It is well known that the productivity and the susceptibility to the tapping panel dryness (TPD) of Hevea brasiliensis are determined by the intrinsic feature of each clone but also by the tapping system. Thus, in order to establish the tapping system that minimizes the TDP occurrence an experiment was carried out in Côte d'Ivoire with four high yielding clones including, PB 235, PB 255, IRCA 18 and IRCA 111. Five tapping systems (d2 6d/7 0/y, d3 6d/7 4/y, d4 6d/7 6/y, d5 6d/7 8/y, d6 6d/7 10/y) were applied to these clones, corresponding to a range of 100 % to 30 % of tapping intensities. The results show that the occurrence of TPD is closely related to the tapping intensity and the tapping system which consists in tapping twice per week with one day rest (S/2 d3 6d/7 4/y), was found to be the best tapping system. A quadratic relation was found between the rate of the TPD and the tapping intensity. Thus the clones IRCA 18 and IRCA 111, like PB 235 and PB 255, belong to the active metabolism clones. These clones also showed their adherence to the group of high rubber producer clones. This experiment also gave further evidence that the production of a tree for the tapping strongly depends on the intensity of tapping. It has also indicated that the intensity of tapping governed the tapping panel dryness rate among the clones having active metabolism, such as IRCA 18, IRCA 111, PB 235 and PB 255.
Rubber cultivation has been handicapped from the outset by the impossibility of obtaining selfrooted clones or rootstock clones. The grafting of clonal buds on not selected rootstock remains at the present time the only method of propagation available for the establishment of plantations. However, this "semi-vegetative" multiplication leads to certain heterogeneity and a reduction in the production of rubber compared to that of the mother tree selected. Fields trials have been conducted in Côte d'Ivoire to assess the relative gain in strength and in rubber production due to the use of self rooted in vitro plantlets of rubber on farm in comparison with conventional mature budded clones. Experimentation was made with trees of clones PR 107, IRCA 18 and RIIM 600 obtained by somatic embryogenesis or microcutting. The results showed a gain in volume of the trunk from 9.93% to 16.83% due to the use of in vitro plantlets. A gain of dry rubber production per tree of 3.5% to 32.35% has also been recorded. The gain in girth and rubber production of in vitro plantlets has been influenced by the metabolic class and the physiological profile of the clone. In light of these results, the use of in vitro plantlet as planting material could be considered to increase the productivity of rubber plantations.
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