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Past assessments of the global carbon balance have shown to have high uncertainty particularly in Mainland Southeast Asia (MSEA) owing to the increasing climate extremes and land use changes. Recently, remotely sensed solar-induced chlorophyll fluorescence (SIF) and near-infrared reflectance of vegetation (NIRv) have been found to have great potential to assess the dynamics of vegetation gross primary production (GPP). Using flux tower GPP from two rubber plantation and one natural forest sites, our study assessed the utility of SIF and NIRv to detect GPP and drought stress for these two major land uses in this tropical monsoon region, and the underlying mechanistic link among SIF, NIRv, and GPP based on path analysis. The results indicate that the native SIF from OCO-2 satellite and its two derived finer-resolution SIF products (i.e., GOSIF and ) outperformed the normalized difference vegetation index (NDVI) and the enhanced vegetation index (EVI) in detecting GPP and drought effects at the rubber plantation sites. Although the young natural forest exhibited much lower EVI and NIRv, it had comparable magnitude of GPP to the rubber plantations, which was captured by SIF. Path analysis shows that the SIF-GPP relationship was mainly controlled by NIRv, which represents the canopy structural effects including both the absorption and scattering traits. In line with site-scale analysis, the regional patterns indicated higher consistency between SIF and NIRv than betwen SIF and EVI for the tropical forests. The quantum yield of SIF () was sensitive to precipitation anomaly, while its negative effect on photosynthesis was offset by high PAR under drought. Both tropical deciduous forest and plantation forest were more vulnerable to drought than evergreen forest. We highlight the usage of NIRv and SIF for monitoring canopy structure and photosynthesis, respectively, and the needs of higher-resolution observations to reduce the uncertainties in quantifying SIF-GPP relationships.

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.

Yield gaps between rubber smallholdings and rubber agro-industries often exist. These gaps are usually important regarding land productivity (kg/ha) but even more important regarding labor productivity (kg/tapper/day). However, technical packages of GAP (good agricultural practices) are available from decades of research in breeding, physiology, agronomy, crop protection and latex harvesting technology. Regarding latex harvesting, the differences between agro-industries and smallholdings are very often even more important than for other disciplines. Reduced tapping frequencies compensated by accurate stimulation intensities or controlled upward tapping are scarcely encountered in smallholdings. Other quality standards are also often less respected, mainly regarding bark consumption, bark wounding and homogenous panel management. In Thailand, smallholders own 85% of the total rubber area. In the southern and eastern regions of the country, climate conditions with heavy rains during the rainy season, associated with rubber price fluctuation, lead farmers to use high frequency tapping systems (S/3 d1 2d/3 or S/3 d1 3d/4 mainly) in order to compensate the reduction of the number of tapping days due to rains. Labor shortage is also a new and increasing issue for farmers who hire tappers. To improve labor productivity in each farm and address the increasing labor shortage, one way might be to reduce the time spent by tappers in the field, using low frequency tapping systems (LFT). LFT systems combine reduction of tapping frequency with Ethephon stimulation. Under accurate stimulation, yield significantly improves at each tapping, leading to a higher labor productivity (g/t/t and kg/tapper/day) and this can at least partly compensate the effect of the reduction of the tapping frequency on production. The objectives of this publication are (i) to assess the efficiency of different LFT systems with Ethephon stimulation on yield, labor productivity and latex physiological parameters and (ii) to select among those systems the ones showing an improved efficiency regarding labor productivity, in order to test them on farm.

This research improved the model for the radial distribution of the sap flux density in rubber trees. The objective of the study was to describe the radial variability of xylem sap flux in rubber trunks. The experiment was conducted in mature rubber trees aged 13 yr from clone RRIM 600, in a non-traditional planting area at the Chachoengsao Rubber Research Center from January to December 2007. The sap flow was measured by the heat dissipation method using home-made radial probes and then calibrated with the cut stems of rubber tree in the laboratory. The radial variability was modeled as a function of the depth into the xylem. Regression analysis was carried out using data on the sap flux density and depth into the xylem. The model used two basic assumptions: (1) young xylem (0–4 cm) has the maximum sap flux density and is stable and (2) sap flux density decreases linearly with the depth into the xylem (and thus older xylem) toward the center of the stem. The results showed that the model for radial distribution can be used to predict the sap flux density and to estimate the tree transpiration of different trunk diameters in rubber trees. The tree transpiration was highly variable with different girths of the rubber trees.

