Conservation and utilization of germplasm of traditional African vegetables in Senegal

A. Seek
TROPICASEM, Dakar, Senegal

Introduction

In Senegal, rain-fed agriculture has traditionally been the predominant system of agricultural production. Vegetable crops that are now widely used (high value added, less vulnerable to erratic climatic conditions, etc.) have gained popularity only over the last 15 years, during which time there has been a considerable increase of cultivated area, and a doubling of production (Ministère de l'Agriculture 1992).

This progress is certainly partly due to the establishment of the Horticultural Development Centre (CDH) in 1972 and the subsequent research work that has been carried out in the field. However, it only applies to species which have been given high priority in the past, namely European vegetables such as tomato, cabbage, potato and onion. Today, more than 30 vegetable species are produced in Senegal, some of them in restricted geographic areas. These species include traditional African vegetables, sometimes referred to as home garden crops or "plants for women". These traditional vegetables are produced over small areas and include several species that have been neglected by research programmes. These crops have not undergone scientific selection and breeding and are maintained in a heterozygous state.

Today, a new awareness of the importance of these plants has led to their gradual integration in research and development programmes. Indeed, in addition to their food value and the fact that they are easy to produce, they also have tremendous industrial and medical potential.

In this paper, we present a brief diagnostic survey of the traditional vegetable species on which our programme is conducting research and then outline their future prospects.

Traditional vegetables in Senegal

Traditional African vegetables include:

· fruit-vegetables such as gombo (okra, Abelmoschus spp.) and jaxatu (African eggplant, Solanum aethiopicum)

· leaf-vegetables such as bissap (roselle, Hibiscus subdarrifa)

· roots and tubers such as cassava and sweet potato.

Food value

Jaxatu. The fruits of the African eggplant (Solanum aethiopicum) are consumed raw or cooked and are usually harvested before they reach maturity. The fruits are berries (light green, dark green, white, blackish) with a bitter taste that varies depending on their saponin content. At full maturity, the fruits turn red or reddish-orange because of their high carotene content. The leaves of the glabrous species or varieties are more nutritious than the fruits and are sometimes consumed in the same way as spinach in the southern part of Senegal.

Gombo. Immature capsules of the plant are harvested 4-9 days after anthesis, before they lignify. They are eaten fresh or dried, as condiments or added to soups. At full maturity, fruits become lignified and, in some varieties, shed their seeds. Some parts of the plant are also used in the textile industry.

Bissap. The dried calyx of red varieties is used to produce acidic syrups that are very popular in Senegal. These varieties are currently processed both domestically and industrially and the export of bracts and the production of chemical dyes are expanding. The major characteristics of the red varieties include their good vegetative performance, their high anthocyanin and nutrient contents and their acidic taste. In contrast, green varieties have no anthocyanin and their calyces are consumed as flavouring for tangy soups and as colourless syrups. Leaves are consumed in the same way as spinach or pounded and added to rice dishes.

These three vegetable species have been found to compare favourably with vegetables such as tomato and eggplant (Solanum melongena), which they seem to exceed in nutritional value, particularly as regards caloric content, dry matter content and carbohydrate content (see Table 1).

Table 1. Comparison of the nutritional values of traditional African vegetables and two temperate species.

Species	Calories	Dry matter	Protein	Fats	Carbohydrates	Other
Gombo	33	10	2	trace	6	-
Bissap	31	11	1.6	0.1	6.7	2.7 (malic acid)
Jaxatu	30	9.6	1.6	0.1	7.2	-
Tomato	26	8.5	1.9	0.1	5.8	-
Aubergine	25	7.6	1.1	0.2	5.8	-

Sources: Toury and Coll 1965; Charrier 1984; Kerharo and Adam 1974.

Jaxatu. This is a fruit-vegetable in the Solanaceae. Solanum aethiopicum is a highly polymorphic species (see taxonomy). Varieties grown in Senegal belong to the Kumba subspecies and produce ribbed berries of bitter taste and variable size (40-200 g). The cultivated area, estimated at about 400 ha more than a decade ago (Van de Plas et al. 1984), has since expanded. However, improved varieties remain few (Soxna and Mbir Ndao from CDH, Ngalam from TROPICASEM). This explains the high quantities of seeds produced by farmers from local cultivars in the major production areas.

The plant is grown all year round and mainly suffers from pests during the hot and humid season. The major insect pests are phytophagous mites, mainly tetranychids and tarsonemids (Seck 1984, 1986a) and piercing-sucking insects. Fungi such as Leveillula taurica and Alternaria stemphylium also pose problems. Seed dormancy is also a constraint (Seck and Sow 1994).

