Salt Tolerance of betA Transgenic Populus simonii×P. nigra and Selection for Superior Transgenic Plants Isolation, Expression and Single Nucleotide Polymorphisms Analysis of Heat Shock Transcription Factor HsfA1d in Populus Cloning and Transient Expression Analysis of Promoter of PnsICE1 from Populus simonii×P.nigra DREB Gene and Its Application in Analyzing Population Structure in Populus simonii Variation Analysis of Growth Traits of Transgenic Populus simonii×P.nigra Clones Carrying TaLEA Gene Isolation and Functional Analysis of FLC Gene from Poplar (Populus simonii×Populus nigra)