Molecular Cloning and Function Analyses of Two Ammonium Transporter Protein Genes from Pyrus calleryana Molecular Cloning and Function Analyses of Two Ammonium Transporter Protein Genes from Pyrus calleryana Molecular Cloning and Expression Analysis of a Phytochelatin Synthase Gene,PcPCS1,from Pyrus calleryana Dcne. Molecular Cloning and Expression Analysis of a Phytochelatin Synthase Gene,PcPCS1,from Pyrus calleryana Dcne.