Structure of floral nectaries in Aesculus hippocastanum L.

Representatives of the family Sapindaceae exhibit high morphological diversity of the nectary structure. The present paper shows for the first time the results of micromorphological, anatomical, and ultrastructural analyses of floral nectaries in Aesculus hippocastanum. We have also described the forage and signal attractants of these flowers, which are important for the ecology of pollination. Using light, fluorescence, and electron microscopy, we demonstrated that the A. hippocastanum nectary forming a lobed disc is histologically differentiated into the epidermis with stomata, nectariferous parenchyma, subglandular parenchyma, and vascular bundles reaching the basal part of the nectariferous parenchyma. The use of histochemical assays revealed the presence of insoluble polysaccharides, lipids, terpenoids, and polyphenols including coumarins in the nectary tissues. Nectar is exuded onto the nectary surface via stomata and the permeable cuticle. As indicated by the observation of the ultrastructure of the nectary cells, transport of pre-nectar into parenchymal cells may proceed via the symplast and apoplast. We have also demonstrated that nectar transfer outside the protoplasts of parenchymal cells has a character of granulocrine secretion. A. hippocastanum flowers produce nectar abundantly; one flower secreted on average 2.64 mg of nectar and the concentration of sugars in the nectar was 33%