[Evolution and change of the species quality and authentic producing areas of Dioscorea opposita Thunb].

Yam was first recorded in Shen nong Ben cao Jing (Shennong's Classic of Materia Medica) which was basically came from Dioscorea opposita since ancient time. Before the Tang dynasty, it basically came from the wild species of Dioscorea opposita. In the Song Dynasty, D. opposita began to be cultivated, but Chinese medical practitioners claimed that the wild species was better than the cultivated one. After the Ming and Qing Dynasties, yam was mostly from cultivated resources. As the species quality changed, the authentic producing area of Dioscorea opposite was recognized to be in Huaiqing, Henan province, since the Ming and Qing Dynasties. Through long-term cultivation, the species produced in Huaiqing began to differentiate and the Chinese medicinal experts of successive ages considered that the product quality of yam is excellent and if it revealed the features of"white, thin, solid quality", that was the best, and this is consistent with the species called"Tiegun (iron stick)" Diascorea opposita cultivated by Huaiqing farmers. Hence, the title of"Huaiqing yam" is not the general title for the authentic species of yam produced in the Huaiqing region (now Jiaozuo city, Jiyuan city, and Xinxiang city of Henan Province), but strictly refers to that species commonly and long recognized by TCM specialists as the Tiegun Dioscorea opposita.