La «domestication» du samba. L'expérience écossaise

Résumé - Le samba (Triplochiton scleroxylon), comme la plupart des arbres forestiers, se bouture bien si les pieds mères sont traités correctement. L'enracinement est lié à l'âge des pieds mères, à leur capacité photosynthétique, à la quantité et la qualité de la lumière, et à la nutrition minérale ; il dépend aussi de l'origine de la bouture, de sa longueur, du nombre de noeuds et de la compétition entre pousses. Un bon enracinement est obtenu sur des boutures disposant d'une feuille de 50 cm² dans un système amélioré de «poly-propagateur» à hydrométrie élevée (après un traitement d'auxine à raison de 40 μg par bouture) pour obtenir un équilibre raisonnable entre carbone et eau. Au Nigeria de fortes variations clonales de croissance montrent qu'une sélection portant sur 4% des clones apporterait un gain génétique de 100%. Un test simple de prédiction de branchaison peut diminuer rapidement le nombre des génotypes à mettre en expérimentation de terrain et permettre d'opérer un choix précoce sur la forme et la croissance. Des floraisons précoces ont déjà pu être obtenues sous serres en Écosse.

Abstract - The "domestication" of obeche: the Scottish experience. Triplochiton scleroxylon (obeche) is an indigenous hardwood of West Africa (fig 1), which has been widely exploited for timber, but only established in small plantations because of poor seed availability and short seed viability. Techniques have however been developed for low temperature (-18°C) seed storage. In addition, vegetative propagation techniques have been developed to provide an alternative form of planting stock. Single-node cuttings (fig 2A) root well when the stockplanis have been managed correctly. Rooting is enhanced by applications of auxin (fig 3), with an optimum dose of 40 μg per cutting. Trimming the leaf to about 50 cm² optimises rooting, giving the best balance between photosynthesis and transpiration (ie carbon/water content). Rooting ability is strongly influenced by stockplant condition and physiological status. In addition, the origin of the cutting within a shoot (fig 4) and the position of the shoot within the stockplant pre-determine rooting ability. The differences in the rooting of cuttings from different shoots are affected by the level of competition between shoots for light (fig 5). Both the level of irradiance and its spectral composition influence this pre-conditioning effect of light through influences on leaf and stem morphology and their physiological status. To promote the implementation of these techniques in developing countries, improved, high-humidity non-mist poly-propagators have been developed. Through vegetative propagation, clones are formed, so offering the opportunity for selection of genetic superiority (fig 2B, C). Pronounced clonal variation in yield and form has been identified in Nigeria (fig 6), with the highest-yielding 4% giving a genetic gain of about 100% above the average of unselected material. A simple predictive test for branching habit has been developed (figs 7 and 8). This can greatly reduce the numbers of genotypes that need to be tested in field trials. Precocious flowering of obeche in glasshouses in Scotland indicates that there are opportunities to induce flowering at an early age and give a shorter generation time in a breeding programme. In this way, 2 generations of obeche seed (fig 2D) have been produced in less than 7 years. In conclusion, there are opportunities for highly productive clonal forestry with this species and a number of other West African hardwoods, provided appropriate forms of silviculture are developed.