COLLECTING SESBANIA SPP. GERMPLASM IN THE VICINITY OF NAIROBI, KENYA

by

Ndungu, J.N; Llewellyn-Smith, R. & Boland, D.J.
International Centre for Research in Agroforestry (ICRAF)
PO Box 30677, Nairobi
Kenya

INTRODUCTION

Sesbanias are nitrogen-fixing legumes and are used in sub-Saharan Africa for soil improvement purposes, stakes, fodder and fuel wood. Sesbania sesban has shown particular promise for improved fallows (Anon 1992) in the Miombo region of Southern Africa where its soil improvement potential in local farming systems is widely recognised. Most sesbania species are small plants of 1–2 years longevity although a few, such as S. sesban, are woody perennials.

The International Centre for Research in Agroforestry (ICRAF) has identified S. sesban as one of the priority species for research and improvement and has undertaken germplasm exploration in Western Kenya and Southern Africa (Ndungu and Boland 1994). These collections were additional and complementary to those made earlier by the International Livestock Centre for Africa (ILCA). These latter collections were conducted mainly in Tanzania which is considered a centre of diversity for the genus Sesbania in Africa.

In parallel with the above exploration, collections were made of two endemic perennial sesbania taxa viz. S. sesban var. nubica and its close relative, S. keniensis, found within the city of Nairobi. Populations of both species are under threat from urban expansion.

The collections afforded an excellent opportunity to consider pertinent germplasm collection issues such as (a) the need for systematic inclusion of urban areas in exploration strategies, and (b) development of strategies for the maintenance of active, base and core ex situ germplasm collections of Sesbania spp.

Terms commonly used in ex situ conservation are defined in Annex 1.

WHY MAKE COLLECTIONS OF SESBANIAS IN AN URBAN ENVIRONMENT ?

Historical and ecological background

In 1899, Nairobi was deemed suitable for the setting up of a base camp and descending point into the escarpment of the Rift Valley, for workers on the Mombasa/Port Florence (Kisumu) railway line. At that time, Nairobi was apparently a mosaic of open grassland, montane closed forest and moist woodland swampy areas. Almost a century later, the City of Nairobi, built around the Nairobi river, has developed into a sprawling urban area. The population of Nairobi, which in 1963 was around 300,000 is rapidly increasing, and will, it is estimated be close to 3 million people by the turn of the century. The city covers an area of approx. 9,300 ha (about 23,000 acres), its streets are tree-lined, and gardens planted mainly with introduced exotic ornamental trees and shrubs.

The natural vegetation in the city area has thus been greatly modified. In many areas it is now difficult to determine by visual analysis, what the native vegetation was like originally. However, small and progressively shrinking pockets of indigenous vegetation still remain undisturbed in parts of Karura, Kiambu, the City Park and the Ngong Forests. They include woodland, forest and grassland species.

Sesbanias in the city are usually found along streams in the woodland and grassland areas. They are usually not found in closed forest areas but can occur occasionally away from streams in woodlands and forests within the city. Because sesbanias are pioneer species they are also found growing in disturbed areas around the city e.g. new building and demolition sites.

Rationale for the collections

The following two issues prompted germplasm collections of sesbanias in Nairobi:

1. Conservation of native flora
The Nairobi area has a unique habitat, dissected by numerous rivers and streams which ultimately flow into the Athi River. It has a great diversity of local ecological conditions of altitude, rainfall and soil types within a relatively small area. In recent times the habitat in Nairobi has changed dramatically with increased urban pressure. The riverine areas typically inhabited by sesbanias are endangered by urban development, and peri-urban agriculture, horticulture and dairy farming. The river systems around the city have been altered in recent times, and many have diminished streamflow or have had their courses diverted. Some have even gone underground for part of their course: some seasonal streams have disappeared. It is common for declared green belt areas (even stream banks) to be suddenly resumed for development. For all these reasons a strong case exists for ex situ conservation (rather than in situ) of the wild native sesbania populations around and within Nairobi, as part of an overall strategy for the conservation of genetic resources of Sesbania species.

2. Type population issues
In botanical terminology, “the type is that element of a tax on to which the name of the tax on is permanently attached” (Davis and Heywood 1993). The principal type is the holotype which is that single specimen chosen by the author to best represent the species. The holotype must be from one, individual plant. Duplicates of the holotype are called isotypes.

The holotype of S. keniensis was collected by Rayner in 1949 from a population now enclosed within the city (collection no. 210). The holotype is lodged in Kew, U.K. and the isotype in the East Africa Museum, Nairobi. The exact location recorded is “the French mission, Nairobi, below Loreto Convent on the banks of the Nairobi river”. The French Mission is now referred to as St. Mary's School. This is a rather precise location and our preliminary observations revealed that only a few trees now remain at this location.

