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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.

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.

Rubber tree (Hevea brasiliensis Mull. Arg.) radial growth dynamics were monitored with displacement sensors, together with latex production, to investigate three aspects of the dual production of latex and wood: (1) the usefulness of fine-scale dendrometric measurements as a physiological tool to detect water shortage through radial growth; (2) the dynamic aspects, both at the seasonal and at the multi-year scale, of the competition between latex and wood production; and (3) the spatial distribution of radial growth rates around the tapping cut. Radial growth of untapped control trees started with the onset of the rainy season and lasted until the onset of the dry season, ceasing completely during the driest period. Displacement sensors provided a sensitive means of detecting water shortage, with a clear correlation between diameter variations and changes in water availability (both daily evapotranspiration and monthly rainfall) over the whole annual cycle. However, the correlation was significantly disturbed in tapped trees. After resumption of tapping, the radial growth rate dropped sharply within two weeks and the effect persisted throughout the whole season, so that the cumulative growth of tapped trees was about half that of untapped trees, with the cumulative growth deficit reaching 80% for the period from mid-June to November. This long-known negative impact of tapping on growth was much stronger in the second year of tapping than in the first, whereas latex production increased significantly between the first and second year of tapping. The increased latex production, which could not be ascribed to climatic conditions, shows that the establishment of an artificial latex sink is a progressive, long-term process likely involving many aspects of metabolism. As expected, ethylene significantly increased latex production in both years; however, ethylene had no effect on the growth rates of tapped trees. Radial growth was differentially affected at different locations around the tapping cut, with growth rates significantly lower in the tapped panel than in the untapped panel, and higher above the cut than below the cut. Thus, caution is needed when deriving whole stem wood production from girth measurements at one location on the stem, especially from girth measurements made close to the tapping cut. This also provides new evidence for the location of the latex regeneration area in the tapped panel, below the cut.