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. Recent studies have shown that the soil resistance to penetration has dramatic effects on the root growth rate as well as on its morphology. Above a mechanical stress of the order of 1 MPa, the root growth rate decays and the root diameter tends to increase. In some cases the root is able to reorganize the surrounding soil by pushing the constitutive grains of the soil matrix. In other cases the too large impedance of the soil leads to a reorientation of the growth axis and even to the complete stop of the growth.
Our long-term objectives are to study this coupling between the root growth and the reorganisations of the soil. This is of particular importance for sandy soils where the cohesion between grains is low compared with clay soils and where the root diameter is of the order of the grain sizes. Moreover a better understanding of how root growth rearranges sand particles in soil would be of great interest for ecological and agronomic purposes like the restoration of structure (porosity network) in compacted soils. This would lead to sustainable functioning and productivity of agroecosystems. Until now many studies have explored the effect of mechanical impedance on root growth in the case of a cohesive soil treated as a continuous material. On the other hand some studies have focused on discontinuous soils and observed the root's morphological parameters like the length, diameter and mechanisms of splitting or bifurcations depending on the obstacles the root encounters.
The aim of the present study is to simultaneously quantify in situ forces and root's morphological characteristics in the case of a discontinuous soil. In the first step of this experimental study the discontinuous soil is modelled by only two fixed grains with a controlled minimum gap between them. The root is a radicle at its first stage of development (no secondary root) and is constrained to grow through this pore along a main vertical direction. The originality of our technique is that the grains are photoelastic discs, i.e. optically birefringent when submitted to a deviatoric mechanical stress. When placed between crossed polarizers, these grains exhibit interference patterns with fringes whose number and locations evolve with the intensity of stresses. Thanks to a preliminary calibration and according to the elastic theory, the locations of optical fringes inside the grains could be related to the intensity of mechanical stresses exerted on them.
Chick pea has been selected as a test plant because its radicle has a large size (approximately 1 mm) compared to other plants and therefore the gap between the grains could easily be controlled (from 0.5 to 1.5 mm) as well as the root/gap ratio. Moreover, the seed has enough reserve to allow the development of the root for several days. During its development, the root exerts a radial pressure on these two grains, thus creating an evolving pattern of optical fringes in these neighbouring photoelastic grains. A CCD camera placed in front of the set-up records the simultaneous evolution of the root and photoelastic fringes while the root is growing. The values of the radial force were inferred from the locations of fringes. We observed the temporal coupling between root diameter increase and radial force evolution for different root/gap ratios. These first results validate the experimental device which can therefore be used with more complex geometries (using several grains) to study the strategies of root development in mechanically stressed environment.