Chemical composition, antioxidant activity and bioactive constituents of six native endangered medicinal orchid species from north-eastern Himalayan region of India

Orchidaceae is one of the largest and most diversified group of flowering plants within the families of angiosperm. Medicinal properties of orchids are less explored, but it is drawing significant attention of the scientific communities recently. The north eastern Himalayan region of India is well-known orchid hotspot with prevalence of two-third of total orchid species of the country. In the present study, an attempt was made to investigate the phytochemical composition of six native, endangered medicinal orchid species of this region namely Dendrobium nobile, Dendrobium moschatum, Dendrobium densiflorum, Acampe papillosa, Coelogyne nitida and Arundina graminifolia. The nutritional as well as phytochemical composition and antioxidant activity of the extracts were analyzed by spectrophotometric methods. Minerals were estimated by atomic absorption spectroscopy (AAS) and flame photometer. Quantification of phenolics and bioactive metabolites from methanol extracts of the selected orchid species were carried out by high performance liquid chromatography (HPLC) and gas chromatography-mass spectrometry (GCMS) methods. Results revealed that Dendrobium moschatum contained highest total carbohydrate (6.49 g 100g⁻¹) and protein (11.30 g 100g⁻¹). Dendrobium densiflorum was rich in total flavonoid content (35.54 mg quercetin equivalent QE 100g⁻¹) and total alkaloid content (11.70 g 100g⁻¹). It also showed highest 2, 2’-azino-bis (3-ethylbenzothiazoline-6-sulfonic acid) (ABTS) scavenging activity (64.4 µg ml⁻¹). Acampe papillosa exhibited maximum total phenol content (4.09 g gallic acid equivalent GAE 100g⁻¹), tannin (3.17 g tannic acid equivalent TAE 100g⁻¹) and saponin (7.20 g 100g⁻¹) content whereas Coelogyne nitida showed 281.7 µg ml⁻¹ 2, 2-diphenyl-1-picryl-hydrazyl (DPPH) scavenging activity. Iron, zinc, copper, manganese, sodium, potassium and calcium contents were higher in the Dendrobium species compared to other medicinal orchids under study. Quercetin (4518.3 mg kg⁻¹ in Dendrobium moschatum) and catechin (1211.1 mg kg⁻¹ in Dendrobium densiflorum) were the major phenolic compounds present abundantly in these orchid species. Metabolite analysis by GCMS showed presence of total twenty-six major bio-active compounds (like dendroban-12-one, embramine, scoparone etc.) in these six orchid species. The current study presents scientific evidence in support of the traditional therapeutic application of the studied orchid species and proves that these are potential sources of biologically and pharmacologically active ingredients.