Morphoanatomical, Histochemical, and Essential Oil Composition of the Plectranthus ornatus Codd. (Lamiaceae)

Abstract

Plectranthus ornatus is a medicinal and aromatic plant used in traditional and alternative medicine. In this study, leaves of P. ornatus were collected in two cities of the state of Pará, "Quatipuru" and "Barcarena", and were used with the objective of analyzing, through morphoanatomical data and histochemical and phytochemical studies of essential oil, the samples present structural differences and differences in their chemical composition. Anatomical and histochemical analyses were performed by transverse, using longitudinal sections of 8 μm to 10 μm to perform epidermal dissociation, diaphonization, and tests to identify classes of secondary metabolites. The essential oils were isolated by hydrodistillation, and the identification of the chemical composition was performed by gas chromatography coupled with mass spectrometry. The anatomical study shows that there is no difference between specimens collected in different locations, and stellate trichomes were identified. The histochemical study detected total lipids and acids, terpenes, polysaccharides, phenolic compounds, tannins, alkaloids, and calcium oxalate. The low essential oil yield may be related to the low density of secretory cells (glandular trichomes), the unidentified compounds in the highest concentration in the essential oil were in relation to the chemical composition of the essential oils, and the major compounds were α-pinene, sabinene, (E)-caryophyllene, caryophyllene oxide, and oct-1-en-3-ol. The results provide new information about the anatomy and histochemistry of P. ornatus.

Keywords: anatomy; natural products; phytochemicals; volatile compounds.

Figure 1

Leaves isolated from Plectranthus ornatus boldo. (A)—adaxial face from specimen Barcarena. (B)—abaxial face from specimen Barcarena. (C)—adaxial face from specimen Quatipuru. (D)—abaxial face from specimen Quatipuru collected in different sizes throughout the first and fifth internodes.

Figure 2

Front view of leaf blade P. ornatus Codd. (A–D). (A)—General view of the adaxial side and cells showing sinuous anticlinal walls (arrow). (B)—Diacytic stomata detail on the abaxial side. (C)—Overview of pluricellular non-glandular trichomes. (D)—Overview of glandular trichomes. Cross-sectional area (E,F). (E)—Glandular trichomes with a small petiole. Scale bar: 50 μm (B,E,F), 50 μm (A,C,D).

Figure 3

P. ornatus Codd. Cross section of the leaf blade. (A)—Front view of homogeneous mesophile. (B)—Foliar Boldo showing glandular trichome. (C)—Midrib. (D)—Collateral vascular bundle detail in the midrib. (E)—Overview of the petiole. (F)—Collateral vascular bundle detail of the petiole. (G)—Crystals of calcium oxalate. Legend: ac—angular collenchyma, cr—calcium oxalate crystals, ade—adaxial epidermis, abe—abaxial epidermis, ep—epidermis, st—stomata, sc—substomata camera, vb—vascular bundle, p—phloem, x—xylem, ngt—non-glandular trichome, gt—glandular trichome. Scale bar: 100µm (A,B,D), 200µm (C,F,G), 500µm (E).

Figure 4

Histochemical tests on the leaf limbo secreting glandular trichomes of Plectranthus ornatus Codd. (A)—White, (B)—Positive result for total lipids with Sudan Black. (C)—Positive result for total lipids with Sudan III. (D)—Positive result for acidic lipids. (E)—Positive result for terpenes. (F)—Positive result for total phenolic compounds. (G)—Positive result for tannin. (H)—Positive result for total polysaccharides. (I)—Positive result for alkaloids. Arrows: Indicate where the reactions happened. Scale bar: 20 μm (G–I) 50 μm (A–F).

Figure 5

Regions of the leaf blade of Plectranthus ornatus used in the anatomical study.