| The genus Fraxinus contains about70species throughout the world They are widely distributed in the Northern Hemisphere, among which20species are distributed in China, including F.mandshurica, F.velutina, F.sogdiana and F.americana etc.Hybrid plant could greatly inerease quality, modestly improve and raise tolerance to stresses. Accordingly, hybrid forestry trees could create remarkable economic and social benefits. However, heterosis of trees has not been used as widely as that of rice and corn. Deficiency of slowly growing of trees and the expense of producing F1seeds are two main reasons leading to this present situation. Therefore, it is urgent to study the genetic mechanism of trees heterosis to promote selection of elite hybrideotton.Based on the hybridization between F.mandshurica and F.sogdiana/F.americana, this paper studied the heterosis of F.mandshurica×F.sogdiana and F.mandshurica×F.americana under drought stress. The main conclusions are as follows:1. F. mandshurica X F. americana, F. sogdiana interspecific hybrid F1drought resistance physiological research(1)Research on drought resistance indicators of F.mandshurica×F.sogdiana and F.mandshurica×F.americana:the hybrid combination were gradual wilting under drought stress and some were withered like burnt. All of them can resume normal after rehabilitation treatment. Drought stress inhibited plant growth, photosynthesis and the synthesis of pigment for photosynthesis. Furthermore, the SOD activity, POD activities increased in different degree under drought stress.3days after rewater, most of the physiological characters can be restored, but still greater than the level before treatment.(2)Research on heterosis of F.mandshurica×F.sogdiana and F.mandshurica×F.americana: The physiological character under drought stress were studied and used to analysis the heterosis of hybrids. For F.mandshurica×F.americana and Fmandshurica×Fsogdiana, growth, photosynthesis, POD activity, SOD activity, the relative conductivity, and photosynthetic pigment content showed heterosis in different degree and varied with their female parents.(3)Comprehensive evaluation of drought resistance on hybrids:39hybrids were evaluated on drought resistance. The order of integrated drought resistance was:110>135>137>36>149>195>14>76>62>142>171>65>136>16>169>72>32>58>131>24>114>29>156>47>15>115>87, of which the best drought resistance heterosis is F.mandshurica×F.americana F1110.2. DNA methylation variation mechanism of F. mandshurica X F. americana interspecific hybrid F1under drought stress (1) CK team:The MSAP ratio, which was the ratio of MSAP sites to the totally amplified sites, in two hybrids (110,115) were15.48%,16.81%, respectively, all of which were lower than those of either their parents(20.12%,24.61%). The results suggested that demethylation was occurred in some amplified5'CCGG sites.(2)Results showed that the average level of methylation was increased under drought stress treatment, especially in115,113,4-3. But the alterations of the DNA methylation state and level induced by drought stress were not increased obviously, and were species specificity.Genomic DNA methylation patterns in parental (113and4-3) and hybrids (115) seedlings were mainly inside cytosine methylation. Genomic DNA methylation patterns in hybrid (110) seedlings were mainly outside of cytosine methylation. The DNA methylation patterns of hybrid F1have been changed and adjusted, suggesting that heterosis of hybrids F1appeared because of the genomic methylation patterns' change and re-adjust.Results showed that the average of DNA methylation in seedlings were increased under drought stress treatment after three days, while the main MSAP pattern was decreasing bands, and female parent(113) changed obviously. The average of DNA methylation in seedlings were increased under drought stress treatment after nine days, while the main MSAP pattern was also decreasing bands, and male parent(4-3) and hybrids (115)changed obviously. The result was similar with the genomic methylation level change.(3)Some variable MSAP profiles were isolated and sequenced, which were analyzed with Blast. Blast analysis results showed that sequences have homology with some functional genes.3. F. mandshurica×F. americana in drought stress digital gene expression profiling analysis(1)About4%of Tag types accounted for nearly65%of the total Tags, but60-70%of Tags type was less than5-9%of the total Tag number. This meant some specific genes play a significant role in the process of the resistance drought stress.(2)Through analysising the differential expressed genes in hybrids110and parent were significant differences in Photosynthesis,Photosynthesis-antenna proteins,Metabolic pathways,Biosynthesis of secondary metabolites,Biosynthesis of unsaturated fatty acids, Carbon fixation in photosynthetic organisms,Pyruvate metabolism. Nitrogen metabolism, Starch and sucrose metabolism,Glycolysis/Gluconeogenesis,Circadian rhythm-plant, Phenylpropanoid biosynthesis,Plant hormone signal transduction etc.
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