Search

International audience; Introduction Light modelling at the scale of organs is essential to account accurately for the complex interactions between biophysical processes such as photosynthesis, stomatal conductance and energy balance. Yet, the calculation of radiative exchanges at fine scales is computationally-intensive and it remains a hindrance to a widespread use of FSPMs despite advances in light modelling using either radiosity (Chelle and Andrieu, 1998) or raytracing (Bailey, 2018). This study shows that simplifications based on the discretization of radiative fluxes allow processing radiative exchanges in a natural environment while maintaining good accuracy on the simulation of biophysical processes such as carbon assimilation.

Functional-structural plant modelling approaches (FSPM) open the way for exploring the relationships between the 3D structure and the physiological functioning of plants in relation to environmental conditions. FSPMs can be particularly interesting when dealing with perennial crops like oil palm, for which research on innovative management practices requires long and expensive agronomic trials. The present study is part of the PalmStudio project, which aims at developing a FSPM for oil palm capable of conducting virtual experiments to test the relevance of innovative management practiceds and/or design ideotypes. We propose a methodological approach which integrates architectural responses to planting density in an existing oil palm FSPM (Perez et al. 2018a b). Combining standard field phenotyping with Lidar-based derived measurements, we manage to evaluate the phenotypic plasticity of the main parameters required for the calibration of the 3D plant model. LiDAR scans were processed using the PlantScan3D software (Boudon et al. 2014) to derive phenotypic traits of leaf geometry that were compared to labour-intensive measurements. Density-based allometries of leaf geometry and biomass are then derived from the observed variations in phenotypic traits and integrate into the FSPM. Our results illustrate the accuracy and the efficiency of Lidar-based phenotyping of leaf geometrical traits. In average, we find less than 3% of difference in leaf dimensions (i.e. rachis length) in comparison with traditional hand-made field measurements. The fast and efficient measurements of usually labour-intensive traits such as leaf curvature allowed estimating the plasticity of leaf geometry in response to density. We find that the main traits affected by density were leaf dimensions (up to 15% and 25% of increase in rachis length and petiole length respectively) and curvature (15% of increase in leaf erectness-related parameter), whereas other structural traits like the number of leaflets per leaf remained unchanged. Simple density-based allometric relationships were then modelled and combined with the existing allometric-based 3D oil palm model VPalm (Perez et al. 2018a). These data also enable the development and the integration in VPalm of a biomechanical model simulating leaf curvature. The methodology presented in this study paves the way for a rapid integration of phenotypic plasticity in FSPMs. Our FSPM is now able to estimate how planting density affect not only plant architecture but also functional processes such as carbon assimilation and transpiration. Ongoing research aims at coupling the current FSPM with a carbon allocation model (Pallas et al. 2013) to simulate the retroactions of functioning processes on plant architecture together with environmental and agronomic conditions.

