Detaling morphological traits of Trollius europeus L. flowers, nectary structure, and holocrine nectar secretion...

Trollius europeus L. flowers produce nectar available to various groups of insects. No anatomical studies of these floral nectaries have been conducted to date. This study presents the structure of nectaries at different levels of organisation and the characteristics of petaloids, nectary leaves (petals), and stamens, including their micromorphology. The analyses of the nectaries were carried out with the use of light, fluorescence, scanning electron, and transmission electron microscopy techniques. The nectary located in a small cavity on the nectary leaf consists of the epidermis, nectar-producing parenchyma, and subnectary parenchyma, which has three large vascular bundles containing phloem and xylem. Amyloplasts with starch granules are present only in the parenchyma surrounding the vascular bundles. The other cells of the nectary parenchyma contain only chromoplasts with large plastoglobules. Since there are no chloroplasts, sugars required for nectar production are assumed to originate from phloem sap. The numerous plasmodesmata in the cell walls indicate a symplastic route of pre-nectar. Nectar is secreted onto the nectary surface in the holocrine mode. After disorganisation of the cytoplasmic structure, the epidermal cell wall is disrupted and the cell contents along with the nectar are released into the depression on the nectary leaf. The secretion of nectar from cells is non-synchronous and lasts 4-5 days. The content of nectar proteins and lipids derived from the cytoplasm of epidermal cells increases the nutritional value of the secretion, which may be important for pollinators.