The Response Of Rhizophora Stylosa To Dual Stress Of Waste Water And Flooding

Mangroves refer to a woody plant community growing in tropical and subtropical estuaries.Growing in low-latitude coastal areas,mangroves are periodically flooded by tide water and constantly eroded by salty and fresh water.Meanwhile,mangroves are also confronted with environmental stress such as waste water pollution.Due to the rise of sea level and the negative impact of human production activities,the area of natural mangrove communities has decreased drastically.Hence,close attention has been paid to mangroves and they are now under key protection worldwide.Mangrove plants in China have relatively complex communities.When different mangrove plants with different ecological sequential structures face environmental stress factors such as salinity,flooding or waste water,their morphological and physiological changes in different degrees,thus affecting the whole mangrove ecosystem.In this paper,Rhizophora stylosa,a typical mangrove plant in China,is selected as the research object.Two different water environments,i.e.seawater and fresh water,are designed.Through conducting an orthogonal experiment and artificially simulating the waste water-flooding stress environment,this paper studies the morphological and physiological responses of Rhizophora stylosa under different waste water concentrations and flooding time,and measures its morphological and physiological changes,so as to explore the adaptive mechanism of Rhizophora stylosa to environmental stress and provide theoretical support for the sustainable development of mangrove ecosystem.The research findings are as follows:（1）In the seawater,with the increase of waste water concentration,the plant height,basal diameter,photosynthesis index and chlorophyll content of Rhizophora stylosa in groups with different flooding treatments increase first and then decrease,and the sudden cut-off point appears at a low waste water concentration.The superoxide dismutase（SOD）,malondialdehyde（MDA）and electrical conductivity of the leaves are positively correlated with waste water stress.As the flooding time increases,the basal diameter,net photosynthetic rate,stomatal conductance and electrical conductivity of Rhizophora stylosa in different groups first rise and then descend,while the plant height,transpiration rate and SOD decrease first and then increase,with MDA content being positively correlated with flooding stress.（2）In the freshwater,as the waste water concentration goes up,the plant height,basal diameter,photosynthesis index and chlorophyll content of Rhizophora stylosa in different groups all present a trend of first increasing and then decreasing,with the sudden cut-off point appearing at a medium waste water concentration.However,SOD activity and electrical conductivity first decrease and then increase.With the prolongation of flooding,the basal diameter,photosynthesis index,MDA content and chlorophyll content of Rhizophora stylosa in different groups first rise and then fall,while the plant height,SOD activity and electrical conductivity first descend and then ascend,and there is a positive correlation between electrical conductivity and flooding stress.（3）According to the one-way and two-way analyses of variance,the interaction between waste water and flooding has no significant effect on the morphological responses of Rhizophora stylosa,the flooding time exerts a slight influence on the morphological responses of Rhizophora stylosa,while the concentration of waste water plays a dominant role in determining the morphological responses of Rhizophora stylosa.The physiological responses of Rhizophora stylosa are influenced by waste water,flooding,and the interaction between waste water and flooding to some extent.（4）Rhizophora stylosa can accept a higher concentration of waste water without being greatly polluted by the waste water in the fresh water environment than in the seawater environment.（5）Under the double stress of waste water and flooding,the optimal growth conditions of Rhizophora stylosa are as follows:a low to medium concentration of waste water(i.e.COD:100-200 mg·L^-1,TN:25-50 mg·L^-1,TP:3-6 mg·L^-1),and a flooding time of 12-24 h·d^-1.