Parkia biglobosa (Leguminosae) en Afrique de l'Ouest : biosystematique et amelioration

Abstract

The purpose of the research presented here is to strengthen the scientific and technical basis of biodiversity conservation, and use of Parkia biglobosa (African locust bean, néré ). The approach to this research included the gathering of technical and ethnobotanical data. This information can be used to support the development of methods and strategies for conservation, use and improvement of this multi-purpose agroforestry tree species in West and Central Africa.Overexploitation of land, particularly in densely populated and dry areas, affects the rejuvenation process of this tree. This results in the general aging of néré orchards, particularly in the southern part of its range of distribution.More than 1600 trees from 5 countries were sampled and various morphometric and phenological observations were used to determine the level of the variation extant in P . biglobosa as well the structure of this diversity. Socio-economic and cultural data were obtained through questionnaires filled out by more than 500 people from different ethnicgroups in Burkina Faso and Benin.The initial hypothesis of the research was: a number of phenotypic characters of the species present clinal variation along longitudinal and latitudinal gradients. This has important implications for selection, conservation and genetic improvement.The research established the place and importance of néré in the daily life of many rural communities and determined the kinds of variation in P. biglobosa and how to capture this variation in an efficient way. This rich traditional knowledge presents a social image of the tree, as a common resource, which reinforces the cohesion of the group and is a symbol of peace, continuity, and harmonious development in these communities.Every part of this multi-pupose tree is utilized. The tree functions to diversify production and sources of income in the agricultural system. The fermented seeds produce a protein-rich condiment, soumbala or dawadawa, that fortifies the taste of most sauces. The pulp around the seeds is rich in sugar and is much appreciated by children. Bark and leaves, as well as seeds and pulp, have medicinal properties and are used in the treatment of more than 40 ailments.Almost every community has a different local name for the tree. Ibis linguistic diversity reflects ancient traditional knowledge about the species. The numerous customs, rites, legends and folklore are evidence of the existence of profound, durable and harmonious interdependence and mutual benefit between persons in the village and trees in the orchard. This knowledge is based on a large number of the tree characteristics such as adaptation, various uses, vigour, resistance to pests and specific diseases, bark texture and fruit production capacity (quantity, quality and periodicity). Various forms of néré are distributed in correlation to the enviromnental conditions.Women, who are the most knowledgeable about the tree, are the repository of the knowledge concerning fruit and seed aspects. They handle the seeds, exchange plant material and prepare the food items. The men maintain the knowledge on sylvicultural practices to optimize production. This includes balancing interaction between the tree and crops to avoid crop pests and diseases. Future collecting of plant material should take into account the importance of the traditional knowledge that contributes to the maintenance of intra- and inter- population diversity. nis concerns vernacular names, tree appearance, fruit production capacity, medicinal properties, origins and particular enviromnental conditions.The variation between individual trees is large, but the variation between populations is relatively small as is the variation between countries. The nature, the level and the structure of the phenotypic diversity observed in P . biglobosa is the result of several factors, among which are genetics and environment.The tree architecture is flexible and approaches the model of Champagnat; it adapts to enviromnental factors, as is the case of many of the Leguminosae. The species has developed various adaptive strategies that are reflected in its phenology and reproductive system. Leaves are always present, but gradually fall in the last weeks before the new leaves unfurl.Vegetative and reproductive parts are in competition for available energy sources. The onset of flowering is staggered from south to north, from November-December in the south to late March in the north, creating a difference of 3 to 4 months. As a consequence, fruiting is staggered too, from March in the south to July in the north. In contrast, in a east-west direction, staggering of flowering and fruiting is less, only about two weeks. Phenological variation along the latitudinal gradient is gradual. Rainfall and subsequent availability of water in the soil plays an important, but not exclusive, role in determining leaf production and flowering. The large individual phenological variations seem to be correlated to very localized soil conditions and to the genotype of individual trees.The reproductive system is allogamous by preference, but autogamy is always possible, even if limited by self-incompatibility. Bees constitute (in Burkina Faso) the main pollinators, together with bats. The activity of bats in the southern region is more important. The pollinators influence fruit set and the structure of inter-population diversity. Man strongly contributes to changes in the level and structure of intra- and inter-population diversity. His practices are very important in the savanas and are related to the importance ascribed to the tree. In forest zones the tree is valued relatively little.Because of the importance of P. biglobosa, it is necessary to deploy national, regional and international research and development to assure its conservation and genetic improvement. It is also needed to promote traditional cultivation practices to profit most of the available resources and to increase production in the existing agricultural system.Given the distribution of observed phenetic diversity, selection of a maximum of individuals from a limited number of populations at the country level, and of some populations on the regional level, represents an adequate strategy to capture a maximum of genetic diversity. Indigenous knowledge, favouring the role of women when aquiring these data, is indispensable to improve the selection. The on-going research to understand the level and the structure of inter- and intraspecific genetic diversity will no doubt supply useful additional information