In this study we proposed to investigate the influence of oil palm architecture on the capacity of the plant to intercept light, by using 3D reconstructions and model-assisted evaluation of radiation-use efficiency. The first objective of this study was to analyse and model oil palm architecture and light interception taking into account genetic variability. A second objective was to explore the potential improvements in light capture and carbon assimilation by manipulating oil palm leaf traits and propose architectural ideotypes.Allometric relationships were applied to model these traits according to ontogenetic gradients and leaf position within the crown. The methodology allowed reconstructing virtual oil palms at different stages over plant development. Additionally, the allometric-based approach was coupled to mixed-effect models in order to integrate inter and intra progeny variability through progeny-specific parameters. The model thus allowed simulating the specificity of plant architecture for a given progeny while including observed inter-individual variability. The architectural model, parameterized for the different progenies, was then implemented in AMAPstudio to generate 3D mock-ups and estimate light interception efficiency, from individual to stand scales.Significant differences in leaf geometry (petiole length, density of leaflets and rachis curvature) and leaflets morphology (gradients of leaflets length and width) were detected between and within progenies, and were accurately simulated by the modelling approach. Besides, light interception estimated from the validated 3D mock-ups showed significant variations among the five progenies.Sensitivity analyses were then performed on a subset of architectural parameters to identify the architectural traits impacting on light interception efficiency and potential carbon assimilation over plant development. The most sensitive parameters over plant development were those related to leaf area (rachis length, number of leaflets, leaflets morphology), but fine attribute related to leaf geometry showed increasing influence when canopy got closed. In adult stand, optimized carbon assimilation was estimated on plants presenting a leaf area index (LAI) between 3.2 and 5.5 m2.m−2, with erected leaves, short rachis and petiole and high number of leaflet on rachis. Four architectural ideotypes for carbon assimilation were proposed based on specific combinations of organs geometry, limiting mutual shading and optimizing light distribution within plant crown.In conclusion, this study highlighted how a functional-structural plant model (FSPM) can be used to virtually explore plant biology. In our case of study, the 3D model of oil palm, in its conception and its application, permitted to detect the architectural traits genetically determined and influencing light interception. The limited number of traits revealed in the sensitivity analysis and the combination of traits proposed through ideotypes could guide further breeding programs. Forthcoming work will be dedicated to integrate in the modeling approach other physiological processes such as stomatal conductance and carbon partitioning. The improved FSPM could then be used to test different scenarios, for instance in climate change context with low radiations or frequent drought events. Similarly, the model could be used to investigate different planting patterns and intercropping systems, and proposed new multi-criteria ideotypes of oil palm.; Cette étude propose d’analyser l’influence de l’architecture du palmier à huile sur sa capacité à intercepter la lumière, en se basant sur des reconstructions 3D de palmiers et en établissant un bilan radiatif sur ses structures végétales reconstruites in silico. Le premier objectif de l’étude était de caractériser et modéliser la variabilité génétique de l’architecture du palmier à huile et de son interception lumineuse. Dans un deuxième objectif l’amélioration potentielle de l’interception de la lumière et de l’assimilation carbonée a été évaluée en modifiant les traits morphologiques et géométriques des feuilles et des idéotypes architecturaux de palmiers à huile ont été proposés.Des relations allométriques ont été utilisées pour modéliser les traits architecturaux en fonction de gradients ontogénétique et de topologie des feuilles dans la couronne. La méthode permet de reconstruire des palmiers à huile virtuels à différents âges au cours du développement. De plus, l’approche allométrique a été couplée à des modèles à effets mixtes pour intégrer au travers de paramètres la variabilité entre et au sein des cinq progénies. Le modèle permet ainsi de simuler les spécificités architecturales des cinq progenies en incluant les variabilités entre individus observés. Le modèle architectural, paramétré pour les différentes progénies, a ensuite été implémenté dans AMAPstudio pour générer des maquettes 3D de palmiers et ainsi estimer leur interception lumineuse, de l’individu à la parcelle entière.Les résultats de ces analyses ont révélé des différences significatives entre et au sein des progenies, dans la géométrie des feuilles (longueur du pétiole, densité de folioles sur le rachis, et courbure du rachis) et dans la morphologie des folioles (gradients de longueurs et largeurs le long du rachis). La comparaison virtuelle des différentes progénies ont aussi montré des efficacités distinctes de l’interception lumineuse.Des analyses de sensibilité ont ensuite été réalisées pour identifier les traits architecturaux influençant l’interception lumineuse et l’assimilation potentielle à différents âges de la plante. Les paramètres les plus sensibles au cours du développement furent ceux reliés à la surface totale foliaire (longueur des rachis, nombre de folioles, morphologie des folioles), mais les attributs géométriques plus fins de la feuille ont montré un effet croissant avec la fermeture de la canopée. Sur un couvert adulte, l’optimum en assimilation carbonée est atteint pour des indices de surfaces foliaires (LAI) entre 3,2 et 5,5 m2.m−2, avec des feuilles érigées, de courts pétioles et rachis et un nombre important de folioles sur le rachis. Quatre idéotypes architecturaux pour l’assimilation carbonée ont été proposés et présentent des combinaisons spécifiques de traits géométriques, limitant l’ombrage mutuel des plantes et optimisant la distribution de la lumière dans la couronne.En conclusion, le modèle 3D de palmiers à huile, dans sa conception et son application, a permis de détecter les traits architecturaux génétiquement déterminés et influençant l’interception lumineuse. Ainsi, le nombre limité de traits dégagés par l’analyse de sensibilité ainsi que les combinaisons de traits révélées au travers des idéotypes pourraient être pris en compte dans de futurs programmes de sélection. En perspective, des travaux dédiés à intégrer dans ce modèle d’autres processus physiologiques, tels que la régulation de la conductance stomatique et le partitionnement du carbone dans la plante, sont à envisager. Ce nouvel FSPM pourrait alors être utilisé pour tester différents scénarii, comme par exemple dans un contexte de changement climatique avec de faibles radiations et des périodes de sécheresse fréquentes. De même, ce modèle pourrait être utilisé pour étudier différentes configurations de plantation et des systèmes de cultures intercalaires, et ainsi proposer de nouveaux idéotypes multicritères

