Carbohydrate storage in wood and bark of rubber trees submitted to different level of C demand induced by latex tapping

Abstract

When the current level of carbohydrates produced by photosynthesis is not enough to meet the C demand for maintenance, growth or metabolism, trees use stored carbohydrates. In rubber trees (Hevea brasiliensis Muell. Arg.), however, a previous study (Silpi U., A. Lacointe, P. Kasemsap, S. Thanisawanyangkura, P. Chantuma, E. Gohet, N. Musigamart, A. Clement, T. Améglio and P. Thaler. 2007. Carbohydrate reserves as a competing sink: evidence from tapping the rubber tree. Tree Physiol. 27:881-889) showed that the additional sink created by latex tapping results not in a decrease, but in an increase in the non-structural carbohydrate (NSC) storage in trunk wood. In this study, the response of NSC storage to latex tapping was further investigated to better understand the trade-off between latex regeneration, biomass and storage. Three tapping systems were compared to the untapped Control for 2 years. Soluble sugars and starch were analyzed in bark and wood on both sides of the trunk, from 50 to 200 cm from the ground. The results confirmed over the 2 years that tapped trees stored more NSC, mainly starch, than untapped Control. Moreover, a double cut alternative tapping system, which produced a higher latex yield than conventional systems, led to even higher NSC concentrations. In all tapped trees, the increase in storage occurred together with a reduction in trunk radial growth. This was interpreted as a shift in carbon allocation toward the creation of reserves, at the expense of growth, to cover the increased risk induced by tapping (repeated wounding and loss of C in latex). Starch was lower in bark than in wood, whereas it was the contrary for soluble sugars. The resulting NSC was twice as low and less variable in bark than in wood. Although latex regeneration occurs in the bark, changes related to latex tapping were more marked in wood than in bark. From seasonal dynamics and differences between the two sides of the trunk in response to tapping, we concluded that starch in wood behaved as the long-term reserve compartment at the whole trunk level, whereas starch in bark was a local buffer. Soluble sugars behaved like an intermediate, ready-to-use compartment in both wood and bark. Finally, the dynamics of carbohydrate reserves appears a relevant parameter to assess the long-term performance of latex tapping systems.