Do Nectar Guides in Flowers of Two Myosotis L. Species (Boraginaceae) Serve as Osmophores?

Osmophores, which constitute scent-releasing glands in flowers, have so far been insufficiently characterised. This study is the first to present the structure of scent glands exhibiting features consistent with those of osmophores in two Myosotis species (Boraginaceae). Petal tissues of Myosotis scorpioides and M. sylvatica were analysed using light microscopy, scanning electron microscopy, and transmission electron microscopy. Histochemical tests were performed to detect fragrance-producing tissues (neutral red), phenols (Toluidine blue O), lipids (Sudan III, Sudan Red B), and starch (Lugol solution). The study has shown that the epidermal cells within the yellow nectar guides (faucal scales) were differentiated, as they comprised palisade-like cells, papillae, and unicellular trichomes. The petal appendages forming white ridges on the petals were covered with papillae. Histochemical assays revealed the presence of scent-producing epidermal cells (palisade-like, papillae, unicellular trichomes), lipids (which is typical of scent glands) and starch during the pre-secretory phase. The ultrastructure of the examined cells exhibited dense cytoplasm, large nuclei, and numerous mitochondria and plastids. Such organelle composition is associated with secretory activity, which is further supported by the presence of numerous secretory vesicles located in the vicinity of the plasmalemma, releasing their contents into the periplasmic space. The histochemical analyses and the observations of the anatomical structure and ultrastructure of the cells indicated that both the faucal scales and the petal appendages exhibit features of osmophores. These findings contribute to expanding the extant knowledge on the structure of scent glands with traits typical of osmophores and the characteristics of the Boraginaceae family.