2 mg/L ammonium nitrogen.The photosynthesis pigment is the most strongest.And in the conditions of 1-5 m M sulfide,45% light intensity and 2 mg/L ammonium nitrogen environment is not conducive to its growth and development,photosynthesis would be inhibited strongly and the balance of C-N metabolism would be disturbed,even be poisoned to death.The above research provides a new research perspective and foundation for further clarifying the mechanism of large scale decline of submerged macrophytes in eutrophic shallow lakes in China.