Rubber trees are the main source of natural rubber (NR). The area occupied by rubber plantations rose from 3.9 million ha in 1961 to 12.5 million ha in 2018. Both the expansion of rubber plantations in marginal zones (prone to biotic and abiotic stress), and long-term rubber tree cultivation in traditional areas, raise questions about the sustainability of NR production in a context of climate change. Our study set out to gain insights into the biogeochemical cycles in rubber plantations, for a better matching of fertilizer inputs to the dynamics of nutrient demand throughout rubber tree growth. Nutrient accumulation in tree biomass is a major component of the biological cycle in tree plantations. We studied the dynamics of biomass and nutrient accumulation in two chronosequences covering the whole lifespan of a plantation in Ivory Coast managed on a sandy soil at the SAPH site, and one on a clayey soil at the SOGB site. In total, 56 trees were destructively sampled in 2-, 5-, 20- and roughly 40-year-old stands. While the use of allometric relationships is common for estimating nutrient stocks in planted forests, this study was the first to provide allometric equations predicting nutrient stocks in rubber tree components. Allometric models were applied to the inventory of 4 commercial stands, for each age at each site, to estimate stand biomass and nutrient stocks. The current annual increments of nutrient stocks in tree biomass peaked between 2 and 5 years after planting. They reached 80 kg ha(-1) yr(-1) for N, 14 kg ha(-1) yr(-1) for P and 34 kg ha(-1) yr(-1) for K at SAPH (53, 7, and 39 kg ha(-1) yr(-1) respectively at SOGB), which highlighted the importance of an appropriate fertilization schedule for young rubber trees. At the clear-cut age (38-40 years), the amounts of nutrients accumulated in tree biomass were 970 kg N ha(-1), 188 kg P ha(-1), 366 kg K ha(-1), 941 kg Ca ha(-1) and 255 kg Mg ha(-1) on the sandy soil at SAPH (907,118, 629 1499, and 375 kg ha(-1) respectively on the clayey soil at SOGB). Contrasting soil properties and management practices at the two sites had a much greater effect on the amounts of P, K, Ca and Mg accumulated in the trees than on N accumulation. Logging practices in rubber plantations can lead to considerable nutrient exports on poor tropical soils. Harvest residues should be distributed uniformly in the plots so that the roots of young trees can quickly gain access to the nutrients released during decomposition.

Industrial clones of Hevea brasiliensis have been traditionally propagated by bud-grafting. Although this technique is routinely used by planters - high multiplication rate, high budding success-it is liable to induce graft incompatibilities symptoms and physiological ageing of the planting material. Recent findings have confirmed that industrial clones produced by rooted (micro) cuttings on their own roots further to in vitro rejuvenation, grow faster and yield more latex than those derived from budding. The self-rooted industrial rubber tree clones planted at SoGB were first rejuvenated in vitro by somatic embryogenesis at Gent University in Belgium. The few emblings obtained were then micropropagated by axillary budding for producing a sufficient number of microcuttings for each clone. However the high constraints associated with the shipment of the in vitro microcuttings from Europe to Africa and poor acclimatization success rates prompted us to use these Belgium-produced in vitro plants as stock plants for mass clonal propagation by rooted cuttings at a much cheaper cost locally in the vicinity of the planting sites. Two industrial rubber tree genotypes produced by rooted cuttings have already been planted to compare their agronomic traits to those of the same clones but derived from traditional bud-grafts. Extending such comparative analyses to a greater number of clones will ultimately answer the basic question of knowing whether industrial rubber clones produced from rooted cuttings have a superior field behavior than the same material propagated by grafting. The respective multiplication rates, the time needed to get plantable material as well as the cost efficiency have also to be taken into consideration.

Industrial clonal plantations of Hevea brasiliensis, more commonly known as rubber tree, have been established for several decades with grafted/budded plants as an alternative to the mass clonal propagation of the mature selected industrial clones on their own roots. Substantial investments have been devoted for many years to the development of tissue culture techniques focussing on somatic embryogenesis an d micropropagation by axillary budding to reach this goal but with limited success. The recent mass production of self-rooted H.brasiliensis industrial clones by nursery methods provides the opportunity to assess the respective pros and cons of in vitro versus nursery vegetative propagation methods for mass producing industrial clones of the rubber tree. (Resume d'auteur)

