Influences Of Elevated CO₂ Concentration And Drought Stress On Water Transport Of Tree Species

The water,growth and competition of trees were influenced by elevated CO₂ concentration,which caused quickly growth of trees might easier to suffer cavitation and embolism and can not surive because of water transport system destroyed.It can seriously influenced the water transport efficiency and security.The water transport of trees under 720μmol·mol^-1CO₂ and drought stress were measured after 7 trees have been exposed to 720μmol·mol^-1CO₂ for different times.The growth index(the growth quantity,leaf area, leaf tissue characteristics and biomass),physiological index(gas exchange parameters, chlorophyll fluorescence parameters,photosynthesis characteristics andδ¹³C),water use efficiency,water consumption characteristic,hydraulic architecture parameters(hydraulic conductivity,specific conductivity,leaf specific conductivity,Huber values),water parameters were measured under 720μmol·mol^-1CO₂ and 380μmol·mol^-1CO-2 with drought stress.The results show that:(1) The 7 trees growth quantity,leaf area,biomass,root/shoot ratio,photosynthesis rate,Ci,Fv/Fm,Fv/Fo,WUE_i and WUE_L increased with elevated CO₂ concentration.Also, the thickness of spongy tissue,thickness of palisade tissue,thickness of leaves,stomatal long diameter,stomatal wide diameter,stomatal area and stomatal long diameter/stomatal wide diameter increased.The sequence of leaf area was Robinia pseudoacacia,Fraxinus chinesis,Syringa oblate,Acer truncatum,Euonymus japonicus,Platycladus orientali and Pinus tabulaeformis.The stomatal density,transpiration rate,dark respiration rate,specific leaf weight,δ¹³C and water consumption was decreased,while stomatal conductance was firstly decreased and then increased along elevated CO₂ concentration.Theδ¹³C of conifer trees were higher than broad-leaves trees and root and branch ofδ¹³C were obviously bigger than leaf.Theδ¹³C of Robinia pseudoacacia and Fraxinus chinesis were gone beyond the range of C₃ plants,and the averageδ¹³C of 7 trees under 720μmol·mol^-1CO₂ were higher than the average of C₃ plants,which is 27.0‰.(2) Thickness of spongy tissue,thickness of palisade tissue,thickness of leaves, stomatal long diameter/stomatal wide diameter,water consumption and biomass decreased along water stress,while cuticle thickness of epidermis of Pinus tabulaeformis and Platycladus orientalis,thickness of epidermis,root/shoot ratio and long-term water use efficiency increased.The water consumption curve of trees was unimodal curve,which different from water consumption rate when the leaf area great changed.The stoma changed slowly because of the interaction between elevated CO₂ concentration and water stress which changed the sensitivity of stoma under only CO₂ concentration or water stress.(3) The total biomass of Platycladus orientalis was not increased with long times under elevated CO₂ concentration.Theδ¹³C of root and branch under elevated CO₂ concentration with 1 year were also bigger than that of 2 years.The leafδ¹³C of Acer truncatum was undulate under controlled cycles of dehydration and rehydration and CO₂ concentrations.The peak of the curve were appeared at serious water station,and the fluctuate range decreased with the circles of the water stress.The water potential were decreased under 720μmolmol^-1CO₂,which reduced the occurrence of cavitation and embolism,delayed the happen of water stress and changed the water stress in plants.(4) The circle 2 was a more complicated and important phase of Acer truncatum under controlled cycles of dehydration and rehydration and CO₂ concentrations.RWC⁰/ RWC_s decreased whileΨ⁰Ï€andΨ¹⁰⁰Ï€increased firstly in circle 1,and then decreased from circle 1 to 3 along elevated CO₂ concentrations.The ability of maintaining turgor pressure, osmotic adjustment,capable of preserving and endurance of water separation capability were increased under elevated CO₂ concentrations,whereas the resistance of water separation capability decreased.Theε^max) increased in normal water condition of Acer truncatum under 720μmol·mol^-1CO₂,and cytoderm more hardness and less elasticity.But in circle 3,ε^max decreased in light and middle drought conditions and increased in serious drought condition.(5) The branch 0,branch 1 and branch 2 of hydraulic conductivity of 7 trees increased along elevated CO₂ concentration.Specific conductivity increased,while Euonymus japonicus and branch 1 and branch 2 of Syringa oblata were decreased.The leaf specific conductivity and Huber values changed ruleless.Branch 0 and branch 1 of hydraulic conductivity increased amplitude were smaller than branch 2.Hydraulic conductivity, specific conductivity and Huber values with relative ramification rate were gradually decreased along water stress,while leaf specific conductivity was not linearity changed under 720μmol·mol^-1CO₂ along water stress.Hydraulic conductivity,specific conductivity and leaf specific conductivity in different area stem segment increased under elevated CO₂ concentration and decreased under water stress.(6) The water potentialΨ₅₀ of 50%of PLC was the inflexion of the vulnerability curve,which increased with relative ramification rate;the inflexion divided the curve,the right part and upside part.The more right the right part keep,the bigger the water potential threshold value.The water potential ofΨ₅₀.which is PLC equal to 0,is the threshold value of occurrence embolism.General considerd theΨ₀,Ψ₅₀ andΨ_max,the embolism vulnerability with relative ramification rate was branch 2,branch 1 and branch 0,1 year of Platycladus orientalis smaller than that of 2 years.The embolism vulnerability in different area stem segment was restricted stem segment bigger than non-restricted stem segment, while 1 year of Platycladus orientalis bigger than that of 2 years.Branch 2 was the first damaged part of trees.It means that the ecological strategy of trees adopting the water stress was trying their best to keep the trunk and low branch part.(7) Use different manners to accommodate to the embolism which caused by decreased water potential,7 trees with relative ramification rate and in different area stem segment,were taking ecological strategy of tradeoff of efficiency and security in different water stress.This strategy include keeping high water transport security,alleviating water transport security to tradeoff the efficiency,and at the same time,reducing the water transport security and efficiency but not end the needing of water to growth and any productivity.