Gombo. Senegalese farmers mainly grow Abelmoschus esculentus. The cultivated area was estimated at 300 ha in 1983 (Mbaye 1994). Varieties grown in the country include local cultivars and a few fixed improved varieties such as Puso, Pop 12 (ISRA) and Clemson Spineless (USA). They differ in qualitative and quantitative traits such as growth cycle, productivity, resistance to pests, colour, the degree of leaf indentation, shape and size of capsules, etc. Hybrid varieties have been developed in Asia and in the USA but are not well known in Africa. Poor adaptability to the ecological and climatic conditions and susceptibility to pests and disease are the major constraints facing the production of this crop, specifically its poor adaptation to the cold season (sensitivity to photoperiod and temperature; Seck 1991) and its high susceptibility to nematodes (Meloidogyne spp.) and to Fusarium oxysporum f.sp. vasinfectum.

Bissap. This important food plant is a tropical Malvaceae of disputed origin. An estimated 100 ha are cultivated annually. Bissap grows both in the wild and as a domesticated plant. The two varieties consumed in Senegal (the red calyx type and the green calyx type) are differentiated by their anthocyanin content, which determines their pigmentation and consequently their utilization. Two varieties have been released by CDH/ISRA, the rest being local cultivars.

The crop is grown during the hot humid season for calyx production and all year round for leaf production when irrigation is available. Bissap production faces the same constraints as gombo, but seems to be resistant to the Meloidogyne nematodes (Messiaen 1975).

Taxonomic considerations

Traditional African vegetables have not always been well studied by taxonomists. However, in the last few years, our taxonomic knowledge of species such as African eggplants and okra has somewhat improved.

Jaxatu. The classification of Solanum species that do not produce tubers is still unclear. Nevertheless, in the course of the last decade, Dr R.N. Lester of the University of Birmingham has developed an identification key and contributed greatly to the taxonomy of S. aethiopicum. This has facilitated the setting up of improvement programmes for this species. Solanum aethiopicum, which grows over a wide geographical area in tropical regions, is a highly polymorphic species. Lester (1982, 1986) has described four subspecies or groups (Kumba, Gilo, Aculeatum and Shum), formerly considered fully fledged species. Despite the great morphological diversity, similarities in important traits, as well as the perfect cross-fertility between the various groups, confirm Lester's results (Seck 1986b). Solanum aethiopicum probably originated from S. anguivi through domestication. The new varieties being developed and disseminated in Senegal are the result of hybridization between genotypes from these groups.

Gombo. The classification of this genus was initiated by Medikus in the 17th century, tackled again in 1924 by Hochereutiner, then restudied in 1966 by van Borssum Waalkes. The latter, who limits the number of species to six, has been challenged by Bates (1968), then amended successively by Hamon (1988) and IBPGR (1990), which identified nine species with three ploidy levels. A new species has been described, A. caillei, a native of West Africa.

Bissap. Bissap is considered a form of the species Hibiscus sabdariffa. There seem to be several botanical varieties, such as var. altissima, a fibre plant (ENDA-PRONAT 1994). Some authors such as Assegninou (1994) have argued that bissap is a hybrid between H. sabdariffa and Hibiscus aspe. There is variation in leaf morphology, for example the number of lobes. There certainly are gaps in our understanding of the taxonomy of the group.

Plant genetic resources and their utilization

Local varieties represent the bulk of the genetic diversity of these species in Senegal. Their phenotypes match the preferences of producers and consumers. These varieties are also well adaptated to local conditions. Multiplication for seed by farmers is a major factor in maintaining this diversity. Farmers' selection generally appears not to be very strong for these species. Each of these plant groups comprises both domesticated and wild species. The wild species must be recognized for their great genetic variability (resistance, hardiness, etc.).

The research programme of TROPICASEM aims to make use of the genetic diversity of these crops to:

· broaden the range of improved varieties available to users
· increase productivity and growth rate (early maturity)
· develop hybrid varieties to address specific problems (resistance, adaptation, etc.).

Some specific objectives of the programme are:

Jaxatu. The programme is advanced and has developed hybrids and open-pollinated varieties that are mainly being disseminated in West Africa.

· resistance to mites and the Alternaria complex, as well as to Stemphylium
· improvement of fruit quality (colour, size, shape, etc.).

· adaptation to the cold season, by making use of interspecific variability
· resistance to nematodes
· resistance to Fusarium.

· improvement of leaf yield (green varieties) and bract yield
· improvement of taste.

1. New information on some major aspects of the biology of the target crops

2. A highly diversified genebank for the species under study, which includes:

a) populations and stabilized pure lines under testing
b) new varieties for future release derived from crosses c) hybrids being finalized for release.

Future prospects

Despite the major research programmes of CDH/ISRA and the private sector, there still remain serious gaps in our knowledge of these traditional vegetable species. Research on the following aspects is still needed:

· morphology and phenology
· reproductive biology
· genetics
· intra- and interspecific genetic variability
· technological packages and methods that enable the evaluation and characterization of populations
· consumers' preferences.

· making an inventory of the major domesticated and/or wild species on an agro-ecological zone basis
· setting up research network(s) to promote information and germplasm exchanges
· carrying out regional or subregional explorations to collect germplasm
· developing descriptors and methods to facilitate characterization and evaluation of populations.

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