The development of new breeding strategies to find more sustainable and productive systems is a major challenge to cope with ceaseless increasing demands for vegetable oils, notably palm oil. Optimizing plant architecture to increase radiation interception efficiency could be an option for enhancing potential oil palm production. Indeed, studies in cereals showed great improvement of yields by altering plant architecture, in combination with agronomic practices. By analogy, we proposed to investigate the influence of oil palm architecture on the capacity of the plant to intercept light, by using 3D reconstructions and model-assisted evaluation of radiation-use efficiency. The first objective of this study was to analyse and model oil palm architecture and light interception taking into account genetic variability. A second objective was to explore the potential improvement in light capture and carbon assimilation by manipulating oil palm leaf traits and propose architectural ideotypes. Data were collected in Sumatra, Indonesia, on five progenies (total of 60 palms), in order to describe the aerial architecture from leaflet to crown scales. Allometric relationships were applied to model these traits according to ontogenetic gradients and leaf position within the crown. The methodology allowed reconstructing virtual oil palms at different stages over plant development. Additionally, the allometric-based approach was coupled to mixed-effect models in order to integrate inter and intra progeny variability through progeny-specific parameters. The model thus allowed simulating the specificity of plant architecture for a given progeny while including observed inter-individual variability. The architectural model, once parameterized for the different progenies, was then implemented in AMAPstudio to generate 3D mock-ups and estimate light interception efficiency, from individual to stand scales. Model validations were performed at different scales. Firstly at organ scale, the geometry of the stem, the leaves and the leaflets were compared between virtual mock-ups and actual plants measured in the field. Secondly, at plant scale with indicators derived from terrestrial laser scanning (TLS) to assess crown dimensions and porosity. These indicators integrated topological and geometrical information related to the amount of light intercepted by an individual. Finally, validations were performed at plot scale using hemispherical photographs (HP) to assess the variability of canopy openness for the five studied progenies. Significant differences in leaf geometry (petiole length, density of leaflets and rachis curvature) and leaflets morphology (gradients of leaflets length and width) were detected between and within progenies, and were accurately simulated by the modelling approach. The comparison of plant area obtained from TLS and virtual TLS highlighted the capacity of the model to generate realistic 3D mock-ups. The architectural variabilities observed at plot scale between and within progenies were also satisfactory simulated. Finally, light interception estimated from the validated 3D mock-ups showed significant variations among the five progenies. Sensitivity analyses (Morris method and metamodelling approach) were then performed on a subset of architectural parameters in order to identify the architectural traits impacting light interception efficiency and potential carbon assimilation over plant development. Daily carbon assimilation was estimated with a photosynthesis model coupled to the radiative balance model, which enabled to integrate the temporal and spatial variations of photosynthetic organ irradiances. The most sensitive parameters over plant development were those related to leaf area (rachis length, number of leaflets, leaflets morphology), although fine attribute related to leaf geometry showed increasing influence when canopy got closed. In adult stand, optimized carbon assimilation was estimated on plants presenting a leaf area index (LAI) between 3.2 and 5.5 m2.m−2, with erect leaves, short rachis and petiole and high number of leaflet on rachis. Four ideotypes were identified with respect to carbon assimilation, exhibiting specific geometrical features that optimize light distribution within plant crown and reduce mutual shading among plants. In conclusion, this study highlighted how a functional-structural plant model (FSPM) can be used to virtually explore plant biology. In our case, the 3D model of oil palm, in its conception and its application, enabled the detection of the architectural traits genetically determined and influencing light interception. The limited number of traits revealed in the sensitivity analysis and the combination of traits proposed through ideotypes could guide future breeding programs. Forthcoming work will be dedicated to integrate in the modeling approach other physiological processes such as stomatal conductance and carbon partitioning. The improved FSPM could then be used to test different scenarios, for instance in climate change context with low radiations or frequent drought events. Similarly, the model could be used to investigate different planting patterns and intercropping systems and propose new multi-criteria ideotypes of oil palm.

Climate change is very likely to impact on oi l palm productivity and raise new questions on how to tackle these changes. The development of new strategies to find more sustainable and productive systems is a major challenge to cope with ceaseless increasing demands of palm oil. Agronomic practices or the selection of genetic material better adapted to future climate could be an option to maintain the high productivity of oil palm. The evaluation of plant and system performances related to climate change is nevertheless difficult to implement, particularly with perennial plants. Functional-structural plant mode ls (FSPM) have been developed to explore the relationships between plant structure, plant functioning and environmental conditions. FSPM are thus practical tools to set up virtual experiments and test hypotheses concerning processes that could otherwise take years in actual field conditions. The main assumption underlying this project is the possibility to enhance potential crop production optimizing plant architecture in relation to radiation us e efficiency. In this way, the present study proposes the basis of an oil palm FSPM, focusing on modelling oil palm architecture and light interception. A modelling approach has thus been developed to reconstruct oil palm architecture from simple field measurements while integrating the genetic variability observed on 5 progenies with contrasting genetic origins. The AMAPstudio software (Griffon & de Coligny, 2014) was used to generate 3D mock- ups and estimate light interception efficiency on virtual plants for each progeny. Model evaluation was performed using terrestrial laser scan (TLS) and hemispherical photographs (HPs) to compare the quality of 3D reconstruction respectively at the individual and stand scale. Significant differences in leaf geometry (petiole length, density of leaflets and rachis curvature) and leaflets morphology (gradients of leaflets length and width) were detected between and within progenies, and were accurately simulated by the modelling approach. The comparison of plant area obtained from TLS and virtual TLS highlight the capacity of the model to generate realistic 3D mock-ups. The architectural variabilities observed at plot scale were satisfactory simulated, as the gap fractions estimated from HP were compliant to the gap fractions estimated from virtual HPs. Finally, simulations revealed distinct light interception efficiency for the different progenies, from plant to stand scale. (Texte intégral)