| With the Populus nigra clones as the experimental materials, the growth and physiological indices were measured under different soil water conditions. These indexes of biomass,leaf water potential,the permeability of plasma membrane,the changes of net photosynthesis rate and proline content on different drought-stress were analyzed. Then we made a comprehensive judgment on the clones'drought-resistance and selected the high drought-resistance varieties and analyzed the mechanism of genetic variation, which could provided scientific basis for the introduction and breeding of Populus nigra and anti-drought afforestation. The results as follows: 1.Different soil water content had obvious effect on the clones'growth condition. The result showed that the height and diameter growth of the seedling were retrained by water stress. The single leaf area reduced obviously and the accumulation of dry material of root, stem and leaf had the decreasing trend. The biomass of these Populus nigra clones of N79,N88,N87,N1,N85,N31,N111,N2 is larger than the compared clones—P.107 and Shannlin NO.3, and the N86 has least biomass. The photosynthesis also weakened correspondingly and the clones of N14 N92 have a slow reaction and small change extent, then it has a good drought-resistance representation. Significant differences existed among clones on growth indexes and drop extent of the indexes with water stress. 2.The leaf water potential decreased under water stress. The change extent of N92,N91 is small, so them could sustain the water stress but the N85 has a reverse representation. The permeability of plasma membrane increased greatly, and the clone of N111 has a small change extent of comparatively conductance and a good membrane stabilization under water stress(medium and severe ). The free proline content has an increased trend. The change extent of these indexes could reflect better the drought resistance of clones. 3.The analysis of the genetic resources of Populus nigra showed that there were significant differences among all the clones and the different water stress conditions on the growth and physiological indices. The difference under different water conditions was more obvious than among the clones. Quantity genetic analysis showed that genetic variation coefficient, performative variation coefficient and hereditary ability of growth properties were greater than physiological properties. The performative variation coefficient of various indexes was greater than genetic variation efficient. These indexes of seedling height,single leaf area,ratio of root/stem and net photosynthesis rate have a biggish heritabilitity. The genetic variation coefficient and performative variation coefficient of the ratio of root/stem is larger but the permeability of plasma membrane's is lower. 4.The growth and physiological characteristics under water stress were studied, index system on comprehensive judgment of drought resistance ability of the clones was suggested and the drought resistance ability of the clones was judged comprehensively. On this basis, the indices of adaptability to drought and of productivity to drought were analyzed. The net photosynthesis rate, single leaf area, plant height, biomass, the ability of osmotic adjustment and the permeability of plasma membrane were suggested to be used as comprehensive judgment indexes in drought-resistance clones selection. With the experimental study and analysis, we found that most of the P.nigra clones had strong adaptability to drought and productivity to drought than the compared clones—P.107 and Shannlin NO.3, which indicated these introductory Populus nigra genetic resources have an abroad adaptability and were suitable for the dry region. On the basis of these comprehensive, appraisal indexes, we used a method of weighted arrangement (TOPSIS) to judge the drought resistance of the clones. The clones that had strongest comprehensive drought-resistance abilities included N9,N15,N19,N89,N91, so them were eximious Populus nigra genetic resources and were suitable for the dry region.
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