Salt Tolerance of betA Transgenic Populus simonii×P. nigra and Selection for Superior Transgenic Plants Cloning and Transient Expression Analysis of Promoter of PnsICE1 from Populus simonii×P.nigra 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)