Prota 2: Vegetables/Légumes
Mem. New York Bot. Garden 35: 252 (1982).
Caesalpiniaceae (Leguminosae - Caesalpinioideae)
n = 13, 14, 2n = 24
Cassia obtusifolia L. (1753), Cassia tora auct. non L.
Sicklepod, African foetid cassia, low senna (En). Séné, pistache marron, casse fétide (Fr).
Origin and geographic distribution
Senna obtusifolia is found throughout tropical Africa with the exception of Madagascar. It is considered an early introduction into Africa from America, where it shows far more variation. In Africa the fruits are broad as in specimens from the Caribbean and southern United States, which suggests a Caribbean origin of the African plants. In Asia plants with broad fruits are widespread, but in the Philippines only plants with needle-like fruits occur. Senna obtusifolia is considered a weed worldwide and an estimated 600,000 ha are infested in Queensland (Australia).
The young, tender leaves of Senna obtusifolia are occasionally used as a vegetable throughout Africa and elsewhere and the plant is cultivated in home gardens for this purpose in several countries including Senegal, Ghana, Cameroon and Ethiopia. Older leaves, if eaten frequently or in large quantities, will cause diarrhoea.
Cattle, sheep, goats and ostriches browse the plants, but the growth stage of the plants may influence acceptability. In both Africa and the United States, mycotoxicosis is an often fatal disease of cattle grazing Senna obtusifolia and other Senna species.
The leaves are used as a laxative and as a poultice to treat skin infections, sores, ulcers and insect bites. The leaves are further used as an anthelminthic and against vomiting and stomach-ache. A decoction of the leaves is used to treat eye complaints in Senegal and Zanzibar. In DR Congo a fish poison is made from the crushed leaves. The roots are used as a laxative and anthelminthic. The seeds are eaten, combined with a leaf decoction, to treat conjunctivitis.
Roasted seeds have been used as a substitute for coffee, leaves for a tea-like infusion. The seeds, the macerated leaves and the roots provide black, blue, yellow and orange dyes. In Sudan the powdered and fermented leaves are used as a condiment. The stems are used to make mats and fences.
In Uganda the seeds are occasionally dried and ground into powder, which is cooked and eaten as a staple food in moderate amounts. The seeds have been eaten in times of famine in the Sahel region as well. As the seeds are reputedly poisonous, cooking or roasting is deemed necessary to make them safe to eat. The flowers are decorative and the plant is commonly planted as an ornamental near towns. In India the seeds are collected from the wild for the industrial extraction of gums (galactomannans) for the food industry.
Fresh leaves of Senna obtusifolia contain per 100 g edible portion: water 79.7 g, energy 251 kJ (60 kcal), protein 5.6 g, fat 0.2 g, carbohydrate 12.5 g, fibre 2.3 g, Ca 589 mg, P 96 mg, Fe 5.9 mg, β-carotene 7.9 mg, thiamin 0.23 mg, riboflavin 0.71 mg, niacin 1.5 mg, ascorbic acid 113 mg (Leung, W.-T.W., Busson, F. & Jardin, C., 1968). The cooked vegetable tastes bitter but has an attractive consistency. The seeds contain 5.3% oil, the major components being linoleic acid 41%, palmitic acid 23% and oleic acid 22%. The seeds contain commercially interesting levels of gums.
The laxative properties of Senna species are attributed to anthroquinones. A yellow phenolic pigment, cassiaxanthone, has been isolated from the roots of Senna species. Mycotoxins, produced by fungi affecting Senna, may be the cause of death in cattle. Myrothecium verrucaria, a fungus isolated from Senna obtusifolia, is used for control of nematodes in food crops and ornamental plants and is widely tested as a herbicide against weeds like water hyacinth (Eichhornia crassipes (Mart.) Solms), Chenopodium album L. and Senna obtusifolia itself. Strains of Fusarium oxysporum and Alternaria cassiae were similarly obtained and showed considerable control of Senna species if applied at the pre-emergence stage.
Annual or perennial herb or shrub up to 2(–2.5) m tall. Leaves alternate, imparrinnate with 3 pairs of leaflets; stipules linear or filiform; petiole without gland, rachis with prominent gland between 1–2 lower pairs of leaflets; leaflets obovate, (1–)1.5–5(–6) cm long, apex rounded or abruptly acuminate, mucronate. Inflorescence 1–2-flowered, with usually very short peduncle. Flowers bisexual, 5-merous; pedicel 1.5–3.5(–4.5) cm long; sepals ovate, c. 5 mm long; petals obovate, 1–2 cm long, yellow; stamens 10, lower 3 largest, 4 somewhat smaller and 3 very small, reduced; ovary superior, linear, curved. Fruit a linear, dehiscent pod up to 23 cm × 0.5 cm, straight or curved, many-seeded. Seeds rhomboid, c. 5 mm long, with a distinct areole. Seedling with epigeal germination; cotyledons semi-fleshy.
Until the beginning of the 1980s Cassia was considered to be a large genus of over 550 species, but then it was split into 3 genera: Cassia sensu stricto, Senna and Chamaecrista. Cassia now has only 30 species, whereas Senna and Chamaecrista comprise about 260 and 270 species, respectively. Senna obtusifolia is closely related to Senna tora L., but the latter can be recognized by its shorter pedicels. The distinction between Senna obtusifolia, Senna occidentalis (L.) Link and Senna tora is not always properly made; names have often been misapplied and vernacular names may apply to all of them. The presence of Senna tora in Africa is doubtful and references to it probably concern Senna obtusifolia.
Senna obtusifolia is a short-day plant, but exact light requirements for flower initiation differ by provenance. It is self-pollinating and interspecific crosses have not yielded viable seed.
Senna obtusifolia is found along rivers and on lake shores, as well as on cultivated land, up to 1700 m altitude.
Details on cultivation of Senna obtusifolia in Africa are unknown. In India seeds for gum production are presently harvested from the wild. As far as is known, Senna obtusifolia is only grown commercially in Korea for medicinal uses, with seed yields as high as 2.6 t/ha. Fertilizer applications of 80 kg N, 30 kg P and 50 kg K were optimal for seed production. Several fungi affect Senna obtusifolia and it is an alternative host of Alternaria cassiae, which affects e.g. cowpea (Vigna unguiculata (L.) Walp.) and several Solanum species.
Genetic resources and breeding
While the wider genetic variation of Senna obtusifolia is found in the New World, there is obvious scope for selection for vegetable use even within the African populations. Farmers already select plants that taste less bitter, are less fibrous and have easy -to-pick leaves. Australia may start a breeding programme based on seed stock collected from all over the area of distribution in the near future. Breeding would aim at high seed and gum yield, good gum quality and adaptation to mechanized cultivation.
Senna obtusifolia, like many other Senna species, is a true multipurpose plant. A proper understanding of the variation will be essential for future developments. Senna obtusifolia will probably remain a minor vegetable. Seed gums are used worldwide for a variety of industrial applications. Increased demand and inconsistency of supply and price has driven industrial users to search for new sources of supply and Senna obtusifolia is a good alternative for locust bean (Ceratonia siliqua L.) and guar (Cyamopsis tetragonoloba (L.) Taub.). A patent restricting the use of Senna obtusifolia gum lapsed in 2002. The medicinal properties also seem to justify more research. However, the weedy nature and the toxic properties require caution.
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Correct citation of this article:
Bosch, C.H., 2004. Senna obtusifolia (L.) Irwin & Barneby In: Grubben, G.J.H. & Denton, O.A. (Editors). PROTA 2: Vegetables/Légumes. [CD-Rom]. PROTA, Wageningen, Netherlands.