General interest in S. keniensis results from its apparent close taxonomic affinity with the commonly used S. sesban. The two species resemble each other but are usually separable using three key features viz. S. keniensis has fewer flowers per inflorescence (2–3), has longer staminal tubes (18–23mm long) and has an almost glabrous appearance. In his original description of the species Gillett (1963) notes: “S. keniensis has in the past been much confused with S. sesban which grows alongside it”. However, Gillett believed that S.keniensis was more closely related to S. goetzii, which is one of three perennial native Sesbania species in Africa, (S. keniensis and S. sesban being the other two).

THE COLLECTIONS

Nairobi is transversed by numerous small streams leading into the Athi River to the southeast of the city limits. The sampling strategy adopted was based on river systems. Somewhat surprisingly, each minor stream usually had only either one of the two species present along the lengths of the streams searched. Each population was usually small and consisted of 5–20 individuals. S. sesban was also found on ridges away from the streams but S. keniensis never so. S. sesban was common on several disturbed non-river sites around the city.

S. keniensis is now endangered at the site of the holotype. A special collection at this site was made around Loreto Convent Msongari and St. Mary's Boys School. Only ten individuals were found along the Nairobi river up to 1 km downstream. About 0.5 km upstream from St Mary's we found a small stand of nine trees of the two species targeted for collection in mixture around a small swampy area. No obvious hybrids were observed although controlled crosses of the species have been successfully made (Heering and Hanson 1993).

The Nairobi collections yielded eight seedlots (accessions) of the two taxa, Sesbania sesban var. nubica and Sesbania keniensis. The details of the collections are summarized in Table 1 with latitudes and longitudes determined by a global positioning system (GPS) receiver.

Most stands of sesbania found in the city were small. Single tree collections were made, the possibility of later bulking seed on a per urban catchment basis being left open. All seed was divided and stored in either base or active collections. Base collections are stored (at -20°C) at the National Genebank of Kenya, and active collections are stored (at 5°C) at the Kenya Tree Seed Centre (KEFRI). Careful documentation of all the seedlots was done using the standard germplasm collection form of the Multipurpose Tree Germplasm Resource Unit of ICRAF.

Table 1. Collection details for Sesbania in the urban environment of Nairobi

¹ Figures in brackets represent number of Sesbania keniensis trees extant at each location.

DISCUSSION

The movement of plant germplasm worldwide, often carried out in uncontrolled ways and without adequate documentation, and its introduction into new habitats (like areas in and around Nairobi), is a major threat to local genepools, and raises the need for collecting germplasm of native species in rapidly developing urban areas. In the present case, it is possible that native Sesbania species may have hybridised with introduced sesbanias, with a loss of their genetic integrity. An serious concern in the present situation was that significant amounts of inbreeding and genetic drift may have occurred through depletion of the original Sesbania stands, resulting in serious genetic erosion.

It is clear that populations of the two native sesbanias viz. S. sesban var. nubica and S. keniensis, in the Nairobi city area are endangered, although losses of such populations are not concidered to endanger the species as a whole. Loss of the type population of S. keniensis is more serious in that it will restrict the efficacy of future research into the exact taxonomic identity of the species and its relationship with its close relatives viz. S. sesban and S. goetzii. Under these circumstances ex situ conservation collections are justified for both S. keniensis and S. sesban. Collections of both species from the type area will provide a good germplasm source for future taxonomic studies and possible use.

S. sesban var. nubica is endemic to the city area. Although, in theory, seed could have been brought into the city in soil and other building materials (river sand for concrete), this is considered unlikely in those sites collected from. Abundant regeneration was noticed in recently bulldozed building sites suggesting possible (accidental) scarification of soil-stored seed in connection with building.

Sesbanias seem to be of little use to city residents although S. sesban is widely used on farms in Western Kenya and many city residents from the country are familiar with the species. In the city sesbanias are cleared to make room for food and fodder crops, and for building and road construction. The use of sesbanias for firewood and in soil improvement, was not noted anywhere in the city.

After adequate evaluation and characterization of the seedlots, the ICRAF Multipurpose Tree Germplasm Resource Unit (MPT-GR) will maintain core collections of S. sesban var. nubica and S. keniensis. To the knowledge of the authors, genetic material from type populations has not earlier been included in core collections of woody species. Inclusion seems appropriate as populations of the species in question still exist at the locality of the original holotype. The importance of inclusion of genetic material from these in the core collection will provide an exact comparative base by which subsequent evolution and variation within the species can be measured. Accordingly, it is recommended that the type population of S. keniensis should be the first accession of the species to be included in a core collection for that species.

ACKNOWLEDGEMENTS

This collection, mission identifier GBK 05/93, was undertaken in collaboration with the Kenya Forestry Research Institute (KEFRI) Tree Seed Centre, and the National Genebank of Kenya (KARI). We wish to acknowledge their support and participation. We thank our Nairobi-based colleagues F. Owino, E. Torquebiau and L. Guarino for useful comments on early drafts of the manuscript. Ms A. Thomsen, FAO also ably helped review an early draft.

LITERATURE CITED

Anon (1992). Annual Report 1991. International Centre for Research in Agroforestry (ICRAF), Nairobi. pp.148.

