Abstract
In entomogamous plants, the presence and function of floral secretory structures, whose main role is to attract pollinators, is strictly associated with the pollination ecology and hence the reproductive success of the plant. The aims of the present paper were to analyse the micromorphology and anatomy of flower nectaries and stigmas in Viburnum opulus and V. lantana and to determine the function and microstructure of inflorescence trichomes in both taxa using light and scanning electron microscopy as well as histochemical assays. It was found that stigmas were formed by papillae, which contained lipids, polysaccharides, tannins, and pigments. Stigmatic secretion proceeded via cuticular pores. Floral nectaries formed a thick layer around the styles, and nectar was secreted through numerous nectarostomata. There were no traces of vascular bundles penetrating the nectary tissue. In turn, numerous tannin deposits were observed in the cells of the glandular parenchyma. Pedicels, hypanthia, and bracts had mainly peltate and capitate glandular trichomes as well as stellate non-glandular trichomes (in V. lantana). The trichomes were shown to contain lipids, mucilage, and tannins. Many similarities in the flower and nectaries microstructure and considerable heterogeneity were observed in the examined Viburnum species. Knowledge of the microstructural characteristics of flowers, nectaries, and trichomes may be important for the phylogenesis and taxonomy of the genus Viburnum and the family Adoxaceae. Additionally, floral and nectaries features are helpful in assessment of the relatedness between taxa and provide better understanding of the floral biology and pollination ecology.
Highlights
The genus Viburnum L. representing the family Adoxaceae comprises approximately 160–200 species of evergreen, semi-evergreen, and deciduous shrubs or small trees growing primarily in the temperate climate zone of the northern hemisphere and in tropical mountains, south-eastern Australia and Tasmania, with their largest diversity in Himalaya and China (Winkworth and Donoghue 2005)
Since the current knowledge of the structure and function of flower nectaries and other floral secretory structures is insufficient, the aim of the present paper was to show for the first time the micromorphological and anatomical organization of the stigmas and floral nectaries in Viburnum opulus L. and V. lantana L. and to determine the distribution, structure, and role of glandular trichomes located in the proximity to the flowers in both taxa
The pentamerous V. opulus flowers gathered in corymb-like inflorescences were differentiated into marginal sterile, large, white flowers equipped with a pistil having smaller or ruptured stigmas and 4 underdeveloped stamens as well as highly numerous, inconspicuous, inner creamy-white hermaphroditic flowers with an unpleasant odour and 5 stamens equipped with long filaments and bright yellow anthers (Fig. 1a, b)
Summary
The genus Viburnum L. representing the family Adoxaceae comprises approximately 160–200 species of evergreen, semi-evergreen, and deciduous shrubs or small trees growing primarily in the temperate climate zone of the northern hemisphere and in tropical mountains, south-eastern Australia and Tasmania, with their largest diversity in Himalaya and China (Winkworth and Donoghue 2005). Besides Viburnum, i.e. the most numerous genera in Adoxaceae, the family comprises only a few representatives: Adoxa L., Sinadoxa C. Viburnum differs markedly from the rest of the family in several aspects, e.g. in having simple, rarely ternate leaves, introrse anthers, single-seeded drupes, and scalariform vessel perforations (Donoghue 1983a, b; Winkworth and Donoghue 2004, 2005). In Adoxa, Sinadoxa and Tetradoxa, multicellular trichome nectaries are located at corolla lobe bases; in contrast, many Sambucus species lack nectaries, whilst in others (S. javanica, S. chinensis) whole
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