Does the Floral Nectary in Dracocephalum moldavica L. Produce Nectar and Essential Oil? Structure and Histochemistry of the Nectary

SIMPLE SUMMARY: Dracocephalum moldavica is an aromatic plant originating from Asia and grown for medicinal, cosmetic, seasoning, beekeeping, and decorative purposes. All types of trichomes present in the aboveground parts of the plant produce essential oil. The floral nectaries of this species have special properties, as they produce not only nectar but also essential oil, which we have described in the present study. The micromorphological studies have shown that nectar release is mediated by nectarostomata, and the histochemical tests have revealed that the essential oil is produced by trichomes and the pavement cells of the nectary epidermis. The secretion of aromatic nectar has great ecological importance in the plant–insect relationship and explains the great interest of bees in the flowers of this species. Our study provides the first description of the production of nectar and essential oil by the nectaries in the family Lamiaceae. ABSTRACT: Dracocephalum moldavica is an aromatic plant with a lemon scent and versatile use. Its flowers produce large amounts of nectar, which is collected by bees and bumblebees. The aim of the study was to investigate the structure of the floral nectary in this melliferous plant, which has not been analysed to date. The analyses were carried out with the use of light, fluorescence, scanning electron, and transmission electron microscopy, as well as histochemical techniques. The four-lobed nectary with a diameter of 0.9–1.2 mm and a maximum height of 1.2 mm is located at the ovary base; one of its lobes is larger than the others and bears 20–30 nectarostomata and 8–9 glandular trichomes. The histochemical assays revealed the presence of essential oil and phenolic compounds in the nectary tissues and in glandular trichomes. The nectary tissues are supplied by xylem- and phloem-containing vascular bundles. The nectariferous parenchyma cells have numerous mitochondria, plastids, ribosomes, dictyosomes, ER profiles, vesicles, thin cell walls, and plasmodesmata. Starch grains are present only in the tissues of nectaries in floral buds. The study showed high metabolic activity of D. moldavica nectary glands, i.e., production of not only nectar but also essential oil, which may increase the attractiveness of the flowers to pollinators, inhibit the growth of fungal and bacterial pathogens, and limit pest foraging.