| The hydro-fluctuation belt is a land-water transition zone with a vertical drop of up to 30m in the range of 145?175 m above sea level on both sides of the Yangtze River after the completion of the Three Gorges Dam?TGRAs?.The hydro-ecological process of winter and summer flood season,which different from the hydrological changes in the natural riparian zone causes plant death due to the destruction of the original habitat,thus leading to many ecological environment problems.Therefore,it is considered that the key to vegetation restoration and reconstruction is to select the species suitable for the survival of the fluctuating zone.Many species are not suitable to survive in the special habitat of dry-wet alternation in the hydro-fluctuation belt.Pinus elliottii is considered by many scholars as a suitable plant.Understanding the changes of different types of drought,flooding stress intensity and time conditions plays an important role in exploring the adaptive mechanism of plants in the special habitats of the the fluctuating zone.In this study,Pinus elliottii,a suitable plant in riparian zone,was taken as the research object,and based on the natural water level movement rule in the fluctuating zone,the photosynthetic characteristics,physiological and biochemical characteristics,non-structural carbohydrates?starch,soluble sugar?and nutrient elements of Pinus elliottii were studied by simulation of drought and flooding stress conditions,respectively,to explore the effects of summer drought and winter flooding stress on Pinus elliottii.The research tries to provide provide scientific theoretical guidance for the selection of suitable plants for vegetation restoration and reconstruction in the hydro-fluctuation belt of the Three Gorges Reservoir Areas?TGRAs?.A simulation experiment was conducted to study the photosynthetic characteristics,physiological and biochemical changes,non-structural carbohydrates and nutrient elements of two-year-old Pinus elliottii seedlings to different drought environment induced by the summer drought of the Fluctuating zone from August 2017 to mid-September 2017 in the TGRAs.Three different treatments were set up:control group?normal water irrigation?,mild drought group?watering once every 5 days?and moderate drought group?watering once every 10 days?.A simulation experiment was conducted to study the 4 indices of two-year-old Pinus elliottii seedlings to different flooding environment induced by the winter flooding characteristics from October 2017 to May 2018 in the TGRAs.On the basis of previous drought treatment,five treatment group were st up:control group?normal water irrigation?,root flooding group 1?former mild drought group,submerged at about 5 cm above the soil surface?,whole flooding group2?former mild drought group,submerged at about 5 cm below the top of the plant?,root flooding group 3?former moderate drought group?and whole submerged group 4?former moderate drought group,submerged at about 5 cm below the top of the plant?.The results are as follows:The results showed in the study of the effects of drought stress on photosynthetic characteristics and physiological and biochemical characteristics of two-year-old Pinus elliottii in summer that after 15 days of drought stress,and the negative effect was more significant with the increase of drought stress time.The net photosynthetic rate of mild drought group was slightly higher than that of moderate drought group;the photosynthetic products consumed by the drought-resistant group were higher than those of the control group.The Pn of Pinus elliottii was closely related to Gs and Tr.Tr decreased significantly after 15 days of drought stress in moderate drought stress group,while Tr maintained at normal level in mild drought stress group during the whole drought stress process.However,there was no obvious regularity with intercellular CO2 concentration before 30 days of drought.The protein content in leaves of Pinus elliottii is proportional to the duration of drought stress.At the end of drought stress,the protein content in different treatment groups showed as follows:moderate drought group>mild drought group>control group,but not significantly compared with the control group;SOD,POD and CAT showed an overall"increase-decrease"change pattern,but the degree of stress was different in different drought treatment groups;MDA content in moderate drought treatment showed a significant upward trend.The results showed in the study of the effects of summer drought on non-structural carbohydrates and nutrient characteristics of two-year-old Pinus elliottii in summer that starch content in leaves of Pinus elliottii in mild and moderate drought groups