Keywords

7.1 Introduction

Hypoxis is a well-known genus of the family Hypoxidaceae [1, 2] and its species are monocotyledons that are commonly distributed in the Southern Hemisphere including sub-Saharan Africa, America, south-east Asia and Australia [3, 4]. The genus Hypoxis is reported to have its centre of variation in South Africa [5, 6] where it occurs in open undisturbed grasslands [7]. There are 96 known species of Hypoxis in Africa, out of which 30 Hypoxis species are found in southern Africa [8], eleven species of which are used for medicinal purposes with H. hemerocallidea and H. colchicifolia reported to be the most popular [2, 9]. Hypoxis genus has generated pharmaceutical interest based on its use as a traditional medicine by indigenous people of eastern and southern Africa [10]. The Hypoxis corm has a catalogue of medicinal uses and also serves as a source of food; it is among the commonly prescribed medicines by traditional healers [11]. Hypoxis species have been used in South Africa as umuthi for hundreds of years by different tribes to treat various ailments [12]. In Zulu traditional medicine, Hypoxis roots or corms are used for treating infertility, urinary infections, intestinal parasites, heart weakness, nausea, cough, palpitations, nervous disorder and vomiting. An infusion made from the tuber of H. colchicifolia is taken as an emetic against fearful dreams. H. rigula and H. hemerocallidea leaves are used as ropes in KwaZulu-Natal. Corms from H. obtuse are used to make black polish which is then applied to the floors of the huts. In the midst of starvation the Xhosa and the Sotho people roast or boil the corms of some Hypoxis species so that they can eat them [9, 13, 14]. Extracts from the corms of Hypoxis are used as ingredients in a wide range of products such as anti-oxidants, anti-inflammatories, anti-diabetics and anti-convulsants [15]. Some of the species, such as H. hemerocallidea, H. stellipilis, H. sobolifera var. sobolifera [16] and H. obtusa [17, 18] to name a few, have been scientifically proven to contain hypoxoside, a phytochemical that has immune regulatory properties and its extracts are now widely used in the pharmaceutical industry [19].

Hypoxis hemerocallidea Fisch. & C.A. Mey (Hypoxidaceae) was previously known as H. rooperi ; however nowadays it is commonly known as the African potato, yellow star flower (English), inongwe (isiXhosa), ilabatheka, zifozonke (isiSwati) and Inkomfe (isiZulu). It is a stemless, geophytic, perennial herb with large corms (tubers) which are dark brown or black on the outside and bright yellow inside [20]; this type of Hypoxis species occurs in open grassland and woodland. It is widespread in South Africa in provinces like Eastern Cape, Free State, KwaZulu-Natal, Mpumalanga, Gauteng and Limpopo; H. hemerocallidea is also found in open grass of Botswana and Lesotho and in savanna regions of Swaziland and Zimbabwe. Studies on the medicinal properties of H. hemerocallidea dated back to 1982 when unknowingly the corms were simultaneously studied for the first time in two countries by scientists in Italy and South Africa [21]. H. hemerocallidea, or African potato, is counted amongst the special indigenous medicinal species of commercial importance in southern Africa [6]. The corm of the H. hemerocallidea has been used in folk medicine to treat a variety of diseases, such as the common cold, flu, hypertension, adult-onset diabetes mellitus, psoriasis, urinary infections, testicular tumours, prostate hypertrophy and internal cancer, HIV/AIDS and some central nervous system disorders [22].

A hydroalcoholic extract of H. rooperi was patented with a long list of beneficial properties such as anti-inflammatory, antibiotic, antiarthritic, antiatherosclerotic, diuretic and stimulant of muscular and hormonal activities [23]. Some biomedical evidence suggests that H. hemerocallidea corm extract may be useful in the management of type 2 diabetes mellitus [9]. Previous studies have shown that crude aqueous and methanolic extracts of H. hemerocallidea exhibited good antibacterial activity against a number of bacteria strains including Escherichia coli and Staphylococcus aureus [13, 24]. The demand for H. hemerocallidea has intensified in recent years, following the isolation and elucidation of a phytosterol diglucoside, hypoxoside, which has various pharmacological activities [22, 25]. Bayley and Van Staden showed that the corms of H. hemerocallidea are the major site of biosynthesis of hypoxoside [26]. Some of the compounds isolated from H. hemerocallidea are desmosterol, β-sitosterol, campesterol, stigmastanol, stigmasterol and β-sitosterol glucoside [27,28,29]. The glucoside hypoxoside was first isolated and characterized from H. obtusa by Marini-Betolo et al. in 1982 four years before the findings by Drewes et al. (1984), and Vinesi et al., in 1990 [27, 28, 30]. Drewes et al., in 1984 reported the presence of hypoxoside in H. acuminata, H. nitida, H. obtusa, H. rigidula, and H. latifolia [27, 29]. The phytosterols including their main constituents, hypoxoside and its active derivative rooperol are now being used in fields of anti-oxidants, anti-inflammatories, anti-diabetes, anti-convulsants, inhibitors of drug marker substances, anti-cancerous and premalignant cancer cells [25]. Furthermore, the pharmacological properties of rooperol in studies conducted by several scientists have demonstrated its potency towards cancer, inflammation, and HIV [21, 26].

