Microscopic structure and histochemical study of Gentiana decumbens L.f

Abstract

Background: Plants of the family Gentianaceae in Mongolia are rich in bitter iridoid glycosides, flavonoids, and polyphenolic compounds, and have been traditionally used in Mongolian medicine to promote digestion, protect the liver, and reduce fever. However, standardized information on the micro-morphology and histochemical characteristics of the widely distributed species Gentiana decumbens L.f. is scarce, limiting the assessment of its quality as a medicinal raw material. Aim: To identify the morphological and anatomical characteristics of the aerial and underground parts of Gentiana decumbens and to localize the distribution of starch, polysaccharides, and phenolic compounds using histochemical methods. Materials and Methods: Plant materials were collected in July 2024 from Tsagaan-Uul soum, Khuvsgul province, and shade-dried. Samples were softened in a water–ethanol–glycerol solution (1:1:1) for 24 hours, and transverse and longitudinal sections of stem, leaf, root, and floral parts (anther, petal, receptacle/bract) were prepared using a hand microtome. Sections were cleared with 10% chloral hydrate. Histochemical reagents included potassium dichromate (K2Cr2O7), ferric chloride (FeCl3), Lugol’s iodine (I₂+KI), thymol + concentrated H₂SO₄, and methylene blue. Slides were mounted in glycerin and examined under an Olympus light microscope at 40× and 100× magnifications, and images were recorded using a 12 MP digital camera. Results: Leaf: The leaf exhibits a dorsiventral structure with a dense palisade parenchyma on the adaxial side and a loosely arranged spongy mesophyll on the abaxial side. Stomata are hypostomatic, and collenchyma is well developed around the vascular bundles. Histochemical reactions were negative for starch and phenolic compounds. Stem: The stem consists of an epidermis, a wide cortex, a continuous ring of sclerenchyma, 12–15 collateral vascular bundles, and a broad central parenchyma. Histochemical tests showed a brownish-yellow coloration with potassium dichromate (K2Cr2O7), indicating the presence of polyphenols and lignin, and a brown coloration with Lugol’s iodine, confirming the presence of starch. Reactions with ferric chloride (FeCl₃), methylene blue, and thymol + H2SO4 were mostly negative in cross sections. However, in longitudinal sections, Lugol’s and thymol + H2SO4 showed positive reactions, suggesting the presence of starch and polysaccharides, respectively. Root: The root displays a primary structure with radial xylem and phloem, a distinct endodermis with Casparian strips, and a pericycle layer. Positive reactions were observed with K2Cr2O7 and FeCl₃, indicating phenolic compounds and lignin. Lugol’s iodine showed abundant starch granules in parenchyma cells. Receptacle / Bract: The receptacle is covered by a thick cuticular epidermis with trichomes, and composed of outer and central parenchyma layers containing numerous small closed (amphivasal-type) vascular bundles. Positive reactions were detected with K2Cr2O7 and FeCl₃, indicating polyphenols, flavonoids, and tannins; Lugol’s iodine confirmed the presence of starch; thymol + H2SO4 showed partial positivity for polysaccharides. Stamen (longitudinal section): The stamen structure includes epidermis, endothecium, pollen sacs, and a distinct vascular bundle. Positive staining was observed with K2Cr2O7, Lugol’s iodine, and FeCl3, confirming the presence of polyphenolic compounds, lignified elements, and starch. Conclusion: The anatomical features of G. decumbens indicate adaptations to dry, high-light environments through well-developed mechanical strengthening (sclerenchymatous ring), efÏcient photosynthetic structure (compact palisade mesophyll), and nutrient storage (starch-rich pith and cortex). Histochemical analysis revealed the localization of polyphenolic compounds and starch/polysaccharides mainly in the root, stem, and floral organs, while absent in the leaves, demonstrating organ-specific accumulation patterns. These findings scientifically support the plant’s traditional medicinal uses for digestive stimulation and hepatoprotection.