Effects of Salicylic Acid on Reactive Oxygen Species Metabolism in Malus robusta Rehd. under Root-zone Hypoxia Stress Cloning and Phenol Degradation of Polyphenol Oxidase Gene MdPPO2B From Malus Robusta Rehd. Effects of Salicylic Acid on Reactive Oxygen Species Metabolism in Malus robusta Rehd. under Root-zone Hypoxia Stress Distribution of apple skin scar viroid in seeds of Malus robusta Rehd. and viroid elimination by NaOH treatment