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The tapping panel dryness (TPD) syndrome of rubber is characterized by the reduction or ultimately total cessation of latex flow upon tapping, due to physiological disorders in the bark tissue. The protein pattern in the cytoplasm from healthy and TPD tree latex cells was compared by electrophoresis. Two polypeptides (P15 and P22) of 15 and 22 kDa, respectively, were found to accumulate in the cytosol of the TPD-affected trees, whereas a 29 kDa polypeptide (P29) appeared de novo. P15 and P22 were identified as REF (Hev b1) and SRPP (Hev b3), respectively, two proteins proposed to be involved in rubber biosynthesis. P29 appeared to be a new member of the patatinlike protein family. Specific molecular probes were designed for a detailed characterization of REF and SRPP gene expression and RFLP mapping. This allowed the demonstration that REF and SRPP display very similar expression profiles. They are highly over-expressed by the tappinginduced metabolic activation, although not by wounding per se, or ethylene or ABA. In addition to this similarity in gene expression, they were found to share one common locus in the genome. No significant difference in REF and SRPP gene expression was observed between healthy and TPD trees, indicating that their TPD-related accumulation in the cytosol was not transcriptionally regulated. Western blot analysis demonstrated that osmotic lysis of the sedimentable organelles (lutoids) in vitro caused the release of REF and SRPP from the rubber particle membrane into the cytosol. A mechanism of cellular delocalization as a consequence of the lutoids instability is proposed to explain REF and SRPP accumulation in the cytosol of TPD trees.

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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.

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Latex is a key product for many tropical countries, of which 80% is produced by smallholders. Latex is produced by the rubber tree (Hevea brasiliensis). Given the 7-year immature unproductive period, establishing a rubber plantation requires considerable investment by smallholders, emphasizing the need for sustainable management. The difficulty of performing an agronomic diagnosis of a tree crop is to obtain an accurate picture of current and past cultivation practices, to be able to assess their impacts on the agro-ecosystem as well as on sustainability. Smallholders do not usually keep records of latex yield or of their technical practices, making it impossible to perform a diagnosis based on productivity. As latex harvesting involves tapping the bark, which leaves scars on the trunk, we hypothesised that these morphological traces would be good indicators of current and past practices and would thus enable a diagnosis based on the economic lifespan of plantation. To this end, we formalised a tapping panel diagnosis that involved reproducing the scars on tapping panel diagrams, and analysing them using two indicators: the amount of virgin bark consumed and the number of tapping years that remained. We validated this tapping panel diagnosis in a sample of 25 smallholder plantations in Cameroon, where we characterised eight tapping management systems reflecting different levels of tapping intensity. The assessment of the respective share of each tapping practice on virgin bark consumption revealed major effects of tapping frequency and of shaving thickness. We showed that the tapping panel diagnosis used as a decision support tool can increase remaining tapping years by 33% to 355%. To conclude, the tapping panel diagnosis formalised here for the first time will be a useful support for the participatory development of innovating tapping management schemes involving both technicians and smallholders. (Résumé d'auteur