STUDY ON THE TRANSFORMATION OF POPULUS SIMONII×P.NIGRA WITH SALT RESISTANCE GENE BET A Cloning and Transient Expression Analysis of Promoter of PnsICE1 from Populus simonii×P.nigra Comparison on the Pulping Property between Juvenile and Mature Wood of Populus simonii×P.nigra Variation Analysis of Growth Traits of Transgenic Populus simonii×P.nigra Clones Carrying TaLEA Gene Salt Tolerance of betA Transgenic Populus simonii×P. nigra and Selection for Superior Transgenic Plants