Myosotis scorpioides L. (Boraginaceae) floral nectary: Micromorphology, fluorescence, and ultrastructure

The small flowers of Myosotis scorpioides are pollinated by various groups of insects feeding on their nectar accumulating at the base of the corolla tube. To date, only few studies have focused on the anatomy and ultrastructure of nectaries in plants from the family Boraginaceae. The aim of this study was to analyse the structure of the M. scorpioides nectary at different levels of organisation, with special emphasis on its ultrastructure, which has not been investigated so far. The studies were carried out with the use of light, fluorescence, scanning electron, and transmission electron microscopes. Undulated discoidal nectaries with a diameter of up to 0.9 mm and a height of about 0.2 mm were localised at the ovary base. There were numerous nectarostomata on the top and abaxial surfaces of the nectary. Phenolic compounds present in the nectary exhibited autofluorescence. The ultrastructural studies of nectary epidermis and secretory parenchyma cells revealed the presence of thin cell walls, different-sized vacuoles, medium-sized nuclei, plasmodesmata in the cell walls, and various types of chromoplasts. Numerous mitochondria were visible in close proximity to the plastids. Additionally, clusters of dictyosomes, well-developed rough ER profiles, and numerous ribosomes and different-sized vesicles often located near the cell walls were observed. Our study conducted at the subcellular level indicated high metabolic activity of the M. scorpioides nectary. The presence of phenolic compounds in the nectary may both serve as a protection of this part of the flower against biological factors and contribute to the intensification of the floral scent acting as a pollinator attractant.