Toward a functional-structural model of oil palm accounting for architectural plasticity in response to planting density.

Abstract : Functional-structural plant modelling approaches (FSPM) open the way for exploring the relationships between the 3D structureandthe physiological functioningof plantsin relation to environmental conditions.FSPMscan beparticularly interesting when dealing with perennial crops like oil palm, for which research on innovative management practices requires long and expensive agronomic trials. The presentstudy is part of the PalmStudio project, whichaimsat developing a FSPMforoil palmcapable of conducting virtual experiments to test the relevance of innovative management practicedsand/or design ideotypes.We propose amethodologicalapproach which integrates architectural responses to planting densityin an existing oil palmFSPM(Perez et al.2018a b). Combining standard field phenotyping with Lidar-based derived measurements, wemanage toevaluate the phenotypic plasticity of the main parameters required for the calibration of the 3D plant model.LiDAR scans were processedusing the PlantScan3D software (Boudon et al. 2014)to derivephenotypic traits of leaf geometry that were compared to labour-intensive measurements. Density-based allometriesof leaf geometry and biomassarethenderived from theobserved variations in phenotypic traits and integrateintothe FSPM.Our results illustratethe accuracy and the efficiency of Lidar-based phenotypingofleaf 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 fastand efficient measurements ofusuallylabour-intensive traits such as leaf curvatureallowed 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 rachislength and petiolelengthrespectively)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 thenmodelled andcombined with the existing allometric-based 3D oil palm model VPalm (Perez et al.2018a). These data also enablethe development and the integrationin VPalm ofa biomechanical model simulating leaf curvature.The methodology presented in this study paves the way for a rapid integration of phenotypic plasticityin FSPMs. OurFSPM is now able toestimatehow planting density affectnot only plant architecture but alsofunctional processes such as carbon assimilation and transpiration.Ongoing research aims at coupling the current FSPM with a carbon allocation model (Pallas et al.2013)tosimulate the retroactions of functioning processes on plant architecture together with environmental and agronomic conditions