Effect of Copper and Zinc on Activity of Polyphenol Oxidase in Larix principis-rupprechtii Study on Characteristics of Polyphenol Oxidase in Pingguoli Pear Study on Characteristics of Polyphenol Oxidase in Pingguoli Pear Study on browning mechanism of drying Pinelliae Rhizoma Study on browning mechanism of drying Pinelliae Rhizoma Analysis on three isozymes in plants of Curcuma L. Analysis on three isozymes in plants of Curcuma L.