Submitted to: HortScience
Publication Type: Peer reviewed journal
Publication Acceptance Date: 2/3/2004
Publication Date: 4/3/2004
Citation: Reed, S. 2004. Pollination Biology of Hydrangea Macrophylla. HortScience. 9:312-315. Interpretive Summary: Hydrangea macrophylla is a popular summer-flowering shrub that is valued for its large brightly colored inflorescences, which are composed of a combination of large showy flowers and small inconspicuous flowers. The showy flowers are generally considered to produce neither pollen nor seeds. The seed-producing inconspicuous flowers are located on the interior of the inflorescence and, in cultivars with mophead inflorescences, are few in number and shielded from view by the showy flowers. While a large number of cultivars of this species are available to consumers, many have been bred for potted plant production or for use in European or Japanese gardens. Since many of these cultivars do not flower well in some areas of this country, cultivars bred for U.S. growing conditions are needed. Breeding efforts in H. macrophylla are hindered by a lack of knowledge of the reproductive biology of this species. The objective of this study was to obtain information on pollination systems in H. macrophylla. A series of experiments were conducted that used fluorescence microscopy to study pollen germination and pollen tube growth. It was determined that the optimum time to make controlled pollinations is from the day flowers open until 4 days later. While self-pollen germinated freely on stigmas, self pollen tubes grew down the style much slower than cross-pollen tubes. This indicates that self-pollinations may not be effective in producing seed in H. macrophylla. The showy flowers of the H. macrophylla inflorescence were found to produce pollen that was as viable as that from the inconspicuous flowers. This indicates that the showy flowers can be used as a source of pollen when making controlled pollinations. Information obtained in this study will facilitate H. macrophylla breeding efforts.
Technical Abstract: Little information is available on the reproductive behavior of Hydrangea macrophylla (Thunb. Ex J.A. Murr.) Ser. The objectives of this study were to investigate time of stigma receptivity, viability of pollen from sterile flowers, and self-incompatibility in this popular ornamental shrub. Pollen germination and pollen tube growth in styles were examined using fluorescence microscopy. Stigma receptivity was examined in cross-pollinations made from 1 day before anthesis to 8 days after anthesis. Maximum stigma receptivity for the two cultivars examined occurred from anthesis to four days after anthesis. Viability of pollen from sterile flowers was evaluated through pollen staining and observations of pollen tube growth. No significant difference in percent stainable pollen between fertile and sterile flowers was observed in any of the six taxa examined. Pollen germination and pollen tube growth were studied in cross-pollinations made using pollen from fertile and sterile flowers of two cultivars. For both cultivars, pollen tubes from fertile and sterile flowers grew to the same length and had entered ovules by 72 hours after pollination. Self-incompatibility was evaluated by comparing pollen germination and pollen tube growth in cross- and self-pollinations. In the five taxa examined, self pollen tubes were significantly shorter than cross pollen tubes in flowers that were examined 72 hours after pollination. This finding indicates the presence of a gametophytic self-incompatibility system in H. macrophylla.