Responses And The Physiological And Biochemical Mechanisms Of Three Wetland Plants To UV-B Radiation

Plants are the core components of the wetland ecosystem,the structure and ecological functions of the wetland ecosystem will be affected necessarily by the growth and reproduction changes of plant under enhanced UV-B radiation.However,the research on growth,physiological and biochemical responses of wetland plants to enhanced UV-B radiation has rarely been reported.Three wetland species,Typha orientalis Presl.,Phragmites australis Cav.and Iris tectorum Maxim.,were used in our study to explore the responses and the physiological mechanisms of wetland plants to enhanced UV-B radiation.The main results are as follows:1.Notable leaf damage symptoms were found in three species under enhanced UV-B radiation,including curling,wilting,leaf edge chlorosis and necrotic spots,and the damage degree increased with the increase of radiation intensity.The maximum radiation sensitivity of leaf damage was found in Iris tectorum Maxim.,followed by the Phragmites australis Cav.and then Typha orientalis Presl.2.The effect of UV-B radiation on plant height is initially promoting and then inhibiting,and the both effects increased with the increase of radiation intensity.As the radiation duration prolonged,the inhibition effect increased.The sensitivity of Iris tectorum Maxim.to UV-B radiation was significantly greater than that of Typha orientalis Presl.and Phragmites australis Cav..Enhanced UV-B radiation significantly reduced the specific leaf area and total plant biomass,the biomass decreased more in the aoveground oarts than that in the underground parts.3.Leaf chlorophyll contents significantly decreased under enhanced UV-B radiation,and the effect accumulated with the exposure time.The average decline rate of chlorophyll in Typha orientalis Presl.and Iris tectorum Maxim.were significantly higher than that in Phragmites australis Cav..The harmful effect of UV-B radiation on PSII of wetland plant leaves increased with the increase of radiation intensity and the extension of exposure time.Under enhanced UV-B radiation,the Fo increased,and the NPQ increased significantly,while?PSII and Qp reduced,and Fm,Fv/Fm and Fv/Fo decreased significantly,with the highest average decline rate in Iris tectorum Maxim.and the lowest rate in Phragmites australis Cav..Under enhanced UV-B radiation,the chloroplast deformed,thylakoid lamellae disordered,swelled and even blurred,starch granules emerged and became larger,and the effect of high UV-B radiation intensity was greater than that of low intensity.The net photosynthetic rate?Pn?,stomatal conductance?Gs?and transpiration rate?Tr?decreased gradually while intercellular CO₂?Ci?increased under enhanced UV-B radiation,and the the average decline rates of these gas exchange pparameters in Typha orientalis Presl.were significantly higher than those in Phragmites australis Cav.or in Iris tectorum Maxim.?27.9%?except that the Ci increase rate was significantly higher in Phragmites australis Cav.than in Typha orientalis Presl.and Iris tectorum Maxim.,suggesting that the main reason for photosynthesis decrease is the nonstomatal factors.4.Oxidative stress intensified in wetland plants under enhanced UV-B radiation,and leaf MDA content increased with the increase of radiation intensity.The activity of SOD,CAT and APX increased,but GR increased initially and then decreased.The average increase rates of SOD or CAT in Iris tectorum Maxim.were significantly higher than those in Typha orientalis Presl.,which were then significantly higher than those in Phragmites australis Cav.While the increase rate of APX was higher in Phragmites australis Cav.than in Typha orientalis Presl.or Iris tectorum Maxim.At earlier stage of the UV-B treatment,the activities of SOD,GR and APX is high,and the activities of CAT and APX continuously increased significantly later on but the GR decreased,suggesting that SOD,CAT,GR and APX all play their role in the early stage,dominated by CAT and APX at the late stage.Leaf AsA content increased,but GSH increased initially and then decreased under the enhanced UV-B radiation,indicating that AsA plays a greater role in radiation resistance of the wetland plants.The average increase rate of leaf AsA content in Typha orientalis Presl.?38.8%?was higher than in Iris tectorum Maxim.?38.6%?or in Phragmites australis Cav.?27.8%?.Leaf GSH contents at early experimental stage increased by 95.2%in Phragmites australis Cav.which was significantly higher than in Typha orientalis Presl.?50.7%?and in Typha orientalis Presl.?25.8%?,while the average decline rate of GSH at the late stage was 46.1%in Iris tectorum Maxim.,42.1%in Typha orientalis Presl.and31.2%in Phragmites australis Cav..5.Enhanced UV-B radiation promoted the synthesis of the ultraviolet absorption substance,resulted in significantly increased flavonoids content,and the average increase rate of flavonoids in Phragmites australis Cav.?22.9%?and in Iris tectorum Maxim.?27.8%?were significantly higher than that in Typha orientalis Presl.?12.7%?.The effect of UV-B radiation on carotenoids content varied depending on radiation intensities and plant species,which decreased significantly in Typha orientalis Presl.?32.7%?,but much less in Iris tectorum Maxim.?0.7%?,and increased under low radiation but decreased significantly under high radiation in Phragmites australis Cav..