Sites of secretion of bioactive compounds in leaves of Dracocephalum moldavica L.: anatomical, histochemical, and essential oil study

Abstract

Dracocephalum moldavica L. is an aromatic plant emitting intense lemon scent. The aboveground parts of the plants constitute raw material for medicine and food industry. In contrast to the comprehensively investigated trichomes, there are only few studies of the histochemical characteristics of the leaves of essential oil-bearing plants from the family Lamiaceae. The present study shows the micromorphology, anatomy, and histochemistry of the leaves of the analysed species. The research aimed to determine the location of essential oil and other specialised metabolites in leaf tissues. The investigations of fresh and fixed material were carried out with the use of light, fluorescence, and scanning electron microscopy. Additionally, the content and composition of essential oil in the leaves were determined with the GC/MS method. The leaf epidermis had non-glandular unbranched trichomes and three types of glandular trichomes: peltate as well as long and short capitate trichomes. The results of the histochemical assays showed positive reactions to lipids and to some secondary compounds such as essential oil, terpenes, phenolic compounds, and flavonoids in all types of the glandular and non-glandular trichomes. The same compounds were found in the epidermis cells of the leaves. The results of the present study indicate that the intense smell of the leaves is associated with emission of essential oil not only by the glandular and non-glandular trichomes but also by the leaf epidermis cells. The main components in the essential oil (0.10%) include geranial, neral, geraniol, nerol, and trans-myrtanol acetate. Since D. moldavica has been used as an adulterant of Melissa officinalis L., the anatomical traits of leaves and the essential oil composition in both species were compared in the study.

Phenolic Constituents of Lamium album L. subsp. album Flowers: Anatomical, Histochemical, and Phytochemical Study

“…In Stachys , Prasium , Sideritis , and Scutelaria , most types of glandular trichomes contained phenolic compounds [ 35 , 36 ]. Similar results were obtained in the case of Dracocephalum [ 29 ], Isodon [ 34 ], Satureja [ 37 ], Melissa [ 38 ], Marrubium [ 39 ], and Hyssopus [ 40 ]. In contrast, no phenolic compounds in glandular trichomes were detected in Ocimum obovatum [ 33 ], Plectranthus ornatus [ 41 ], and Salvia aegyptiaca [ 42 ].…”

“…This type of trichomes contained tannin-like phenolic compounds, as shown by the reaction with Toluidine blue O. A similar distribution of phenolic compounds in non-glandular trichomes on leaves of another Lamiaceae representative—i.e., Dracocephalum moldavica —was reported earlier by Dmitruk et al [ 29 ].…”

“…The conical one-celled (ii) trichomes located in the corolla tube as well as some epidermis cells and papillae were found to contain flavonoids emitting secondary yellow fluorescence. The flavonoid content in some leaf epidermis cells and glandular trichomes and in the basal cells of non-glandular trichomes was determined previously in D. moldavica [ 29 ].…”

“…Flavonoids have also been identified in glandular trichomes in other Lamiaceae species—e.g., in Salvia officinalis [ 43 ]; 11 species from the genera Stachys , Prasium , Sideritis , and Scutellaria [ 35 ]; Dracocephalum moldavica [ 29 ]; Marrubium vulgare [ 39 ]; and Thymus quinquecostatus [ 44 ].…”

“…The content of phenolic compounds in corolla epidermis, its structures, and the stamens was determined using fresh free-hand sections and the following histochemical assays: ferric trichloride for total phenolic compounds [64]; potassium dichromate for tannins [65]; and Toluidine blue O (pH 4) for phenolic compounds, tannins [66][67][68][69], and pectins, the staining of which is limited at their lower content in the cell walls [68,70] (pp. [25][26][27][28][29][30][31][32][33][34][35][36][37][38][39][40].…”