Abstract: To a better understanding of the evolutionary mechanism between reproductive organs and pollinators in Viburnum, this paper reports reproductive characteristics of Viburnum macrocephalum f. keteleeri, including floral characteristics, anatomical features of reproductive organs, pollen viability, pollen/ovule ratio (P/O), pollinators, pollen tube growth path, breeding system, and fertilization. This species possesses a compound umbel including fertile and infertile flowers. The fertile flower has one pistil and five stamens; each pistil has a dry stigma and an ovary that contains one anatropous ovule. The stamens and pistils of the infertile flower are normal initially but degenerate later during flowering; degenerated stamens are mainly characterized by disappearance of stamens, short filaments, absence of filaments, or different size of anthers, whereas degenerated pistils have smaller or ruptured stigmas; occasionally, pistils and stamens become petal-like. Pollen viability of individual flowers declined significantly 4–5 d after pollen dissemination. Within population, pollen viability of all flowers rapidly decreased at the end of April. Pollen/ovule ratio (P/O) was 12800–18700. Pollinators for this species include bees, flies, butterflies and beetles, but bees and butterflies are the main ones. Artificial pollination treatments demonstrated that this species was self-incompatible and seed production depended on pollinator visits. Pollen dissemination of fertile flowers mainly occurred during 9:00 am–4:00 pm, and the peak of pollinator visits was 11:00 am–3:00 pm. Pollen grains usually germinated within 1 h after pollination and pollen tubes penetrated the stigma through the papilla clearance and then continued to grow along the transmitting tissue in center of style. Some pollen tubes reached the ovary 18 h later and grew into the ovule through micropyle within 20 h after pollination. In addition, formation of sterile flowers, adaptive floral structures, low production of fruits, and growth characteristics of pollen tubes were discussed.

Key words: Viburnum macrocephalum f. keteleeri, reproductive organs, anatomical structure, breeding system, pollination biology, pollen tube development