3044

11: Auxiliary plants

Sesbania rostrata Bremek. & Oberm.

Ann. Transvaal Mus. 16: 419 (1935).

LEGUMINOSAE - PAPILIONOIDEAE

2n = 12

Sesbania pachycarpa DC. (1825) pro parte, Sesbania hirticalyx Cronquist (1952).

Sesbania (En).

Sesbania rostrata occurs naturally throughout tropical Africa. It is cultivated, mainly on experimental scale, in West Africa and East and South-East Asia.

Sesbania rostrata is used as green manure in wet-rice cultivation. It has also shown potential for incorporation in alley-cropping systems. It is suitable as a fodder for both ruminants and non-ruminants. The leaves are processed into leaf meal. Dry stems are used as fuel e.g. in Madagascar.

The above-ground parts of 50 days old Sesbania rostrata grown in northern India contained per 100 g dry matter: N 2.9 g, P 0.3 g, K 1.6 g, S 0.4 g.

Erect, robust, softly woody, non-aculeate annual or short-lived perennial, 1—3 m tall. Stem pithy, sparsely pilose, glabrescent, with vertical rows of pustules usually evident above the leaf axils and producing warty outgrowths on older stems, submerged portions clothed with matted fibrous roots. Leaves paripinnate, (4.5—)7—25 cm long; stipules linear-lanceolate, 5—10 mm long, reflexed, pilose, very persistent; petiole 3—8 mm long, pilose; rachis up to 19 cm long, sparsely pilose; stipels present at most petiolules; leaflets opposite, in (6—)12—24(—27) pairs, oblong, 0.9—3.5 cm x 2—10 mm, the basal pair usually smaller than the others, apex rounded to obtuse to slightly emarginate, margins entire, glabrous above, usually sparsely pilose on margins and midrib beneath. Inflorescence an axillary raceme, shorter than subtending leaf, 1—6 cm long, (1—)3—12(—15)-flowered; rachis pilose; peduncle 4—15 mm long, pilose; pedicel 4—15(—19) mm long, sparsely pilose; bracts and bracteoles linear-lanceolate, 5—8 mm long, sparsely pilose, caducous; calyx campanulate, 5—7.5 mm x 4—5 mm, sparsely pilose, teeth 1—2 mm long, subulate, sparsely pilose; standard suborbicular, 12—16(—18) mm x 11—14(—15) mm, yellow or orange, speckled dark purple or reddish, apex emarginate, appendages with short, triangular, upward-pointing or slightly incurved, free tips, less than 1 mm long; wings 13—17 mm x 3.5—5 mm, yellow, a small triangular tooth and the upper margin of the basal half of the blade together characteristically inrolled; keel 12—17 mm x 6.5—9 mm, yellow to greenish, basal tooth short, triangular, slightly upward-pointing with small pocket below it on inside of the blade; stamens 10, vexillary stamen free, bent sharply near the base, staminal sheath longer than free parts of filaments, auricled; ovary sparsely pilose on upper margin or glabrous, style glabrous, stigma small. Pod in outline falcate, 15—22 cm x 3.5—5 mm, beak slender, up to 3.5 cm long, thicker at the centre than at the sutures, up to 50-seeded. Seed subcylindrical, 3—3.5 mm x 2.5—3 mm x 2—2.5 mm, brown, greenish or dark reddish-brown; hilum in a small, central, circular pit.

Under favourable conditions, Sesbania rostrata grows very fast, reaching a height of 2 m in 60 days, accumulating 8—11 t above-ground dry matter per ha.
Sesbania rostrata nodulates with three groups of rhizobia. Stem nodules are formed following infection with strains of Azorhizobium caulinodans such as TCSR-1 and ORS-571. This symbiosis is highly specific. Azorhizobium caulinodans differs from Rhizobium and Bradyrhizobium strains in its ability to fix atmospheric nitrogen as a free-living organism and is closely related to Xanthobacter. Azorhizobium caulinodans may infect many Sesbania species, but forms an effective symbiosis almost exclusively with Sesbania rostrata. A second group of rhizobia belongs to Rhizobium and forms root nodules only; it infects and fixes atmospheric nitrogen in symbiosis with many Sesbania species. The third group comprises a few strains of Rhizobium and forms effective stem and root nodules in Sesbania rostrata and only root nodules in several Sesbania species. Information on the ability of Sesbania rostrata to fix atmospheric nitrogen in the presence of soil nitrogen is conflicting. Some studies indicate that nodule numbers and N-fixation rate are only slightly reduced by soil nitrogen and N-fertilizer applications of up to 100 kg/ha. Other studies have found a reduction in the number of stem nodules proportional to the N-fertilizer gift, and no formation of root nodules. Acetylene reduction assays have indicated that the rate of nitrogen fixation of stem nodules was reduced to only 10% of the unfertilized control by an N-fertilizer application of 30 kg/ha, and to even lower levels with higher applications up to 60 kg/ha. Plant height and fresh weight, however, were highest with 30 kg N-fertilizer per ha.
Sesbania rostrata is a quantitative short-day plant, with a critical photoperiod of 12—12.5 hours.

Sesbania rostrata is one of the 3 taxa of Sesbania Adanson that form stem nodules, the others being Sesbania speciosa Taubert from East Africa, cultivated and naturalized in Indonesia and New Guinea and Sesbania sesban (L.) Merrill var. punctata (DC.) J.B. Gillett (synonym: Sesbania punctata DC.) from tropical Africa.

Sesbania rostrata occurs naturally in marshes, floodplains, on muddy river banks and the edges of pools, but has also been recorded in open savanna. It occurs up to 1600 m altitude and tolerates waterlogged soils and flooding to over 1 m deep. In cultivation, Sesbania rostrata is almost always associated with wet rice.

