Variability of leaf traits related to photosynthesis was assessed in seedlings from 14 tree species growing in the tropical rain forest of French Guiana. Leaf photosynthetic capacity (maximum rate of carboxylation and maximum rate of electron transport) was estimated by fitting a biochemical model of photosynthesis to response curves of net CO2 assimilation rate versus intercellular CO2 mole fraction. Leaf morphology described by leaf mass per unit leaf area (LMA), density and thickness, as well as area- and mass-based nitrogen (N) and carbon (C) concentrations, were recorded on the same leaves. Large interspecific variability was detected in photosynthetic capacity as well as in leaf structure and leaf N and C concentrations. No correlation was found between leaf thickness and density. The correlations between area- and mass-based leaf N concentration and photosynthetic capacity were poor. Conversely, the species differed greatly in relative N allocation to carboxylation and bioenergetics. Principal component analysis (PCA) revealed that, of the recorded traits, only the computed fraction of total leaf N invested in photosynthesis was tightly correlated to photosynthetic capacity. We also used PCA to test to what extent species with similar shade tolerances displayed converging leaf traits related to photosynthesis. No clear-cut ranking could be detected among the shade-tolerant groups, as confirmed by a one-way ANOVA. We conclude that the large interspecific diversity in photosynthetic capacity was mostly explained by differences in the relative allocation of N to photosynthesis and not by leaf N concentration, and that leaf traits related to photosynthetic capacity did not discriminate shade-tolerance ranking of these tropical tree species.

-This study looked at the interactive effects of tree architectural stage of development (ASD) and light availability on different plant traits (growth parameters, leaf morpho-anatomy and photosynthetic capacities) in the tropical species Dicorynia guianensis. A qualitative architectural analysis was used to categorize tree individuals sampled along a natural light gradient. The results show that some traits could have an ASD-dependence at the whole plant and leaf level without control of light. The changes observed relate to vigour thresholds the plant has to reach to shift from one ASD to another (i.e., the number of nodes and the internodes length per Growth Unit). Light conditions do not modify these thresholds but may modify the time they are crossed. Tree height was found strongly modulated by light conditions; hence, at a similar height, individuals may belong to different ASD. At the functional level, a decrease in N m , and A maxm was observed with increasing light availability, while N a increased and A maxa remained unaffected. An ASD effect was also observed on A maxa and LMA but not on A maxm . These results demonstrated a weak ability of photosynthetic plasticity in response to light conditions, and that variations of leaf photosynthetic variables according to ASD can be explained by modifications in leaf nitrogen and LMA. Questions on the reliability of a height-based sampling strategy for evaluating the phenotypic plasticity of trees in relation to light conditions are raised.Dicorynia guianensis / leaf structure / functional leaf traits / plasticity / tree structure Résumé -Relations entre architecture des arbres et traits fonctionnels foliaires de l'angélique Dicorynia guianensis Amshoff (Caesalpiniaceae) en forêt tropicale humide. Les relations entre stades architecturaux de développement (ASD), morpho-anatomie foliaire et capacités photosynthétiques ont été étudiées chez Dicorynia guianensis, une espèce forestière de Guyane. Les ASD ont été définis à l'aide de critères qualitatifs par une méthode simple de description architecturale puis échantillonnés le long d'un gradient naturel de lumière. Les résultats montrent que chaque ASD peut être identifié par un syndrome de caractères quantitatifs. Ces caractères évoluent d'un stade à l'autre, et dans les différents milieux, selon une séquence ordonnée d'événements qui se manifestent pour des seuils de vigueur donnés. Les conditions lumineuses ne modifient pas ces seuils mais avanceraient ou différeraient dans le temps leur passage. La hauteur des individus et le LMA sont fortement modulés par la lumière. Ainsi, pour une même hauteur, des individus peuvent correspondre à des ASD variés. Au niveau fonctionnel, l'augmentation du rayonnement incident s'est traduite par une diminution de N m , et de A maxm et par une augmentation de N a , tandis que A maxa n'a pas été affectée. Un effet marqué de l'ASD a été constaté sur A maxa et LMA mais pas sur A maxm . Ces résultats révèlent une faible plasticité photosynthétique chez D. guianensis et mon...

-Growth and net assimilation rate of seedlings of ten Guianan tree species grown under five light regimes. Seedlings of ten species of tropical rainforest trees (Bocoa prouacensis, Carapa guianensis, Carapa procera, Jacaranda copaia, Platonia insignis, Qualea rosea, Recordoxylon speciosum, Sterculia excelsa, Virola surinamensis, Vouacapoua americana) occurring in successional phases were grown in shade tunnels under five relative light levels (65 %, 45 %, 25 %, 10 % and 4 % of full sunshine). The study was designed to assess the behaviour of the species in juvenile stages versus different light growth conditions. Mainly net assimilation and growth rates were recorded. For seedlings growing under 65 % of full sunlight, maximum net assimilation rates were fairly variable between the species and ranged from 3.0 to 7 μmol m -2 s -1 and the total dry matter production ranged from 8 to 286 g per plant for plants grown for 20 months. The net assimilation rate of Carapa guianensis decreased by almost 40 % between 45 % and 65 % of full sunlight. The growth performance was substantially reduced at the lowest irradiance, but the pattern varied highly between species with increasing light intensity. The