3041

11: Auxiliary plants

Senna didymobotrya (Fresenius) Irwin & Barneby

Mem. New York Bot. Garden 35: 467 (1982).

LEGUMINOSAE - CAESALPINIOIDEAE

2n = 28

Cassia didymobotrya Fresenius (1839), Cassia verdickii de Wildeman (1900), Cassia nairob(i)ensis L.H. Bailey (1941).

Candelabra tree, wild senna (En). African wild sensitive plant (Am).

Senna didymobotrya is a native of tropical East and Central Africa, from Ethiopia and Sudan to Angola and Mozambique. It was introduced into tropical Asia and America as a green manure and cover crop, and later as an ornamental. It sporadically naturalized in frostless regions, including Malesia. It is now grown throughout the world as an ornamental.

Senna didymobotrya was introduced as a cover crop and green manure in India, Sri Lanka, Peninsular Malaysia and Java and has been used as a shade tree in tea plantations. In sites where Erythrina spp. do not grow well, Senna didymobotrya may be a valuable substitute. It is now also popular as an ornamental plant owing to its bright yellow flowers and black-green bracts.
In Africa, Senna didymobotrya is commonly used as a stupefacient poison for fishing and as ornamental plant. Medicinally, it is widely used as a purgative and an anti-malaria medicine. A decoction of the leaves is used against stomach complaints.

The aboveground biomass of Senna didymobotrya grown as ground cover in Sri Lanka was found to contain 0.7 g N per 100 g fresh material. Leaves and roots contain a number of anthraquinones, choline, and the trisaccharide raffinose. In vitro cultures of Senna didymobotrya produced chemical compounds that can be converted into low-energy sweeteners and insecticides. When in flower or bruised, the plant emits an unpleasant smell said to be very reminiscent of mice.

Usually a several-stemmed shrub or small tree, 0.5—5(—9) m tall. Branches terete, striate, pubescent to villous, rarely subglabrous. Leaves simply paripinnate, narrowly oblong-elliptical in outline, 10—50 cm long; stipules broadly ovate-cordate, 6—17 mm x 8—10 mm, acuminate, palmately veined, reflexed, tardily caducous; petiole terete, 1—8 cm long, rachis up to 40 cm long, both pubescent and eglandular; petiolules up to 3 mm long; leaflets in 8—18 pairs, chartaceous, elliptical-oblong, 2—6.5 cm x 0.5—2.5 cm, 2—3 times longer than wide, base oblique, apex rounded but mucronate, pubescent to glabrescent, marginal veins distinct. Inflorescence an erect, axillary, 20—30-flowered, spike-like raceme, 10—50 cm long; peduncle terete, 5—8 cm long, pubescent; bracts broadly ovate, 8—27 mm x 5—15 mm, black-green, at first imbricate and enclosing the flower buds; bracteoles absent; pedicel slender, 3—10 mm long, densely pubescent; sepals 5, subequal, oblong-obovate, 9—14 mm long, puberulous, green; petals 5, slightly unequal, at first incurved, later on more spreading, ovate to obovate, 17—27 mm x 10—16 mm, with a slender, about 1 mm long claw, glabrous, bright yellow, delicately veined; stamens 10, filaments shorter than anthers, anthers of 2 lower stamens 9—11 mm long, 3 upper stamens staminodial, anthers of 5 median stamens about 5 mm long; ovary and stipe velvety pubescent; style slender, glabrous, recurved, about 1 cm long; stigma punctiform. Fruit a flat, 9—16-seeded pod, linear-oblong, 7—12 cm x 1.5—2.5 cm, glabrescent, short beaked, dehiscent or indehiscent when dry, depressed between the seeds, sutures raised, blackish-brown. Seed flattened, oblongoid, apiculate, 8—9 mm x 4—5 mm x 2.5 mm, smooth, pale brown; areole elliptical, 3—4 mm x 0.7—1.5 mm.
Juvenile stems tend to be somewhat tender and should be staked. When growth is very rapid, plants are apt to become straggly. Senna didymobotrya withstands lopping well. It flowers profusely twice a year; in temperate regions it flowers throughout the summer. The bracts, stipules and indumentum of Senna didymobotrya are quite variable. In the axil of the leaves an abortive inflorescence is often present.
In the older literature, this species is best known as Cassia didymobotrya. Until the beginning of the 1980s, Cassia L. was considered to be a genus with over 500 species. The large genus Cassia L. emend. Gaertner has now been subdivided into 3 genera: Cassia (trees, some filaments curved, bracteoles present, no areoles on seed), Senna Miller (herbs, shrubs or trees, all filaments straight, bracteoles absent, areoles on seed) and Chamaecrista Moench (herbs or shrubs, all filaments straight, bracteoles present, no areoles on seed). Cassia now has about 30 species, Senna and Chamaecrista comprise about equal numbers of species.

In its natural habitat Senna didymobotrya is often ruderal in riparian montane wooded grassland or evergreen bushland, at 900—2400 m altitude. It tolerates light frost.

Senna didymobotrya is easily propagated by seed; cuttings are said not to be successful. The seed may be sown in the nursery or directly in the field. When planted as a small shade tree in tea it is spaced at about 5 m x 5 m.
The plants can be lopped several times per year to provide green manure. Lopping is preferably done when the plants are in flower, when the nutrient content in the leaves is high. The plant yields a fairly large amount of loppings. About 5 t of green material provides 35.5 kg nitrogen. In temperate areas, potted ornamental plants are overwintered in greenhouses.
Senna didymobotrya is hardy and quite free from diseases and pests.

Senna didymobotrya used to be a ground cover and green manure crop, appreciated mainly as an alternative plant in locations where Erythrina spp. did not flourish. It has now been largely replaced as a green manure crop by species such as Tephrosia candida Roxb. ex DC., Tephrosia purpurea (L.) Pers. and Tephrosia vogelii Hooker f. Its potential as an ornamental pot plant is being developed.

Botta, B. & delle Monache, G., 1993. Cassia didymobotrya (wild senna): in vitro culture, biotransformation and the production of secondary metabolites. In: Bajaj, Y.P.S. (Editor): Biotechnology in agriculture and forestry. Vol. 21, 4. Medicinal and aromatic plants. Springer, Berlin, Germany. pp. 64-86.
de Wit, H.C.D., 1955. A revision of the genus Cassia in Malaysia. Webbia 11: 241-242.
Holland, T.H., 1931. Alternative green manure plants. Tropical Agriculturist 76: 135-136.
Irwin, H.S. & Barneby, R.C., 1982. The American Cassiinae. A synoptical revision of Leguminosae tribe Cassieae subtribe Cassiinae in the New World. Memoirs of the New York Botanical Garden 35(2): 467-468.
Steyaert, R., 1952. Cassieae. Flore du Congo belge et du Ruanda-Urundi. Vol. 3. Institut National pour l'Étude Agronomique du Congo Belge (INÉAC), Brussels, Belgium. pp. 504-506.
Wilkinson, C.H., 1937. Ground cover on tea estates in Dimbula. Tea Quarterly 10: 206-209.

B. Sunarno

Sunarno, B., 1997. Senna didymobotrya (Fresenius) Irwin & Barneby. In: Faridah Hanum, I & van der Maesen, L.J.G. (Editors): Plant Resources of South-East Asia No 11: Auxiliary plants. PROSEA Foundation, Bogor, Indonesia. Database record: prota4u.org/prosea