Study on Polyphenol Content and Antioxidant Activity from Twelve Clones of Catalpa bungei Leaves Changes of Total Phenol Content and the Activities of PPO, POD and PAL during the Browning in Phalaenopsis Explant in Vitro Changes of Total Phenol Content and the Activities of PPO, POD and PAL during the Browning in Phalaenopsis Explant in Vitro