Title: DEM analysis of mechanical interactions between a growing root and a granular soil

Author (Talk): MAHMOUD FAKIH, Phd student - Montpellier university

Abstract:

Among numerous factors that explain the variability of root architecture, including genetics, environment and developmental instability, the mechanical interaction between growing roots and granular soils has often been under-estimated. We hypothesize that the heterogeneous structure of soil at the particle scale, evidenced by the broad distribution of forces, can significantly influence root growth trajectories and its ramification system. We use discrete-element numerical simulations to investigate, the growth of a single root inside a granular material. The root is modelled using chains of connected spheroline elements. The growth is initiated from a circular element placed at the free surface of a granular bed prepared by random pluviation. This circle plays the role of meristem, which is constantly replicated at a given rate and pushed forward under the action of elastic forces. As a result, the root grows as a line of fixed thickness equal to the diameter of the circle and with prescribed stiffness and bending moment. The orientation of the meristem at every growth step is determined by the dynamics of the whole root under the action of its internal elastic forces and reaction forces exerted by the grains. Since the model is two-dimensional and the pore space is not open, we use two different diameters for the particles: a large diameter for the interactions between particles and a smaller diameter for the interactions between particles and root. The difference between the two diameters corresponds to the width of a gap at contact points through which the roots can pass. The ratio of this gap to the root diameter is considered as a model parameter. Several parameters being involved in the mechanical behaviour of the root-grains system, we first carried out a parametric study of their influence on forces at the root tip and the overall root trajectories. By analysing the evolution of reaction forces exerted by grains on the root tip, we find a broad distribution of the forces experienced by the root apex. The distribution has the same functional form when the forces are normalized by the mean force for each stiffness. It is characterized by a decreasing power law for forces below the mean and an exponential fall-off for forces above the mean. This distribution seems thus to reflect the broad distribution of forces inside the granular material. The modelling approach developed in this work will be applied to characterize the shape of the roots and the influence of soil properties on their development. The numerical results are compared to experimental data from the Rhizoscope genotypic platform developed at CIRAD in Montpellier (France), which allow observing real plant roots growing in a packing of glass beads. Authors : Mahmoud Fakih ( Phd student Montpellier University, visitor in MIT) Farhang Radjai (MIT) Jean Yves Delenne (INRA-France) Thierry Fourcaud (Cirad-France)