Search

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 first trends indicated that the mean residence time (MRT) of soluble sugars did not vary (36-45 h in June and October), whereas the dynamics of quebrachitol were different, indicating possible different sources of C for this compound believed to play a key role in osmotic regulation.

International audience; Beech (Fagus sylvatica L.) is a shade tolerant species of broadleaved temperate forests from the northern hemisphere. However, how it manages to survive low irradiance conditions is debated. This must involve both carbon balance components (mainly photosynthesis vs. respiration) and efficient utilization / partitioning of the available C resource among the different sinks (mainly growth vs. reserve storage). This survival strategy was investigated through a combined experimental and modelling approach. Four-year-old beech seedlings growing in the understorey of natural Scots pine stands were 3D-digitized in summer for architecture, including woody parts (growth units) and leaves. Then seedlings were harvested in autumn and winter to quantify biomass and carbohydrate reserve contents of each organ, both above and below ground. Roots were separated into 3 classes: taproot, main lateral roots and fine roots, and shoot segments according to their age. These data were used to construct 3D-digital mockups of the young trees and to parameterize and initialize the carbon-based functional-structural model PIAF-1. Additional beech-specific parameters relevant to phenology and gas exchange rates were derived from similar experiments or literature. Meteorological data (daily radiation and temperature) were generated for each tree based on recorded weather data. Transmitted solar irradiance above each seedling was calculated from hemispherical photographs and the global radiation above the stand. The first results suggest that under the most restrictive light conditions, the reserve store was just sufficient for the spring outgrowth. The major factor allowing the seedling to pass the short but critical near-zero-reserve phase appeared to be the leaf growth strategy, with surface area expanding prior to structural hardening. The reserve mobilization strategy showed only as a second factor, with the mobilization rate likely not limiting in contrast to the total reserve store.

International audience; Beech (Fagus sylvatica L.) is a species of temperate broadleaved forests known to be very shade-tolerant. How does it manage to sustainably cope with very limited light resource? Processes must involve both C balance components (mainly photosynthesis vs. respiration) and efficient utilization/partitioning of the available C resource among the different sinks (mainly growth vs. reserve storage). We investigated them by combined experimental and integrated modelling approach.

International audience; The European beech (Fagus silvatica L.) is a late-successional shade tolerant species. It is often cultivated and managed with the shelterwood system in Europe, meaning that its natural regeneration takes place in shade. Under low light conditions, seedling size must stay below a threshold where carbon gain and carbon balance remain positive. This is not so easy to determine experimentally. Therefore, a simulation approach using the carbon-based functional-structural model PIAF-1 (Lacointe and Donès, 2007) was used to investigate the relationship between carbon balance and growth of young beech trees under low light availability. Two-year-old beech seedlings were planted in March 2007 in the understorey of natural Scots pine stands in Fontfreyde, Chaîne des puys, France. In summer 2009, nine beech seedlings were sampled under light availability ranging from 7 to 28% of incident light. To acquire their architecture, aboveground parts of each tree were 3D-digitized, including woody parts (growth units) and leaves using PiafDigit (Donès et al., 2006). Then seedlings were harvested in autumn and winter to quantify biomass and starch and sucrose contents of each organ, above and below ground. Roots were separated into 3 classes: taproot, main lateral roots and fine roots. Each part was scanned and the length and diameter of roots were assessed using WinRhizo (Regent Instruments Inc., Canada). These data were used to parameterize and initialize PIAF-1. The digitized aboveground parts were converted from growth units to cylindrical segments bearing leaves in their true place and orientation. Belowground parts were reconstructed based on diameter and length of each root class. Meteorological data (hourly light and temperature) were generated for each tree based on recorded weather data. Transmitted solar irradiance above each seedling was calculated from hemispherical photographs and the global radiation above the stand. Each seedling was visualized on a PIAF-1 3D visualization interface. The quality of the reconstructed seedlings was assessed by comparing the graphical outputs from PIAF-1 with those from VegeSTAR (Adam et al., 2002) using Tree Analyser (Phattaralerphong et al., 2005) to compute synthetic geometrical properties such as total leaf area or density. Potential growth, amount of storage, and reserve capacity of stem, leaf and root are still being analyzed. Results of simulations will be reported and discussed.

A method for computing leaf area of isolated trees from perspective photographs was developed. The method is based on gap fraction inversion. Photographs are discretized into picture zones where gap fraction is computed from image processing. Canopy volume and leaf area density associated with each picture zone are computed from geometrical considerations and inversion of gap fraction equations. Total leaf area and the vertical profile of leaf area are computed from the product of associated volume and its density. The method has been implemented in software called Tree Analyser, written in C++. The method has been tested by comparison with direct estimation of leaf area of three-dimensional (3D) digitized trees of walnut, peach, mango, olive and rubber. Estimated leaf area was sensitive to picture discretization, individual leaf size and leaf inclination distribution. Optimal size of picture discretization was 17 times projected leaf size. Total leaf area was estimated by using a set of eight photographs taken around the tree in the main horizontal directions: deviation ranged from -11% in peach tree to +5% in rubber tree. The method allows fast and nondestructive monitoring of leaf area of individual tree canopies. The next version of the method will include the estimation of 3D leaf area distribution within the canopy.