Sesbania rostrata is mainly propagated by seed. Treatment of seed with concentrated sulphuric acid for 30 minutes improved the germination rate to more than 90%. Subsequently, treated seed should be washed with ample water to avoid overheating. Scrubbing seed with sand, or a hot water treatment are much less effective. Seed is broadcast, requiring 40—60 kg/ha. Sesbania rostrata is either planted in rice paddies or on the bunds of rice fields and waste land near rice fields. Vegetative propagation by stem cuttings is possible, as the nodulation sites on the stems consist of adventive root primordia. Using of cuttings instead of seed results in a quick establishment of the crop and may double the N accumulation in a 6-week growth period, or reduce the growth period by 2 weeks.
It is only necessary to apply a solution of an appropriate Rhizobium strain in locations where Sesbania rostrata has not been grown before. Spontaneous inoculation in the field is generally adequate for a high rate of nitrogen fixation. Although Rhizobium strains for stem inoculation are highly specific, they are easily established in the soil, as they can be transferred via the seed-coat. They show a high rate of survival under flooded and dry conditions. Natural infection of stems probably occurs through wind, rain splash and insects.

Grown as a green manure crop, Sesbania rostrata is allowed to grow for 45—65 days depending on its growth rate. When it is left to grow longer than about 55 days, the lignin content increases which decreases the decomposition rate of plant biomass. During the short-day season, it may be left to grow longer as it starts flowering early, resulting in a lower growth rate. The green manure crop is ploughed in just before the rice crop is sown or transplanted. Initial decomposition is rapid, with 30—45% of the leaf material decomposing in 10 days after incorporation. Decomposition then slows down considerably, reaching 50% after 35 days, while the half-life of stems and root-stubble is about 110 days.
When Sesbania rostrata is grown for green manure, applying P and K fertilizers at the rate normally given to rice may increase nitrogen fixation by 30% and improve the availability of N, P and K to the subsequent rice crop.
At the International Rice Research Institute, Los Baños, the Philippines, the average rice grain yield was about 6 t/ha after incorporation of a Sesbania rostrata crop grown for 45—60 days, which is the same as the yield obtained with urea applied at a rate of 50—60 kg N/ha. Under favourable conditions the amount of N accumulated in the green manure crop is about 100 kg/ha in 50 days and 160 kg/ha in 60 days. The residual effect of Sesbania green manure application on soil organic matter and N levels seems limited.
It has been proposed to plant Sesbania rostrata on field bunds. Prunings of these plants would provide cuttings for vegetative propagation, or be a source of readily available green manure. These plants could also be a source of seed.

The most common diseases affecting Sesbania rostrata are damping-off caused by Pythium spp. and Rhizoctonia spp., leaf spot caused by Cercospora spp. and viral leaf mosaic. The root-knot nematode Meloidogyne attacks the root system. In dry conditions of the West African Sahel, nematode attack may be so serious that it is impossible to grow Sesbania rostrata.

Germplasm collections of Sesbania are maintained at the International Rice Research Institute, Los Baños, the Philippines, by the Institut de Recherche Scientifique pour le Développement en Coopération, Office de la Recherche Scientifique et Technique Outre-Mer (ORSTOM), Dakar, Senegal, and by the Commonwealth Scientific and Industrial Research Organization (CSIRO), Australia. A small number of accessions is maintained at the Southern Regional Plant Introduction Station, Griffin, Georgia, the United States. No breeding programme is known to exist.

Stem-nodulating legumes such as Sesbania rostrata have a high potential as green manure in wet-rice production systems, because of their very fast growth and high rate of atmospheric nitrogen fixation, especially under wet conditions. Its tolerance of flooding gives Sesbania rostrata a competitive advantage over most other legumes. Selection of cultivars resistant to diseases is urgently needed.

Alazard, D., Ndoye, I. & Dreyfus, B., 1988. Sesbania rostrata and other stem-nodulated legumes. In: Both, H., de Bruijn, F.J. & Newton, W.E. (Editors): Nitrogen fixation: hundred years after. Gustav Fisher, Stuttgart, Germany. pp. 765-769.
Becker, M., Diekmann, K.H., Ladha, J.K., De Datta, S.K. & Ottow, J.C.G., 1991. Effect of NPK on growth and nitrogen fixation of Sesbania rostrata as a green manure for lowland rice (Oryza sativa L.). Plant and Soil 132: 149-158.
Becker, M., Ladha, J.K. & Ottow, J.C.G., 1988. Stem-nodulating legumes as green manure for lowland rice. Philippine Journal of Crop Science 13: 121-127.
Buresh, R.J., Garrity, D.P., Castillo, E.G. & Chua, T.T., 1993. Fallow and Sesbania effects on response of transplanted rice to urea. Agronomy Journal 85: 801-808.
Joshua, D.C., Saradha Ramani, Suarez, E. & Shaikh, M.S., 1992. Growth, stem nodulation, and nitrogen content of Sesbania rostrata plants treated with different rates of urea. Journal of Plant Nutrition 15: 1353-1358.
Lewis, G.P., 1989. Sesbania Adans. in the Flora Zambesiaca region. Kirkia 13: 32-34.
Pandey, R.K., 1991. A primer on organic-based rice farming. International Rice Research Institute, Manila, the Philippines. 201 pp.
Ventura, W. & Watanabe, I., 1993. Green manure production of Azolla microphylla and Sesbania rostrata and their long-term effects on rice yields and soil fertility. Biology and Fertility of Soils 15: 241-248.

I.B. Ipor

Ipor, I.B., 1997. Sesbania rostrata Bremek. & Oberm.. 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