| 19 Populus tomentosa clones seedlings were selected in this experiment. Growth rhythm, leaf anatomical structure under different soil water condition were studied, leaf water physiology, photosynthetic and transpiration characteristic under different growth phases were analyzed, growth, physiology and biochemistry response to gradual soil water drought stress and rewatering were also studied. 40 Populous tomentosa clones tree with 5 years were also carried out in this study. Morphology, growth and physiology characteristics of trees contrast to seedlings were both analyzed. The objective of this study was selecting fast-growing, drought resistant and water-saving Populus tomentosa superior clones, furthermore provide practical basis for breeding of Populus tomentosa.Populus tomentosa is a kind of light loving tree with the characteristics of high water consumption and drought resistance. Light compensation point (19 to 37μmol·m-2·s-1) and light saturation point (1800μmol·m-2·s-1) are higher than other Populus. The differences of water consumption of Populus tomentosa among different clones were significant. Water consumption per seedling was 92 to 269 g·d-1, and daytime water consumption rate was 11.1 to 218.8 g·m-2·h-1. With the gradually increasing drought stress, Clones 46, 83 and BL1 showed strong characteristics of drought resistent capability; Clone BL8, 42,and 28 showed strong characteristic of water-saving capacity;. Leaf water potential, net photosynthesis and water use efficiency were declined slower than other clones; water consumption dropped quickly and content of SOD, POD and MDA were increasing sharply. During the severe drought stress stage, Clones 30 and 50 showed stronger abilities to bear hunger with lower hydration compensation(-3.38±0.29 MPa and -3.50±0.40 MPa), on the contrary, Clones BL1, 83 and BL5 showed weaker abilitiy to bear hunger with higher hydration compensation(-1.37 to -1.05 MPa).Index system for drought resistant and water-saving Populus tomentosa was established. All the determination of the indicators used in this study were selected by Principal components analysis, and the result showed that crown-height, crown-width and branching angles of trees play little roles in evaluating the growth amount of trees. Thickness of on and under epidermis and specific leaf weight had smaller impact on drought resistance. Contents of SOD and MDA were instability in different phases, therefore, these indicators should be chosen carefully.Subordinate function values analysis was applied to evaluate Populus tomentosa clones comprehensively. It concluded that BL2 is the ideal clone for seedling. It had the characteristics of fast-growing (height, 122.2 to139.3 cm; diameter of seedling, 1.109 to 1.261 m), stronger drought resistance (cuticular, 1.26 to 1.28μm; fence and spongy tissue were separately 2.05和4.85), and it also had stronger water-saving capacity with lower leaf transpiration rate(4.09 to 4.36 mmol·m-2·s-1) and water consumption(169.09 to 226.55 g·d-1), higher stomatal resistance(3.64 to 4.66 mmol·m-2·s-1), higher instantaneous water use efficiency(2.87 to 3.05μmolCO2·mmolH2O-1) and leaf carbon isotopes ratio(-30.629 to -30.099‰). When the seedlings of BL2 were subjected to drought stress, they showed stronger drought and water-saving capabilities with lower hydration compemsation point(-2.66 to -1.75 MPa). In addition, Clone 64, 30, 26 and 69 were also some of superior clones of those clones studied in this research for trees. They were fast-growing (the diameter at breast height, 4.0 to 4.5 cm; height, 6.0 to 6.5 m) with high survival rate (88 to 94%), stronger drought resistance (leaf thickness, 60 to 91μm; cuticular thickness, 0.83 to 1.33μm; and net photosynthesis rate, 18.2 to 20.0 mmol·m-2·s-1), and water-saving capacity (instantaneous water useefficiency,.5.8 to 6.5μmolCO2·mmolH2O-1; carbon isotopes ratio of leaf and branch, -27.2 to -25.5‰). Key words: Populus tomentosa Clones, drought resistance and water-saving, water use...
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