To the best of our knowledge no work has been reported on the essential oil chemical composition of H. hemerocallidea, Therefore, this study is aimed at extracting essential oils from both (fresh and dry) parts of Hypoxis hemerocallidea leaves and corms, to determine the chemical profile and then evaluate the medicinal potential of the essential oils. We therefore report the chemical composition of H. hemerocallidea essential oils from this study for the first time.

7.2 Materials and Methods

7.2.1 Plant Material

H. hemerocallidea plants were collected in the fields of Bathurst location near Grahamstown in the Eastern Cape Province. The plant samples were then sent to Rhodes University for identification; they were taxonomically identified by Mr. T. Dold and the voucher specimen was deposited in Selmar Schonland Herbarium Grahamstown (GRA) at Rhodes University; the collection number was PR/PL03.

7.2.2 Extraction of Essential Oils

600 g of fresh or dry (leaves and corms) of H. hemerocallidea were subjected to a hydro-distillation method for approximately 5 h using the Clevenger apparatus [31]. The extracted essential oils were then collected and stored in airtight amber glass bottles in a refrigerator at 4 °C until the time of analysis [32].

7.2.3 Analysis of Essential Oils

GC-FID was performed on a HP5890-II instrument, equipped with a DB-5MS (30 m × 0.25 mm; 0.25 μm film thickness) fused silica capillary column. Hydrogen was used as carrier gas adjusted to a linear velocity of 32 cm/s (measured at 100 °C). Split flow was adjusted to give a 20:1 ratio and septum sweep was a constant 10 mL/min. The oven was programmed as follows: 60–240 °C at 3 °C/min. The samples were injected using the splitless technique using 2 μL of oil in hexane (2:1000). Injector and detector were set at 250 °C. The GC was equipped with FID and connected to an electronic integrator HP 5896 Series II. The percentage yield of the samples was computed from the GC peak areas without using correction for response factors.

GC-MS was performed on a HP-6890 GC system equipped with a HP-5MS fused capillary column (30 m × 0.25 mm; 0.25 μm film thickness), coupled to a selective mass detector HP-5973. Helium (1 mL/min) was used as carrier gas; oven temperature program: 60–240 °C at 3 °C/min; splitless during 1.50 min; sample volume 2 μL of the oil solution in hexane (2:1000). Injector and detector temperature was 250 °C. EIMS: electron energy, 70 eV; ion source temperature and connection parts: 180 °C.

7.2.4 Identification of Essential Oils

Identification of compounds was done by matching their mass spectra and retention indices with those recorded in NIST08 library and by comparison of retention indices and mass spectra with literature values [33,34,35].