The forest tree species Hevea brasiliensis is extensively planted in the humid tropics to meet the increasing demand for natural rubber. Huge quantities of planting stock are therefore needed. The seed option remains the easiest and cheapest way to establish plantations of rubber trees but those show a great variability for vigor and also for latex yield. The rationale of producing clones for overcoming this variability was already obvious in the early 1910's but due to the difficulties encountered at that time for rooting shoots, grafting was used as an alternative cloning method. The striking increase in yield noticed from the graft-derived clonal plantations warranted their large scale development. Eventually, the budded clones by virtue of their much higher and uniform productivity supplanted the seedlings in most industrial plantations. However, grafting is also associated with drawbacks and for decades efforts aiming at mass producing selected rubber tree clones on their own roots by rooted cuttings have been pursued. However, this approach was progressively abandoned due to disappointing rooting results and, from the 70's onwards, priority has been given to in vitro methods which were booming during this period. But despite 40 years of heavy investments, industrial H. brasiliensis clones could still not be mass micropropagated in vitro efficiently enough to meet the requirements of large scale production. The situation may change radically soon, however, due to the development of new nursery techniques adapted to the mass clonal production by rooted cuttings of any H. brasiliensis selected genotype. Efforts to improve the techniques as well as the establishment of new field trials are underway in order to determine if self-rooted rubber tree clones are more productive than grafted ones. This old issue is becoming of overriding importance considering the increasing pressure on land availability reducing thereby the prospects for expanding rubber tree plantations.

The importance of Hevea brasiliensis (rubber tree) as a cash crop keeps increasing warranting the development of new and more efficient techniques than the bud-grafting traditionally used for mass producing superior planting material. In vitro production of rubber trees by somatic embryogenesis and the good field performance (fast growth, high yield) of the resulting emblings have been reported for years, but so far only on an experimental scale. In vitro mass production of self-rooted rubber clones by somatic embryogenesis or microcuttings is hindered by a lack of responsiveness of most of the selected genotypes and by prohibitive production costs. Given this situation, macro propagation by rooted cuttings following in vitro propagation of mature selected clones was attempted by the SoGB estate in Cate d'Ivoire as a possible alternative to in vitro techniques only. Two industrial mature clones, clone A (70yrs-old) and clone B (53yrs-old), were first rejuvenated in vitro by somatic embryogenesis then micropropagated in greater numbers by microcuttings. After acclimatization, these in vitro-rooted microcuttings were potted in individual containers to be managed intensively as stock plants to produce macro cuttings. After 3 weeks under suitable rooting conditions in the nursery, rooting rates of 74.6% (1203/1613) for clone A and 76.5% (198/259) for clone B macrocuttings were obtained. The cuttings produced vigorous and taproot-like adventitious roots. After successful acclimatization, all the rooted cuttings developed true-to-type and 4 months later reached a sufficient height of 25-30 cm to be field planted. In addition to higher field vigor and true-to-typeness, cutting-derived rubber tree clones can be produced more quickly, on less nursery space at lower cost and in easier working conditions than traditionally bud-grafted plants. Further investigations are underway for assessing at the field level the comparative advantages of rooted cuttings versus grafted clonal offspring with respect to traits of major economic importance such as latex yield.

L'importance de l'hévéa (caoutchouc), Hevea brasiliensis, en tant que culture de rente ne cesse d'augmenter justifiant de s'intéresser à de nouvelles techniques de clonage plus efficaces que le greffage (écussonnage) traditionnellement utilisé pour la production industrielle de matériel de plantation de qualité supérieure. Les bonnes performances sur le terrain (croissance rapide, haut rendement) des hévéas produits par embryogenèse somatique n'ont été constatées jusqu'à présent qu'à l'échelle expérimentale. La propagation de masse in vitro par embryogenèse somatique ou microbouturage de clones d'hévéas sur leurs propres racines reste pénalisée par un manque de réactivité de la plupart des génotypes sélectionnés et par des coûts de production prohibitifs. Face à cette situation, la propagation par bouturage de clones matures sélectionnés issus de micropropagation in vitro a été tentée par la SoGB en Côte d'Ivoire comme une alternative possible à l'utilisation exclusive des techniques in vitro. Les deux clones matures industriels, A (70 ans) et B (53 ans), ont d'abord été rajeunis in vitro par embryogenèse somatique puis micropropagés en plus grand nombre par microbouturage. Après acclimatation, les microboutures enracinées in vitro ont été rempotées dans des pots individuels pour être gérées de manière intensive comme pieds-mères destinés au bouturage. Après 3 semaines en conditions horticoles adéquates, les taux d'enracinement obtenus pour les boutures des clones A et B ont été respectivement de 74,6 % (1203/1613) et 76,5 % (198/259). Les racines adventices néoformées étaient généralement vigoureuses. A l'issue d'une phase d'acclimatation réussie, les boutures se sont développées de façon conforme pour atteindre 4 mois plus tard une hauteur de 25-30 cm suffisante pour être plantées au champ. En sus d'une plus grande vigueur et conformité sur le terrain, les clones issus de bouturage peuvent être produits plus rapidement, sur des surfaces plus réduites à moindre coût et dans des conditions de travail plus faciles par rapport aux plantes issus d'écussonnage. Des analyses en cours devraient permettre d'établir les avantages comparatifs des boutures par rapport aux plants écussonnés en ce qui concerne d'autres caractères à fort impact économique tels que le rendement de latex.