Growth and physiological responses to drought stress and comprehensive evaluation on drought tolerance in Leuce clones at nursery stage

In order to explore the responses of white poplar clones to drought stress and select superior clones with characteristics of fast growth and strong drought resistance, the growth and physiological characteristics of nine white poplar clones grown in pots under different drought stresses were investigated. The soil moisture content was greater than 80% of field capacity for the normal level (control), 50%-70% for mild drought, 30%-50% for moderate drought, and less than 30% for severe drought. The results showed that the increment of height, basal diameter and biomass, and root-shoot ratio of clones had significant differences among the nine clones. Two clones, 89 and YX2, showed fast growth. The chlorophyll content increased under mild drought stress,but then decreased rapidly under moderate and severe drought stress, and the chlorophyll content of clone 89 and YX2 showed the smallest decline under drought stress. The contents of the free proline and the soluble sugar in nine clones showed the trend of increasing gradually with the development of soil drought, and that of clone Z3, YX2 and 89 was significantly higher than the other clones. The cell membrane permeability and MDA content had a steady increase with the soil water content gradually reducing, and that of clone Z3,YX2 and 89 was significantly lower than others under the same drought condition. The activities of SOD increased under mild and moderate drought stress, and then decreased under severe drought stress. The activities of SOD in the clone 89 and YX2 were always higher than that in control. Comprehensive assessments of drought resistance based on the principal component analysis and subordinate function values analysis indicated that the sequence of drought resistance in nine clones from high to low was as follows: 89, YX2, LM50, Z3, Y3, 150, 27, 26, and 1316. Therefore, it was preliminarily considered that the clone 89 and YX2 had significant drought resistance and fast growth, which could be used as the important materials for afforestation in arid area and breeding resource in stress resistance research.