7.3 Results

7.3.1 Chemical Composition of Essential Oils

The essential oils extracted from the leaves and corms (fresh and dry) of H. hemerocallidea were pale yellow in color with an unpleasant odor; the percentage yields of the oils are as follows: 2.0% for fresh leaves, 2.2% for dry leaves, 2.8% for fresh corms and 2.5% for dry corms. Constituents identified in the leaves and corms (fresh and dry) of H. hemerocallidea together with their Kovat indices and percentage composition are listed in Table 7.1. A total of 51 components were identified in the fresh leaves GC-MS chromatogram of the essential oil of H. hemerocallidea with 22 components accounting for 97.3%, 27 components accounting for 96.7% in dry leaves, 27 components accounting for 95.8% in fresh corms and 26 components accounting for 93.0% in dry corms. The fresh leaves essential oil had the following main components: sabinene (27.6%), linalool (15.3%), terpinene-4-ol (8.6%), δ-3-carene (6.4%) and trans-β-ocimene (5.3%), while in dry leaves essential oil the main components were β-terpineol (9.2%), β-caryophyllene (11.5%), myrcene (7.5%), terpinen-4-ol (6.6%), γ-terpinene (6.5%), linoleic acid (6.5%), and β-selinene (5.2%). Hexadecane (8.1%), cis-nerolidol (7.7%), β-phellandrene (7.5%), n-hexadecanoic acid (6.9%), trans-β-ocimene (6.4%), octadecane (6.4%), β-bourbonene (6.2%) and α-terpinolene (5.1%) were the major components in the essential oil of fresh corms. In the essential oil of the dry corms, linalool (25.4%), α-terpineol (13.8%), α-terpinolene (9.8%), cis-nerolidol (6.8%) and n-hexadecanoic acid (6.6%) were the major components. The GC-MS analysis of the fresh leaves essential oil also showed that there was the presence of monoterpenes (55.6%), oxygenated monoterpenes (26.5%), sesquiterpenes (9.6%), aromatics (2.7%), esters (1.5%) and alcohols (1.4%), while in the essential oil of dry leaves monoterpenes (29.8%), oxygenated monoterpenes (23.1%), sesquiterpenes (28.7%), aromatics (4.0%), alcohols (0.3%), esters (4.3%) and carboxylic acids (6.5%) were found to be present. The essential oil of fresh corms was composed of monoterpenes (19.0%), sesquiterpenes (6.2%), oxygenated sesquiterpenes (7.7%), saturated hydrocarbons (22.4%), aromatics (28.3%), ketones (2.1%) and carboxylic acids (10.1%) while in the essential oil of dry corms the detected chemical classes of compounds were monoterpenes (10.7%), oxygenated monoterpenes (39.2%), oxygenated sesquiterpenes (6.8%), saturated hydrocarbons (10.4%), aromatics (13.1%), ketones (6.2%) and carboxylic acids (6.6%) as displayed in Table 7.2. Some of the major components identified in the essential oils of H. hemerocallidea are shown in Fig. 7.1.

Table 7.1 Chemical constituents from the different parts of H. hemerocallidea essential oils
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Table 7.2 Chemical classes of compounds identified in the essential oils of H. hemerocallidea
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Fig. 7.1
figure 1

Some of the major components identified in the essential oils of H. hemerocallidea

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7.4 Discussion

7.4.1 Chemical Composition of Essential Oils

The present study is the first report on essential oil composition of H. hemerocallidea. Monoterpenes, oxygenated monoterpenes, sesquiterpenes and oxygenated sesquiterpenes were the dominating groups in the oil profiles of the leaves and corms (i.e. fresh and dry) of H. hemerocallidea contributing in a total of 10.7–55.6%, 23.1–39.2%, 6.2–28.7% and 6.8–7.7% respectively. These figures were largely due to sabinene (0.9–27.6%), myrcene (4.1–7.5%), δ-3-carene (0.5–6.4), trans-isolimonene (1.5–4.6%), limonene (1.1–4.3%), β-phellandrene (1.3–7.5%), trans-β-ocimene (0.7–6.4), γ-terpinene (2.6–6.5%), α-terpinolene (0.4–9.8%), linalool (15.3–25.4%), β-terpineol (2.1–9.2%), terpinen-4-ol (6.6–8.6%), α-terpineol (3.5–13.8%), β-bourbonene (3.0–6.2%), β-caryophyllene (2.2–11.5%), α-caryophyllene (1.8–4.8%), β-selinene (2.4–5.2%), δ-cadinene (3.2–4.2%) and cis-nerolidol (6.8–7.7%). Non-terpinic aliphatic carboxylic acids, saturated hydrocarbons and aromatics accounted for 6.5–10.1%, 10.4–22.4% and 2.7–28.3% respectively, with the main representatives being α-ionone (1.5–5.3%), hexadecane (2.7–8.1%), octadecane (2.0–6.4%), n-hexadecanoic acid (6.6–6.9%) and linoleic acid (3.2–6.5%). Additionally, there were also other chemical groups which were present in the essential oil of H. hemerocallidea such as esters, ketones and alcohols, their percentages being 1.5–4.3, 2.1–6.2 and 0.3–1.4% respectively.

7.5 Conclusion

The findings of this study show that the H. hemerocallidea fresh/dry leaves and corms contain essential oils which varied in yields and chemical composition. The most abundant components identified in the essential oils were sabinene, linalool, α-terpineol and β-caryophyllene. The essential oils also had monoterpenes, monoterpenoids and sesquiterpenes as the most dominant chemical classes of compounds.