Search

Rubber trees (Hevea brasiliensis) are the main source of natural rubber, extracted from latex, which exudes from the trunk after tapping. Tapped trees require large amounts of carbon (C) to regenerate the latex after its collection. Knowing the contribution of C sources involved in latex biosynthesis will help in understanding how rubber trees face this additional C demand. Whole crown 13CO2 pulse labelling was performed on 4-year-old rubber trees in June, when latex production was low, and in October, when it was high. 13C content was quantified in the foliage, phloem sap, wood, and latex. In both labelling periods, 13C was recovered in latex just after labelling, indicating that part of the carbohydrate was directly allocated to latex. However, significant amounts of 13C were still recovered in latex after 100 d and the peak was reached significantly later than in phloem sap, demonstrating the contribution of a reserve pool as a source of latex C. The contribution of new photosynthates to latex regeneration was faster and higher when latex metabolism was well established, in October, than in June. An improved understanding of C dynamics and the source–sink relationship in rubber tree is crucial to adapt tapping system practices and ensure sustainable latex production.

International audience; Rubber Flux aims at providing a complete picture of CO2, water and energy budget of a rubber plantation in Eastern Thailand. Beyond the evaluation of the fluxes, our purpose is to partition them among the different components of the plantation ecosystem (canopy, trunks, roots, under storey, soil) and the different functions (photosynthesis, respiration. evapotranspiration ) in order to understand the factors controlling the carbon, water and energy budgets of the ecosystem. The experiment is situated at the Chachoengsao Rubber Research Station located in Phanom Sarakham district, about 140 km east of Bangkok The observation site is located in a 6 ha plot at the center of the 350 ha station. The plot itself is planted with a monoclonal stand (clone RRIM 600). Trees were 13 years old in 2007. The average height was 20 m and average girth at 1.7 in was 60 cm. Initial planting density was 500 trees/ha and actual stand density was 454 trees/ha in June 2006. Trees are tapped for latex production for 5 years. Carbon fluxes of rubber plantation ecosystem are continuously measured by the eddy covariance method (ED). Evapo-transpiration (ET) is measured by ED and water balance together. Meanwhile, amounts of carbon (C) stored in the trees will be evaluated by measuring biomass increment of the plantation, in combination with estimations of the carbon content at the different compartments. ED methodology was adapted from a similar experiment developed by our research group on another tropical tree crop plantation, coconut tree, in Vanuatu islands. The flux tower is 25 in high. Three-demensional (3-D) sonic anemometer Young 81000V 20 Hz is used together with alt open path gas analyser (LI-7500; LI-COR, Inc., Lincoln, Nebraska, USA). Raw data are collected and pre-processed by the "Tourbillon" software (INRA-Bioclimatologie, Bordeaux, France) for a time-integration period of 300 s. Raw data are post-processed using EdiRe software (University of Edinburgh, UK) into half-hourly values. All data are despiked according to variance filters, planar fit is applied (parameters are calculated monthly), and vapour is corrected for buoyancy. Weather station measures semi-hourly net radiation (Rn), photosynthetically active radiation (PAR), diffuse PAR (PARdiff), reflected PAR (PARreflected), global radiation (Rg), Air temperature (Ta), Relative humidity (Rh), wind speed, wind direction, rainfall, vertical profile of air temperature (TCs). Energy balance will be assessed by measurements of net radiation (Rn) and estimation of the energy partitioning among heat fluxes and heat storage. Results obtained at the ecosystem scale by these methods will be compared to gas exchanges measured at the level of the different compartments (canopy, trunk, root system, soil, etc). Thereby, the validated CO2 and H2O fluxes will be used to model gas exchanges of rubber plantation ecosystem according to climate and other environmental parameters as well as crop management.

International audience; In Thailand, rubber is the major tree crop covering more than 2 millions hectares. Its impact on environment, and particularly CO2 balance and water use, is a major issue. A complete flux-measuring system has been installed in a 13 years old tubber plantation (clone RRIM 600) in eastern Thailand (Chachoengsao Rubber Research Center, RRIT-DOA) in 2006. Carbon fluxes of rubber plantation ecosystem are continuously measured by eddy-covariance method (CO2 exchanges between the ecosystem and the atmosphere). Evapo-transpiration (ET) are measured by eddy-covariance and water balance together and partitioned between tree transpiration and soil evaporation. Sapflow is measured by heat dissipation method (Granier 1985, 1987) using home-made 20 mm-long radial probes, continuously heated (0.2 W). Meanwhile, amounts of C stored in the trees are evaluated by measuring biomass increment along the life cycle of the plantation, in combination with estimations of the carbon content of the different compartments. These measurements will provide the annual balance of C within plantations at different ages. Results obtained at ecosystem scale by these methods will be compared to gas exchanges measured at the level of the different compartments (canopy, trunk, root system, soil...). Particularly, Farquhar's photosynthesis model (Farquhar et al. 1980) is parameterized across the canopy. Leaf water potential, sapflow and canopy transpiration are computed to calculate canopy conductivity. Thereby, validated CO2 and H2O fluxes will be used to model gas exchanges of rubber plantation ecosystem according to climate and other environmental parameters as well as crop management.

Carbohydrate reserve storage in trees is usually considered a passive function, essentially buffering temporary discrepancies between carbon availability and demand in the annual cycle. Recently, however, the concept has emerged that storage might be a process that competes with other active sinks for assimilate. We tested the validity of this concept in Hevea brasiliensis Mull. Arg. (rubber) trees, a species in which carbon availability can be manipulated by tapping, which induces latex regeneration, a high carbon-cost activity. The annual dynamics of carbohydrate reserves were followed during three situations of decreasing carbon availability: control (no tapping), tapped and tapped with Ethephon stimulation. In untapped control trees, starch and sucrose were the main carbohydrate compounds. Total nonstructural carbohydrates (TNC), particularly starch, were depleted following bud break and re-foliation, resulting in an acropetal gradient of decreasing starch concentration in the stem wood. During the vegetative season, TNC concentration increased. At the end of the vegetative season, there were almost no differences in TNC concentration along the trunk. In tapped trees, the vertical gradient of starch concentration was locally disturbed by the presence of the tapping cut. However, the main effect of tapping was a dramatic increase in TNC concentration, particularly starch, throughout the trunk and in the root. The difference in TNC concentration between tapped and untapped trees was highest when latex production was highest (October); the difference was noticeable even in areas of the trees that are unlikely to be directly involved in latex regeneration, and it was enhanced by Ethephon stimulation, which is known to increase latex metabolism and flow duration. Thus, contrary to what could be expected if reserves serve as a passive buffer, a decrease in carbohydrate availability resulted in a net increase in carbohydrate reserves at the trunk scale. Such behavior supports the view that trees tend to adjust the amount of carbohydrate reserves stored to the level of metabolic demand, at the possible expense of growth.

absent