Natural Rubber links global industry to millions of family farms. To ensure the sustainability of plantations, conditions should remain favourable during one or two decades. How can such conditions be ensured when the environment is changing? From farm-survey, ecological studies and experiments a multi-disciplinary methodology is developed in Thailand to identify significant indicators to assess the long-term adaptation and sustainability of smallholder's plantation systems. Risks and adaptations related to climate. The increase in global temperature will affect directly agriculture, particularly in the tropical area (IPCC 2007). Drought is the more likely climatic risk as rubber plantations expand towards drier areas, and due to the more irregular rain patterns. The hydraulic properties of rubber clones are considered a key for their adaptability to drought. It is also relevant to assess the effects of drought on soil biology, which maintains soil fertility. Extreme events, like flooding, are likely to increase as well but their actual occurrence and effects on rubber trees and farms is not well documented. As rainfall is becoming more erratic, farmers tend to adopt irregular tapping patterns. Assessing the resulting physiological status of trees will help identify adaptive strategies. Changes in socio-economic context. Rubber price fluctuations have greatly affected the sector recently. This is a risk for farmer's budget but also an incentive for flexible production systems. Plantations are extending towards new areas where their sustainability is questioned, whereas new investors get involved. These changes will affect the functioning of both the field-plant system and the production system. There is a need to identify the way socioeconomic factors interact with biophysical factors to determine farmers' strategies regarding risks induced by global changes. Environmental issues. The impact of rubber plantations on natural resources and associated environmental services has been poorly studied. Environmental impacts affect the plantation in terms of soil sustainability (soil fertility preservation related to soil functional diversity) and generate externalities. We consider the risk linked to the extension of rubber plantations in sub-optimal regions and the risk of soil degradation in the traditional area of cultivation after 50 years of continuous rubber cropping.

Poster + résuméPoster + résumé; absent

Transpiration at the leaf and stand level is mainly controlled by climatic conditions. The diurnal dynamics of stand transpiration and stomatal conductance were analyzed in rubber trees. The experiment was conducted using 13 yr-old rubber trees from clone RRIM 600 under natural conditions at the Chachoengsao Rubber Research Center during January to December 2007, with sampling in the dry season, the rainy season and early in the dry season. At the stand level, the stand transpiration was monitored using sap fl ow measurement and estimated evapotranspiration from meteorological data. At the leaf level, the stomatal conductance was measured using a portable photosynthesis system on the leaves exposed to the sun. The leaf water potential was measured using a pressure chamber and the soil water content was measured by the gravimetric method. Climatic measurements were recorded using a weather station above the canopy. The plant water status was correlated with the climatic conditions and soil water content. The diurnal dynamics of stand transpiration and stomatal conductance were mainly controlled by the vapor pressure defi cit and net radiation. The stomatal conductance was more sensitive to climatic variations than stand transpiration. There were different time lags between the stomatal conductance and stand transpiration throughout the year. The relationship between the climatic factors and transpiration varied throughout the year.

The impact of drought conditions on the water relations of rubber trees is important because the trees could shed leaves while they are being continuously tapped. The aim of this work was to study the effect of soil drought and the tapping of latex on the water relations of rubber trees. This study distinguished between untapped trees and trees affected by tapping activity. The experiment was conducted on 14-15 year-old rubber trees from the clone, RRIM 600, at the Chachoengsao Rubber Research Center (CRRC). Climatic measurement, soil water content, predawn and midday leaf water potential, stand transpiration and percentage loss of hydraulic conductivity (PLC) were measured from January 2009 to January 2010, especially in the dry season. Under drought conditions, during the rainy season, stand transpiration and predawn leaf water potential decreased in response to soil drought in August. The midday leaf water potential and PLC did not change following soil water stress. In the dry season, the plant water status was related to climatic variables and soil drought. The results indicated that drought induces a reduction in the plant water status of rubber trees. Tapping activity did not have a marked infl uence on the water balance of the rubber trees. However, tapping reduced the sap fl ux density and stomatal conductance; thus, attention must be paid to tapping in the dry season, particularly in dry areas.

The effect of high temperature on photosystem II efficiency was studied in leaves of 16 month-old Phalaenopsis 'Sweetheart' plants grown under controlled cabinets. The various temperatures applied to leaves were 28, 35, and 47°C, for 30, 45, and 60 minutes, respectively. The distal half of individual leaves was divided into a shaded zone and illuminated zone and monitored the photosynthetic yield (PSII efficiency) by Walz MINI-PAM system. A PSI Fluorcam 700MF chlorophyll fluorescence imaging system was used to measure the dark-adapted Fv/Fm of the different leaf zones before and at the end of high temperature treatment, and then after 24 hours of recovery at 28°C. The Phalaenopsis leaves maintained a high photosynthetic yield (within 83% to 100% of maximum values) over a wide range of leaf temperatures, from 21°C to 45°C. The critical temperature, at which photosynthesis yield began to decrease sharply, was approximately 45°C. Leaf zones exposed to high temperature (47°C leaf temperature) showed an approximately 22% recovery in mean Fv/Fm after being transferred to 28°C growth cabinet for 24 h. After the 47°C leaf treatment, CF imaging system revealed the variation in the Fv/Fm distribution increased significantly and after 24 h of recovery reduced this variation substantially. The main source of variation in the Fv/Fm results was variation between leaves rather than within leaves. 'High temperature stress test' based on CF technology could be developed to sort Phalaenopsis plants by temperature tolerance levels to guarantee the certain quality for commercial application.

