Using 13CO2 Pulse-Labelling to Track Carbon from Photosynthates to Latex in Rubber Trees (Hevea brasiliensis Muell.Arg)
Thèse soutenue en 2022 - Kasetsart University, Thailand
Rubber trees are the main source of natural rubber, extracted from the latex that exudes from the trunk after tapping. Tapped trees require large amounts of carbon (C) to regenerate latex, which represent an additional C demand because of the amount of latex that flows out and the richness in C of rubber. Therefore, C supply is key for sustainable latex production, but the source of latex C remains poorly known. Does it come directly from the leaves where CO2 is assimilated or from starch stored in the wood? Pulse labelling trees with 13CO2 are valuable approaches to study carbon allocation; however, it is challenging for trees in the field. Thus, the objectives of this study were to determine the contribution of C sources in latex biosynthesis (recent photosynthates versus stored carbohydrates) and if there is a shift in their respective contribution and in C dynamics during the tapping period. Large temperature-controlled chambers (35–45 m3) that enclosed the entire crown of a tree, provided a reliable estimate of tree crown photosynthesis, and ensured efficient 13CO2 labelling were developed and tested. Whole tree crowns of 4-year-old rubber trees were pulse-labelled with 13CO2 for 30–40 minutes. Labelling was performed in June when latex production was low (start of tapping) and in October, when it was high. The 13C contents were quantified in the foliage, phloem sap, latex and trunk wood. The labelling experiment showed that, in both labelling periods, 13C was recovered in the latex just after labelling, indicating that part of the photosynthates were directly allocated to latex. However, significant amounts of 13C were still recovered in latex after 60 days and the peak was reached significantly later than in phloem sap, demonstrating the contribution of a ‘reserve’ pool as another source of latex C. In June, the latex C came from a pool where recent C (photosynthates) mixed with older C (stored starch) but a significant part of recent C was nevertheless invested in storage. In contrast, in October, the recovery of 13C was faster and stronger in latex, indicating a high contribution of recent C coming directly from leaf photosynthesis. The contribution of new photosynthates to latex regeneration was therefore faster and higher when the latex metabolism was well established in October compared to June. To conclude, 13C pulse-labelling proved efficient to study the origin of the latex C. Overall, latex C comes from a pool where newly assimilated C mixes with older one, but their respective contribution varies seasonally. An improved understanding of C dynamics and source-sink relationships in rubber trees is crucial to adapt tapping system practices and ensure sustainable latex production.
