Chilling and Heat Requirements of Main Cultivars of Ping‘ou Hybrid Hazelnuts (Corylus heterophylla×Corylus avellana) Correlations of the Nut and Catkin Quantity Properties of Corylus heterophylla×C.avellana Effects of Different Treatments on Hardwood-Cutting Rooting and Related Oxidase Activity Changes during Rooting of Corylus avellana Studies on Physiological Characteristics of Corylus avellana L. during Storage and Germination