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Measuring the photosynthesis of entire tree crowns and pulse labelling trees with 13CO2 are valuable approaches to study carbon acquisition, transfer, and allocation; however, it is challenging for trees in the field. The objective was to develop and field test large 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. The chambers, made of transparent polyethylene film pulled tightly over a frame, were equipped with an air conditioner, fans, and air blowers. Air temperature, relative humidity, and photosynthetic photon flux density were measured outside and inside each chamber. Six of 4-year-old rubber trees (Hevea brasiliensis) were pulsed-labelled with 18 L of 13CO2 in June and October 2016. The mean air temperature inside the chambers was 1.2 °C higher and the relative humidity 8 % lower than the outside air. The crown photosynthesis, calculated from the decrease in the CO2 concentration inside the chamber, was in the range 140 - 249 µmol s–1 and was significantly related to photosynthetic photon flux density, total leaf area of the tree, and average net CO2 assimilation at leaf level. The labelling efficiency, estimated as the ratio of the amount of 13C recovered in the foliage immediately after labelling divided by the amount of 13C delivered to the tree, was in the range 43 - 68 %. The designed chamber was suitable to estimate crown photosynthesis and perform 13CO2 pulse labelling of 5-m-tall trees in the field.

The carbon (C) content of dry latex is about 80%. Then, the rubber trees must mobilize huge amount of C to regenerate the latex exported at each tapping. Does the latex C come directly from the primary sources, the leaves where C is assimilated through photosynthesis, or from reserve pools as wood starch, or both? Knowing the actual C sources and knowing the pathways towards latex is necessary to manage the tapping systems. Stable isotopes and especially 13C are widely used in plant science as tracers. We realized a field labeling of full crowns of 3y-old tapped rubber trees with 13CO2 to trace the carbon from its assimilation in the leaves to the tree sinks and particularly to latex. Such experiment has never been done so far on rubber trees. Three trees (RRIT 408) were labelled in June and three other in October, using a specifically designed chamber. We sampled leaves, phloem, wood and latex to analyze their 13C content and determine the dynamics of carbon allocation from leaves to latex. Latex is being sampled during one year. The first results showed that 13C was recovered later in latex than in phloem, indicating that most the latex C does not come directly from recent assimilation. The dynamics showing a peak of 13C in latex 10-15 days after labelling in June are consistent with the hypothesis that newly assimilated C is mixed in a pool of older carbon (reserves) before being used to regenerate latex. However, the dynamics in October showed an earlier (6-8 days after labelling) and much higher peak. This showed that when the regeneration metabolism was well established the transfer of recent assimilates into latex was faster. In both cases 13C was still recovered in significant amount more than 40 days after labelling, demonstrating the contribution of reserves. The dynamics of 13C recovery in soluble compounds (sugars and quebrachitol) in the phloem and in the latex C-serum will provide further information on their transport and use in laticifer cells. The f

One of the main challenges for the future of Natural Rubber production is the scarcity of skilled manpower to tap the trees. The only way to cope with such issue is to reduce the tapping frequency. The key is the carbon supply to the latex producing tissues. With low tapping frequencies, the latex exported at each tapping day is higher than in traditional systems. Then the trees must mobilize huge amount of carbon at each tapping. Does the latex carbon come directly from the primary sources, the leaves where C is assimilated through photosynthesis, or from reserve pools as wood starch, or both? Knowing the actual C sources and knowing the pathways towards latex is then necessary to manage the tapping systems. Stable isotopes and especially 13C are widely used in plant science as tracers but studying their natural abundance also provides insightful information on tree physiology. We first compared the seasonal variations in natural abundance in tapped and untapped latex of RRIM600 trees to the variations in their leaves which vary according to climate and phenology. We showed that latex δ13C was higher and varied much less than that of leaves in tapped trees. The lack of correlation between variations in δ13C in leaves and in latex suggests that recent photosynthates are mixed in a large pool of stored carbohydrates that are involved in latex regeneration after tapping. We then did a field labeling of full crowns of 3y-old tapped rubber trees with 13CO2 to trace the carbon from its assimilation in the leaves to the tree sinks and particularly to latex. Such experiment, using a specifically designed chamber, has never been done on rubber trees. Three trees (RRIT 408) were labelled in June and three other in October. We sampled leaves, phloem, wood and latex to analyze their 13C content and determine the dynamics of carbon allocation from leaves to reserves and latex. The latex samples will be followed during one year. The presentation describes the methodology and pre