Floral Nectary and Trichome Structure of Hoya cagayanensis, Hoya lacunosa, and Hoya coriacea (Apocynaceae, Marsdenieae)

Hoya R.Br. is a genus of ornamental plants with an attractive flower shape and unique scents. Anatomical studies are required to understand their structure that produces, and stores, the metabolites released by plants. The present study was conducted to determine the type and position of floral glands in three Hoya species: Hoya cagayanensis C.M Burton, Hoya lacunosa Blume, and Hoya coriacea Blume. The investigations were carried out using light microscopy (LM), scanning electron microscopy (SEM), and histochemical staining tests. Secondary nectaries were present in the corona lobe portion, while simple trichomes (unicellular) were found in the petals' adaxial epidermis of all the studied species. Conical trichomes were found in H. cagayanensis, cylindrical trichomes in H. lacunosa, and falcate trichomes in H. coriacea. In the cells of secondary nectaries, this study revealed proteins, lipids, polysaccharides, and starch grains; however, only lipids and proteins were observed in simple trichomes. Secondary nectaries and simple trichomes were described for the first time in all the studied species, contributing new insight into Hoya's anatomical and micromorphological floral glands.