Aeschynomene afraspera (PROSEA)

Plant Resources of South-East Asia Introduction

List of species

Aeschynomene afraspera J. Léonard

Protologue: Bull. Jard. Bot. Etat Brux. 24: 64 (1954).

Family: Leguminosae - Papilionoideae

Chromosome number: 2n= 80

Synonyms

• Aeschynomene aspera auct., non L.,
• Sesbania leptocarpa auct., non DC.

Vernacular names

• Sola pith (En)
• Thailand: sano (refers to related species as well).

Origin and geographic distribution

A. afraspera is believed to have originated in sub-Saharan Africa between Senegal and Sudan. It is widely distributed in the lowlands of western, central, north-eastern and southern Africa. In 1986 it was introduced into the Philippines and since then has been grown experimentally across South and South-East Asia.

Uses

The potential use of A. afraspera as a fast-growing nitrogen source for wet-rice fields has only recently been noted. Since the late 1980s it has been widely used as a pre-rice green manure crop on experimental stations and in extension demonstration trials. So far it is only occasionally used by farmers in South and South-East Asia.

In its region of origin A. afraspera is grazed by ruminants. In Zambia it is valued as a palatable forage legume. In Senegal the pith of the stems is used as insulation material and medicinally it is applied to stop bleeding.

Properties

The fresh biomass of young A. afraspera contains 10-20% dry matter and has a C:N ratio of 10-16. Per 100 g dry material the above-ground parts contain: N 2.5-3.7 g, P 0.28-0.55 g, K 1.3-2.3 g, lignin 9-13 g. As a green manure A. afraspera mineralizes rapidly. The weight of 1000 seeds is 21-29 g.

Description

• Erect to suberect, branching, herbaceous, annual shrub, 1-3 m tall. Root and stem hollow or pith-filled. Stem glabrous, soft, with abundant, spirally arranged, white or pale green adventitious root primordia, which may develop into hemispherical, green nodules.
• Leaves alternate, composite, stipulate, sensitive; petiole and rachis 3-18 cm long, 1.5-3 cm in short, axillary branchlets; leaflets (20-)50-100, linear-oblong, 8-20 mm × 1.5-3 mm, 4-7 mm long in leaves on axillary branches, entire or finely denticulate, glabrous.
• Inflorescence an axillary raceme, 2-6 cm long with 1-6 flowers; bracts glabrous, 3-8 mm × 2-5 mm.
• Pedicel 5-9 mm long (in fruit up to 12 mm), pubescent; calyx bilabiate, glabrous or slightly pubescent on the outside, 6-8 mm long and 4-4.5 mm wide; corolla pale to bright yellow; standard elliptical to obovate, 9-12 mm × 7-10 mm; wings free, 7 mm × 2 mm; keel petals pubescent, about elliptical, loosely adnate, 9-11 mm × 3-4 mm.
• Pod 5-8 cm × 7-8 mm, venose when young, very warty and dark brown to black when mature, with 6-10 1-seeded segments.

Growth and development

Initial growth until 5-leaf stage is slow. With the onset of stem nodulation and/or closure of the canopy A. afraspera grows rapidly, reaching 0.6-1.5 m in 2 months. Plants growing in isolation are sub-erect to spreading, with abundant branching. In a dense stand, plants grow erect with a single stem. Under flooded conditions, a shallow taproot with abundant intervascular aerenchyma develops and root primordia grow into adventitious roots.

In the Philippines A. afraspera starts flowering 65 days after sowing during the short-day season, and after 80 days when daylength exceeds 12 hours. With prolonged soil flooding, the otherwise short flowering period can extend to over 2 months. Fruit ripening causes drying and brown discolouration of leaves and stems, ending the growth cycle.

The most distinctive characteristic of A. afraspera is the presence of nitrogen-fixing nodules, not only on the roots but also on predetermined, sub-epidermal primordia of adventitious roots on stems and branches. Upon infection with rhizobia via rain splash or insect activity the root primordia can develop into nitrogen-fixing nodules. Since root nodules are scarce under anaerobic conditions in flooded soils, A. afraspera has to rely on stem nodules to fix atmospheric nitrogen. Root primordia on stems become visible in 2-week-old plants, and profuse stem nodulation is apparent 3-5 weeks after germination. Up to 400 nodules can be found on the stem of a 2-month-old plant and 70-80% of the nitrogen in the biomass is reportedly derived from biological nitrogen fixation, indicating the high efficiency of the symbiosis in stem nodules. Though the roots of A. afraspera are nodulated by several rhizobium strains, the rhizobia nodulating both roots and stems seem to be highly host-specific. Strain ORS 322, isolated from a stem nodule of A. afraspera at the Office de la Recherche Scientifique et Technique d'Outre-Mer (ORSTOM) in Senegal, effectively nodulates only A. afraspera and A. nilotica Taub. It is believed to belong to the genus Bradyrhizobium, but also seems closely related to purple photosynthetic bacteria (Rhodospirilliaceae) as it forms bacteriochlorophyll "a" and is capable of photosynthesis. The new generic name Photorhizobium has been proposed. Preliminary observations indicate that the nitrogen-fixation rate of stem nodules is less reduced by available soil nitrogen than is the case in root nodules.

