Nectar guides and floral nectary in Lamium album L. subsp. album: structure and histochemistry in light, fluorescence, and electron microscopy

Abstract

Lamium album is a native common plant growing in Eurasia. It is used in medicine and cosmetics and is highly valued in apiculture. The aim of the study was to investigate the structure of the floral nectary in three stages of flower development. Additionally, histochemical studies of the nectary and nectar guides present on the lower corolla lobe were carried out. No detailed analyses of nectary tissues in this species have been conducted to date. The present analyses were performed with the use of light, fluorescence, and scanning electron microscopy. The nectary gland in the flowers of Lamium album subsp. album formed an incomplete ring at the ovary base. The nectarostomata were arranged in clusters only in the adaxial epidermis of the anterior part of the nectary. During the secretory activity of the nectary (1st day of flowering), numerous small vacuoles and cells with large lobulate nuclei with surrounding plastid clusters were observed in the epidermis and glandular parenchyma cells. The vascular bundles contained xylem and phloem elements. Corolla wilting (3rd day of flowering) was accompanied by destructive changes in the nectary parenchyma, leading to the formation of empty spaces and appearance of cell remnants on the nectary surface. The histochemical analyses revealed the presence of starch and phenolic compounds as well as acidic and neutral lipids, which are characteristic of essential oils, in the nectary tissues. The nectar guides were composed of large yellow papillae containing phenolic compounds and acidic and neutral lipids, which were also present in glandular trichomes and abaxial parenchyma cells. The present study has demonstrated that the scent of Lamium album subsp. album flowers is produced with the involvement of essential oils contained in adaxial and abaxial epidermis cells, glandular trichomes, and nectary tissues.