Cloning of nia Gene and Its Differential Expression Induced by Different Nitrogen Forms in Sugar Beet (Beta vulgaris L.) Construction and Analyses of SSH cDNA L ibraries of Rose Floral Color andScent Mutant Isolation and Characterization of Two NaCl-Inducible Vacuolar Na+/H+ Antiporter Genes (PtNHX1, PtNHX6) in Populus Construction and Analyses of SSH cDNA L ibraries of Rose Floral Color andScent Mutant