Date : 19/07/2012

Internship proposal for : Master 1 or Master 2

Laboratory
PMMH
UMR 7636, ESPCI
10 rue Vauquelin
75005 Paris
Website
Main discipline : Physics
Lab director : Philippe Petitjeans

Mentor
Evelyne Kolb
email : Cet e-mail est protégé contre les robots collecteurs de mails, votre navigateur doit accepter le Javascript pour le voir
phone : +33 1 40 79 58 04

Subjects
1.: root growth
2.: mechanical stress
3.: morphogenesis

Tools and methodologies
1.: time-lapse photography
2.: image analysis
3.: particle image velocimetry

Summary of lab's interests
The mechanical and topological properties of a soil like the global porosity and the distribution of void sizes greatly affect the development of a plant root, which in turn affects the shoot development. In particular, plant roots growing in heterogeneous medium like sandy soils or cracked substrates have to adapt their morphology and exert radial forces depending on the pore size in which they penetrate. Our team is interested in the coupling and retroaction between the root growth and the reorganization of the soil, i.e. what are the forces a root is able to develop on its environment and how the mechanical stress affects the root growth (morphogenesis)? This subject emerges from a multidisciplinary collaboration between Evelyne Kolb (PMMH, ESPCI), a physicist initially working on granular materials, Christian Hartmann (IRD), a soil scientist expert in real soil compaction and rehabilitation, and Patricia Genet (BIOEMCO,ENS), a biologist specialized in mycorrhizae.

Summary of project
To study such soil/root interactions we examine root penetration of medium in a model system made of a single pore between solid constituents. Till now, we worked with a model experiment in which a pivot root (chick-pea seeds, Cicer arietinum L.) of millimeter diameter had to grow in the constriction between two grains in a two-dimensional cell. The root diameter increase was limited by the steric constriction imposed by the gap between the two disks forming the solid phase. The essential point of this experimental set up was that the disks were made of a photoelastic material allowing us to monitor the radial compression force developed by root growth. Thus we measured in situ, simultaneously and up to five days i) the morphological changes induced by the steric constriction and ii) the root's radial forces exerted on the surrounding material (i.e. photoelastic disks) [Kolb, Hartmann, Genet, Plant and Soil 2012]. For this lab training we propose now to study the impact on root growth of a tridimensional (3D) pore of various rigidities. The root has to grow inside a polymeric tube, which radii, length and Young modulus can be varied. This tube exerts a radial and circumferential stress on the growing root. It mimics a 3D pore between reorganisable grains. By image analysis we will quantify how this localized stress will affect the root growth. In particular we will measure the axial growth velocity and the changes of root diameters before and after the tube location and compare these morphological parameters to controls for roots growing without any applied mechanical stress and to the extreme cases where the tube is completely rigid (glass tube). We will also measure the swelling of the flexible tube and use PIV technique (particle image velocimetry) on it to measure the local strains and possibly infer the radial stress exerted by the root. We also want to relate this macroscopic ana lysis to microscopic observations of cell morphology by using cell staining and post mortem observations to compute deformations of cell walls and change in the number of cell files.