Isolation and expression analysis of a plasma membrane Na+/H+ antiporter from Nitraria tangutorum Clning and Sequence Analysis of a Na+/H+ Antiporter Gene in HalophyteLimonium gmelinii Plasma Membrane Na+/H+ Antiporter Is Involved in Plant Salt Tolerance Clning and Sequence Analysis of a Na+/H+ Antiporter Gene in HalophyteLimonium gmelinii