increased significantly after 15 and 7 days,respectively,with the increase of drought stress time.Drought stress reduced the contents of nutrient elements C,N,P,K,Cu and Zn in Pinus elliottii.C content in leaves under drought stress was moderate drought group>mild drought group>control group after 45 days of drought;under moderate drought stress,the N content in leaves decreased significantly,which inhibited the absorption of N by leaves of Pinus elliottii;after 15 days of drought,the P content in seedlings of drought group decreased to below that of control group,and the decline rate of moderate drought group was greater than that of mild drought group.For K element,the contents of Cu and Zn in mild and moderate drought groups decreased significantly after 15 days and 7 days respectively,and the decline rate in moderate drought group was higher than that in mild drought group.The contents of Mn and Fe in leaves increased gradually with the increase of drought stress time,and changed significantly after 15days of drought.The results showed in the study of the effects of winter flooding on photosynthetic characteristics and physiological and biochemical characteristics of two-year-old Pinus elliottii in winter that with the increase of flooding time,the net photosynthetic rate of root-flooded group 1 and full-flooded group 2 showed a"decline-gentle"trend,but the net photosynthetic rate of root-flooded group 3 and full-flooded group 4 showed a downward trend,and after 90days,the net photosynthetic rate was lower than that of the former.After 150 days of flooding,the net photosynthetic rate of Pinus elliottii decreased by 30.9%,33.0%,51.9%and 62.3%respectively when compared with the control group.Meanwhile,water flooding significantly reduced stomatal conductance and transpiration rate.By comparing the relationship between Ci and Ls,it is concluded that the decrease of net photosynthetic rate of Pinus elliottii was mainly determined by stomatal factors in the early stage,and non-stomatal factors in the later stage.The content of soluble protein in root-flooded group was mostly not significantly different from that in control group during the flooding stage,while the content of soluble protein in leaves of Pinus elliottii in full-flooded group increased significantly at the early stage?90 days before flooding?,but decreased gradually at the later stage.7 days before flooding,the activity of SOD in Pinus elliottii did not change significantly.However,after 7 days of flooding,plants maintain the dynamic balance of ROS production and removal by increasing SOD,CAT and POD activities in vivo,and reduce the damage to plants.The MDA content of Pinus elliottii leaves in different flooding conditions was not significantly different from that of the control group within 30 days before flooding,but after 90 days of flooding,the MDA content of Pinus elliottii leaves increased significantly and gradually stabilized to a certain level.The results showed in the study of the effects of winter flooding on non-structural carbohydrates and nutrient characteristics of Pinus elliottii that starch content in leaves of different treatments had different responses to flooding stress.After 150 days of flooding,starch content in leaves of different flooding groups decreased by 11.0%,11.8%,12.5%and29.1%respectively when compared with the initial values.Contrary to starch,flooding stress increased the soluble sugar content in the leaves of Pinus elliottii seedlings.Meanwhile,flooding changed the C/N ratio in the leaves.After 150 days,the C/N ratio ranged from 0.731to 0.789?control?,0.824 to 0.8859?root flooding group 1?,0.788 to 0.971?full flooding group2?,0.744 to 0.826?root flooding group 3?and 0.737 to 0.794?whole flooding group 4?,respectively.However,the initial nutrient contents of leaves of Pinus elliottii was different to a certain degree.The nutrient content can be divided into three stages:early storage stage?the first stage:0?7 days,none,sometimes?,rapid release stage?the second stage:7?60 days?and late slow release stage?the third stage:60?150 days?.During the flooding process,the concentrations of C,N and P in the leaves of the four groups were lower than those of the Non-flooding group,but with the prolongation of the flooding duration,the concentrations of N and P in the leaves did not always decrease,but gradually stabilized.At the same time,the accumulation of copper in leaves decreased by 54.7%.The variation of zinc availability in flooded soils is complex.After 150 days of flooding,the nutrients of Mn,Fe,Cu and Zn decreased by 8.9%?28.1%,9.2%?50.3%,8.8%?66.5%and 8.7%?46.0%,respectively. |
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