Other botanical information

In the past A. afraspera (strictly African) has been confused with A. aspera L. from tropical Asia. A. aspera is often glandular-pubescent, the calyx is 7-10 mm × 5-6 mm, the standard 10-16 mm × 8-15 mm, the wings 7-12 mm × 4-5 mm and the fruit segments bear spiny warts. At least 18 species of the genus Aeschynomene L. have been shown to produce stem nodules, including the Asian species A. indica L. In most species stem nodulation is less profuse than in A. afraspera.

Ecology

A. afraspera is found from 0-900 m altitude in tropical areas with a distinct dry season and a monomodal rainfall distribution. It is a semi-aquatic pioneer plant of marshes and temporarily wet places. Seeds require high soil moisture or flooded conditions for germination, but more than 2 cm of standing water prevents seedling growth. It can form dense stands in soil depressions that are waterlogged during the rainy season, and in coastal freshwater lakes and rivers. It occasionally appears as a weed in rice fields. Provided with sufficient plant-available phosphorus (at least 10 ppm Olsen P), A. afraspera will grow in a wide range of soils, from pure dune sands along rivers to peat soils in mangrove swamps. Soil reaction can range from alkaline in salt flats to highly acidic in acid sulphate soils.

Propagation and planting

Dormancy and an extremely hard seedcoat prevent easy germination. For agronomic use, seeds need to be mechanically scarified or immersed for 30-60 minutes in concentrated sulphuric acid. Vegetative propagation is possible using stem cuttings with root primordia. Cuttings 15-20 cm long from the basal stem show the highest survival rate and best growth. Vegetative propagation may not be economic for green manure purposes, and is mostly used to establish seed production plants, e.g. along the bunds of wet-rice fields.

Planting can be done throughout the year: e.g. at the International Rice Research Institute, Los Baños, the Philippines, planting date had little effect on yield and a limited effect only on the rate of atmospheric nitrogen fixation.

Husbandry

The ability of A. afraspera to form above-ground nodules and to fix nitrogen in waterlogged and marginal soils largely determines its value as a green manure in wet rice. Due to its soft structure, A. afraspera green manure is easily incorporated into the soil and mineralizes rapidly even under flooded conditions. After 6-8 weeks of growth it is ploughed in and rice is transplanted 1-7 days later. In eastern India it is sometimes sown as an intercrop between rows of rice and trampled into the soil before it starts shading the rice. Relay planting of A. afraspera has been used successfully to exploit the short fallow period between two rice crops in multiple cropping systems.

Diseases and pests

Few diseases and pests are reported. This may be related to the limited use of A. afraspera in agriculture so far. A bacterial wilt is reported to affect biomass production in some areas. The leaf-eating larvae of the Lepidopterous species Eurema lecabe can become a problem when A. afraspera is grown in the short-day season. Stunted growth of upland A. afraspera may in some cases be associated with cyst-forming nematodes. However, under favourable wet conditions, A. afraspera seems to largely outgrow disease and pest-related damage. When grown for longer than 75 days A. afraspera can effectively control the rice root nematode Hirschmanniella oryzae.

Yield

An 8-week-old crop grown in a pure stand can accumulate a dry biomass of 4-6 t/ha with a corresponding N yield of 80-200 kg/ha, provided sufficient water and soil P are available; 70% or more of this may be the result of biological nitrogen-fixation. As an intercrop, 35-60 kg N/ha can be accumulated. A rice crop following A. afraspera may recover 90% of the nitrogen contained in the green manure, compared with a recovery rate of 60% from urea. Reported increases in rice grain yield due to the incorporation of a 6-8-week-old A. afraspera green manure range from 0.8-3.2 t/ha.

Genetic resources and breeding

The biofertilizer germplasm collection at the International Rice Research Institute, Los Baños, the Philippines, maintains 45 Aeschynomene species, including 3 accessions of A. afraspera. Other collections of Aeschynomene are maintained at the Office de la Recherche Scientifique et Technique d'Outre-Mer in Dakar (Senegal), and at Tamil Nadu Agricultural University (TNAU) in Coimbatore, India. No breeding programmes are known to exist.

Prospects

Flood tolerance, rapid growth, high nitrogen-fixing activity and rapid decomposition and mineralization give A. afraspera a high potential as a short-duration green manure in wet-rice systems. Growing concern for agricultural sustainability coupled with the rising cost of mineral N fertilizer mean that it may become more important in South and South-East Asia. Its high nitrogen content and good palatability make it a promising forage crop as well.

Literature

• Alazard, D., 1985. Stem and root nodulation in Aeschynomene spp. Applied and Environmental Microbiology 50: 732-734.
• Alazard, D. & Becker, M., 1987. Aeschynomene as green manure for rice. Plant and Soil 101: 141-143.
• Becker, M., 1990. Potential use of the stem-nodulating legumes Sesbania rostrata and Aeschynomene afraspera as green manure for lowland rice (Oryza sativa L.). PhD Thesis, Justus Liebig University, Giessen, Germany. 113 pp.
• Berhaut, J., 1979. Flore illustrée du Sénégal [Illustrated flora of Senegal], Vol. 5, Editions Clairafrique, Dakar, Senegal. pp. 26-27.
• Ladha, J.K., Pareek, R.P. & Becker, M., 1992. Stem nodulating legume-Rhizobium symbiosis and its use in lowland rice. Advances in Soil Science 20: 147-192.
• Watanabe, I., Roger, P.A., Ladha, J.K. & Van Hove, C., 1992. Biofertilizer germplasm collection at IRRI. The International Rice Research Institute, Manila, the Philippines. 66 pp.

Authors

M. Becker