communication orale; International audience

The objective of this study was to apply the QTL mapping by Markers-Assisted Selection approach for early selection on growth and latex yield in rubber breeding. The plant material consisted of 196 progenies derived from the F1 family RRIM 600 x PB 217. A genetic linkage map was built for this family with 229 SSR markers (microsatellites) and 198 AFLP markers. Phenotyping was carried out over a 6 years period on a field trial of 5 hectares, with around 2,400 trees measured individual. The two major QTLs were detected repeatedly. The QTL g16-6 was associated with latex yield near the position of marker a131. This QTL explained up to 66% of the phenotypic variance. It was also associated to other traits that were strongly correlated to production such as inorganic phosphorus and dry rubber content (latex diagnostic), as well as plugging index. This finding indicates the existence of one major gene (or a cluster of genes) located on linkage group g16 and involved in the intensity of metabolic activity of latex cell. A second important QTL associated with girth of the trunk (growth) was detected (QTL g3-60) at the position of marker a312. It explained up to 31% of the phenotypic variance. The discovery of the two major QTLs g3-60 and g16-6 suggest that two major genes act as limiting factors in the genetic determinism of the growth in girth during favorable condition and of rubber production in low-intensive tapping systems. These loci should become privileged targets for the identification of key-genes in rubber. The possible use of their neighboring markers for developing Markers-Assisted Selection (MAS) in the short run appears very reasonable. It should contribute to improve the accuracy of the first selection stage which is the weakest point of the rubber clonal selection scheme.

Shoots of the tropical latex-producing tree Hevea brasiliensis (rubber tree) grow according to a periodic pattern, producing four to five whorls of leaves per year. All leaves in the same whorl were considered to be in the same leaf-age class, in order to assess the evolution of photosynthesis with leaf age in three clones of rubber trees, in a plantation in eastern Thailand. Light-saturated CO(2) assimilation rate (A (max)) decreased more with leaf age than did photosynthetic capacity (maximal rate of carboxylation, V (cmax) , and maximum rate of electron transport, J (max)), which was estimated by fitting a biochemical photosynthesis model to the CO(2)-response curves. Nitrogen-use efficiency (A (max)/N(a), N(a) is nitrogen content per leaf area) decreased also with leaf age, whereas J (max) and V (cmax) did not correlate with N (a). Although measurements were performed during the rainy season, the leaf gas exchange parameter that showed the best correlation with A (max) was stomatal conductance (g (s)). An asymptotic function was fitted to the A (max)-g (s) relationship, with R (2) = 0.85. A (max), V (cmax), J (max) and g (s) varied more among different whorls in the same clone than among different clones in the same whorl. We concluded that leaf whorl was an appropriate parameter to characterize leaves for the purpose of modelling canopy photosynthesis in field-grown rubber trees, and that stomatal conductance was the most important variable explaining changes in A (max) with leaf age in rubber trees.

The objective of this study was to analyse the genetic determinism of growth and latex production in rubber tree by a QTL approach. The plant material consisted of 196 progenies derived from the F1 family RRIM600 x PB217. A genetic linkage map was built for this family with 229 SSR markers (microsatellites) and 198 AFLP markers. Phenotyping was carried out over a 6-year period on a field trial of 5 hectares, with around 2400 trees measured individually. A high broad sense heritability was found for latex production, and a major QTL directly associated to production was discovered (Hbg16a131). This QTL explained from 32% to 59% of the genetic variance of the production traits. It was also associated to other traits that were strongly correlated to production such as inorganic phosphorus and dry rubber content (latex diagnostic), as well as plugging index. This finding indicates the existence of one major gene (or a cluster of genes) located on linkage group 16 and involved in the genetic determinism of latex production. A second important QTL associated to the girth of the trunk (growth) was detected (Hbg3a312). Depending on the age of the trees, it explained from 16% to 28% of the genetic variance of the girth. By contrast, no significant QTL was found for sucrose content of the latex, as estimated independently from the production, so suggesting that sucrose genetic determinism might not include any single gene having by itself some important effect. The application of one ethephon stimulation did not evidence any specific QTL. Those results pave the way for Markers-Assisted Selection (MAS) as applied to clonal selection in rubber. Genotyping the candidates to selection for the 2 QTLs with important effects should allow a much more accurate estimation of genetic values as soon as at the first selection stage.