The Physiological And Ecological Effects Of Nitrate On Aegiceras Corniculatum L.Seedlings Under Mercury Stress

In view of the current situation of heavy metal pollution in mangrove wetlands and the possible eutrophication caused by large number of sewage,this paper takes the Aegiceras Corniculatum L.Seedlings,a typical salt-secreting mangrove plants,as the research object,Hydroponic experiment of two factors and three levels was performed at room temperature(The nitrate concentration was set to 2?15?25 mmol/L,mercury concentration was set to 0?1?10 mg/L),Through the determination of photosynthetic index,nitrogen metabolism index,antioxidant enzyme activity,total mercury content in various organs,and subcellular distribution of mercury in leaves of Aegiceras Corniculatum L.Seedlings,the physiological and ecological effects of nitrate on seedlings under mercury stress were explored.The results show that:1.Mercury stress causes the chlorophyll a content,chlorophyll b content,the ratio of chlorophyll a to b,net photosynthetic rate(Pn),transpiration rate(Tr)and stomatal conductance(Gs)to decrease,The Photosynthesis of plants were inhibited.After adding nitrate,the Pn,Tr,Gs and chlorophyll content of the seedlings all increased first and then decreased with the increase of nitrate concentration.The appropriate concentration of nitrate can alleviate the stress of mercury on photosynthesis.2.The concentration of nitrate accumulation in different organs of the seedlings is:leaf>stem>root.Leaf is the main organ of plant nitrate assimilation.Mercury treatment resulted in the increase of nitrate content in stems and leaves,decreased the nitrate content in roots and Nitrate reductase activity of leaves.increased plant transfer coefficient of nitrate,mercury stress inhibited the absorption of nitrate in plant roots,reduced Nitrate reductase activity of leaves and results the accumulation of nitrate in the aboveground part of the plant.After adding nitrate,the content of nitrate increased in each organ of the seedlings,the Nitrate reductase activity of leaves increased first and then decreased,and the plant transfer coefficient of nitrate was decreased.The application of nitrate can promote the absorption and transport of nitrate and alleviate the inhibitory effect of mercury on Nitrate reductase activity of leaves.The increase of the plant transfer coefficient of nitrate indicated that the root may be the key part of nitrate alleviating mercury toxicity.3.Mercury stress led to a decrease in nitrogen content in various organs of Aegiceras Corniculatum L.seedlings,decreased the carbon content and cabon to nitrogen ratio in roots and leaves,increased the cabon to nitrogen ratio in stem,and had no significant influence on the content of sulfur in various organs.Mercury inhibited the assimilation of nitrogen nutrition and destroyed the balance of plants carbon and nitrogen metabolism.After adding nitrate,the content of nitrogen in leaves and stems increased,the content of carbon in leaves and the content of sulfur in roots increased first and then decreased.The appropriate concentration of nitrate can alleviate the stress of mercury on nitrogen assimilation and promote the carbon metabolism of plants.The increase of sulfur content in roots may be an important mechanism for nitrate alleviating mercury toxicity in plants.The lower cabon to nitrogen ratio of leaves under high nitrogen treatment showed that excessive nitrate is not conducive to the growth of plants under mercury stress,also led to the imbalance of carbon and nitrogen metabolism in plants.4.The content of MDA in the leaves of Aegiceras Corniculatum L.was increased under mercury stress,the activity of SOD decreased,and the activity of POD was increased first and then decreased,mercury treatment lead to the imbalance of production and elimination of reactive oxygen species,POD plays a leading role in the removal of reactive oxygen species under low concentration of mercury.After adding nitrate,the content of MDA in the leaves of seedlings decreased first and then increased,the activity of SOD increased first and the decreased,and the activity of POD decreased gradually.The appropriate concentration of nitrate can reduce membrane lipid peroxidation and alleviate the toxic of mercury,SOD plays a dominant role in the catalytic activity of reactive oxygen species.However,excessive nitrate is not conducive to the reduction of membrane lipid peroxidation and the ability to remove active oxygen from plants.5.The content of mercury in various organs of Aegiceras corniculatum seedlings increased with the increase of mercury treatment concentration,but the plant transfer coefficient of mercury decreased significantly,The accumulation of Hg in each organ:root>stem>leaf,root is the main organ of mercury accumulation in the seedlings,mercury is not easily transported to the above-ground part of the plant.After adding nitrate,the contents of mercury in all organs of Aegiceras Corniculatum L.rose first and then declined,The appropriate concentration of nitrate can promote the absorption of mercury by plants and increase the plant's tolerance to mercury stress.6.The distribution proportion of mercury in the subcellular components of the leaves of seedlings is:Cell wall>Soluble>Organelles>Cell membrane.The mercury content in the cell wall accounts for 58.01%-75.06%of the total amount.Aegiceras Corniculatum L.Seedlings reduce the toxicity of mercury to leaces through the precipitation of cell walls.After adding nitrate,The concentration of mercury in the cell wall of the leaf decreased and the concentration of mercury in the soluble component increased,most of the mercury was still distributed in the cell wall components.The addition of nitrate promoted the internal detoxification of the leaves from the cell wall precipitation to vacuolar regionalization and isolation,and enhance plant tolerance to mercury stress.