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Ecophysiologie et diversité génétique de Faidherbia albida (Del.) A. Chev. (syn. Acacia albida Del.), un arbre à usages multiples d'Afrique semi-aride. Fonctionnement hydrique et efficience d'utilisation de l'eau d'arbres adultes en parc agroforestier et de juvéniles en conditions semi-contrôlées. Tome 1 : Partie synthèse

Abstract : Faidherbia albida (Del.) A. Chev. (syn. Acacia albida, Del.) is a multipurpose tree of the legume family, widespread in Africa. Its strategy in the face of drought appears very original: it is present in semi-arid zones but its phenology is reversed in relation to the rains: it buds at the end of the rainy season, bears fruit and grows during the dry season, then loses its leaves at the beginning of the new rainy season. Very little is known about its hydrous functioning. We conducted seasonal monitoring of the water functioning of adult trees in agroforestry parks (Burkina Faso, West Africa; 920 mm rainfall). The basic water potential remained high throughout the dry season, indicating that the trees were permanently accessing soil horizons with very good water supply. Roots were down to the vicinity of the water table (-7 to -15 m depending on the site). The isotopic oxygen content of the soil water, groundwater and raw sap indicated that the trees absorbed mainly in the vicinity of the water table, except during the rainy season, when they tilted towards the surface. The trees transpired intensely, especially at the beginning of the dry season (about 400 l/day for a 65 cm diameter tree, measured by a sap flow method). But stem density was low, and annual transpiration of the tree component of the plots remained below 5% of the rainfall. During the dry season, the soil moisture stress increased moderately (slight decrease in base water potential, 50% reduction in total soil-leaf specific hydraulic conductance). The edaphic and atmospheric components of water stress probably contributed to the closure of stomata during the dry season. We tracked changes in intrinsic water use efficiency (ratio of net assimilation to stomatal conductance, A/g) using leaf carbon isotopic composition. A/g decreased during the dry season despite stoma closure. The photosynthetic capacity could therefore be affected, and we looked for a nutritional cause. Foliar nitrogen content actually fell by 50% during the dry season: F. albida absorbed and probably fixed nitrogen only within a narrow "window" corresponding to leaf initiation, when the surface horizons are still moist and surface nitrogen remains mobilized. Radial growth stopped early in the dry season, before leaf fall, and may also have been dependent on reduced photosynthetic capacity. Leaf fall started after the new rains, and would therefore be independent of the edaphic drought: on the other hand, the narrow "window" of assimilation and fixation would predispose to leaf senescence during the dry season, and could explain their abscission. The strategy of using water from juveniles that have not yet reached the water table is certainly very critical, and could explain why some fast-growing sources show low survival rates in dryland trials. We used pan-African provenances with contrasting initial growth patterns to describe the diversity of ecophysiological characters of F. albida at the seedling stage. We tested the factors: provenance, soil water availability and experimental site (in a greenhouse in Nancy-France or in a nursery in Ouagadougou-Burkina Faso), factors that are likely to affect the efficiency of integrated water use (W: ratio of dry biomass produced to water consumed). Nitrogen fixation can also modulate W, playing either on A or on the carbon allocation for symbiosis. Nitrogen fixation was estimated by the natural abundance of nitrogen 15, and modulated by phosphorus input. Vigorous provenances had more leaf area and transpiration and invested less in root growth. Differences in vigour between provenances were very small at the more restrictive Ouagadougou site: vigorous provenances lost much of their initial growth advantage, showing less hardiness. These elements seem essential to interpret their poor survival in the dry zone. Inter-provenance differences in W, A/g and carbon isotope discrimination were significant but moderate. A/g was better for the most vigorous provenances, a fact confirmed by carbon isotope discrimination. But the relationship between W and carbon isotope discrimination differed between provenances, probably due to differences in non-photosynthetic processes. Vigorous provenances, which fixed more nitrogen, had lower W values for the same carbon isotope discrimination value. Carbon losses, perhaps related to the requirements of symbiosis, could explain this. The relationship between W and carbon isotope discrimination, tested at the two sites, differed mainly because of differences in air saturation deficits. The relationship between A/g and carbon isotope discrimination seemed to be maintained on the contrary. We conclude that there was a close relationship between A/g and carbon isotope discrimination, demonstrating the relevance of using carbon 13 in the field to estimate A/g, but not W. Faidherbia albida ultimately appears to be a phaetophyte species in the adult state, which makes it relatively free of edaphic water stress. Its very late bud burst has little impact on its water functioning, but greatly affects nutrition, and probably photosynthesis and growth. In the juvenile stage, the stomata close very rapidly in response to moderate edaphic drought, showing a drought "avoidance" strategy. We conclude that F. albida represents an excellent plant model for studying the impact of drought at different physiological stages.
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Olivier Roupsard. Ecophysiologie et diversité génétique de Faidherbia albida (Del.) A. Chev. (syn. Acacia albida Del.), un arbre à usages multiples d'Afrique semi-aride. Fonctionnement hydrique et efficience d'utilisation de l'eau d'arbres adultes en parc agroforestier et de juvéniles en conditions semi-contrôlées. Tome 1 : Partie synthèse. Ecologie, Environnement. Université Henri Poincaré - Nancy I, 1997. Français. ⟨tel-00426396⟩

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