Manipulations of production systems in rubber tree which were intended to improve sucrose translocation in tapped bark resulted in an increase of latex sucrose and of latex production and reduced the incidence of nonyielding laticiferous tissue. This was achieved by shortening the tapping cut from full to half spiral, by changing the descending direction into an ascending mode of tapping or by annual change-over of tapping panel allowing for a longer time the regeneration of bark removed above the location of the cut. The increase of latex yield did not result in a significant decrease in the growth of tr~es over a period of three years.Clonal differences in nonyielding bark appeared to be related to differences in sucrose depletion by tapping. In clone PB 235 which exhibited low latex sucrose, a reduction of tapping frequency resulted in an increase in sucrose level and in a decrease of bark "dryness" tending to an increase in total yield.The tapping manipulations examined did not affect latex flow characteristics such as the plugging index of latex vessels and the bursting index of lutoids. The results stress the importance of photosynthate allocation for the physiology of laticiferous system productivity and indicate the possibilities of improving assimilate economy in rubber trees.Hevea latex, the rubber containing cytosol of latex vessels (ARCHER et al.
1963, DICKENSON 1965) is commercially extracted by controlled cutting of secondary phloem tissue of the trunk, known as tapping, which is practised at regular time intervals, normally twice a week. The yielding peculiarities of the rubber tree thus consist in that the economical product is continuously removed from the site of its formation and its analysis makes it possible to examine metabolic factors of productivity.Saccharides are supplied to latex vessels as sucrose (TvP:~ and RESING 1968, TUPff 1985) and its utilization in latex metabolism is limited by the activity of latex invertase which in turn is strongly pH dependent (Tvrff 1969). The tapping results in a decrease of sucrose content in the latex (BEALING and GHUA 1972, TuP~: 1973a) which indicates that the sucrose exported and utilized in latex metabolism is not fully replaced by translocated sucrose. The sucrose depletion is genotype dependent (TuP~ 1973b, Low
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
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