Anon (1993). Diversity for development - the strategy of the International Plant Genetic Resources Institute. International Plant Genetic Resources Institute (IPGRI), Rome.

Brown, A.H.D. (1989). Core collections: a practical approach to genetic resources management. Genome 31: 818–24.

Davis, P.H. and Heywood, V.H. (1963). Principles of Angiosperm Taxonomy. (Oliver and Boyd: Edinburgh and London).

Doute, R., Ochanda. N., and Epp, H. (1980). Forest Cover Mapping in Kenya using Remote Sensing Techniques. Ministry of Environment and Natural Resources. Publication by the Kenya Rangeland Ecological Monitoring Unit (KREMU).

Frankel, O.H. (1984). Genetic perspectives in germplasm conservation. In: Arber, W., Llimansee, K., Peacock, W.J. and Starlinger, P. (Eta). Genetic Manipulation: Impact on Man and Society (Cambridge Univ. Press: UK).

Frankel, O.H. and Brown, A.H.D. (1984). Plant genetic resources today: a critical appraisal. In: Holden, J.H.W. and Williarns, J.T. (Eta). "Crop Genetic Resources: Conservation and Evaluation (George Allen and Unwin: London).

Gillett, J.B. (1963). Sesbania in Africa (excluding Madagascar) and southern Arabia. Kew Bulletin No. 17 pp.91–159.

Hodgkin, T. (1994). Core collections and the conservation of genetic resources. Paper to Asian Workshop on Sesame, Nagpur, India, 28–30 September 1993. (In press)

Heering, J.H. and Hanson, J. (1993). Karyotype analysis and interspecific hybridization in three perennial Sesbania species (Leguminosae). Euphytica. (In press).

Ndungu, J. and Boland, D. (1994). Sesbania seed collections in southern Africa exploring models for collaboration between CGIAR and NARS. (In press).

Srnith, N.J.H., Williarns, J.T., Plucknett, D.L. and Talbot, J.P. (1992). Tropical forests and their crops. (Comell University Press: Ithaca and London).

Vaughan, D.A. and Chang, Te-Tzu (1992). In situ conservation of rice genetic resources. Economic Botany-46 (4): 368–83.

ANNEX 1. DEFINITION OF TERMS

1. Kinds of germplasm collections
Anon (1993) provides some good definitions of the different kinds of collections that can be held in gene banks. Each sample of germplasm (seedlot) representing a cultivar, breeding line, or collected field sample which is held in storage is referred to as an accession. The kinds of accessions in collections are:

base collection - comprises a set of genetically different accessions of a given gene pool conserved for the long term, ideally under long-term storage conditions. An international base collection of a gene pool can be regarded as the sum total of all genetically different accessions conserved ex situ in gene banks around the world. Base collections are not normally used as a routine distribution source.

active collection - is a collection of germplasm used for regeneration, multiplication, distribution, characterisation and evaluation. Ideally germplasm in the active collection should be maintained in sufficient quantity to be available on request. Active collection germplasm is commonly duplicated in a base collection and is often stored under medium to long-term storage conditions.

working collection - is generally for short-term use by the holder of the collection. Such collections are commonly used by breeders or researchers. Long-term maintenance is not a priority in working collections.

field collection - or field gene bank is a collection of living plants (e.g. fruit trees, glass house crops and perennial field crops). Germplasm which would otherwise be difficult to maintain as seed can be kept in field collections.

in vitro collection - is a collection of germplasm kept as plant tissue. In some cases the tissue is stored at very low temperatures such as under liquid nitrogen (cryopreservation).

core collection - attempts to combine the maximum genetic variation of a species within a manageable number of samples. Core collections offer an opportunity to improve the accessibility and increase the use of collections. They are not a substitute for base and active collections.

The concept of core collection is relatively new. It was first introduced by Frankel in 1984 and later refined by Brown to refer to a representative group of accessions which, with minimum redundancy, includes the maximum genetic diversity of a species and its wild relatives (Frankel 1984, Frankel and Brown 1984, Brown 1989). The concept was first developed to help crop breeders cope with burgeoning collections in gene banks so that they could quickly assess variation to be included in breeding programmes. A review of progress in developing concepts on core collections has been made by Hodgkin (1994). So far most attention has been given to cultivated crops with less attention given to wild species such as sesbanias.

Brown (1989) gave the following criteria for a good core collection:

Utility criteria

1. the core should contain no redundant entries
2. the origins of its entries are authentic unless no choice is available
3. it is sufficiently large to yield reliable conclusions for the collections as a whole, yet is of manageable size
4. it is able to predict sources of useful variation.

Genetic criteria

1. the major sub specific tax a and geographic regions are represented
2. emphasis is given to representing the more broadly adapted rather than intensely specialised alleles
3. within the constraints of criteria (a) and (b), genetic diversity, especially as measured by the number of alleles per locus is maximal.

Forest Genetic Resources No. 22. FAO, Rome (1994)
Manuscript received June 1994