Mechanics of roots in modeled soil substrates

The presence of zones of high mechanical resistance is one of the most common physical limitations to soil exploration by roots, which has direct impacts on yield crops. In order to decipher the elementary mechanisms involved in the mechanical feedback between the plant root growth and the granular structure of the soil [1], we investigated the mechanical properties of the root as well as the root trajectory in different impeding substrates. First, we studied the longitudinal Young modulus of non-lignified plant roots tested in compression for different external environments. Control experiments on non-drying roots placed in isotonic osmotic solutions showed no evolution of the root’s Young’s modulus whose value was around 2 MPa. On the contrary, the experiments performed in air exhibited a dramatic increase of the root’s Young’s modulus with the drying time, sometimes by a factor of 35. Moreover, the Young’s modulus was observed to scale as a decaying power-law with the root’s cross-section measured at different times of drying. We interpreted our results in the framework of the mechanics of cellular foams [2]. This explains why the rigidity of thinner roots whose drying process is more efficient, increases faster with time. Second, we began to characterize the root growth, trajectory or branching pattern, in different model substrates exerting either a localized constriction on the root axis or repeated mechanical stresses. We focused on the elementary event of a root growing in a short elastic tube mimicking the passage of a root in the pore between the movable grains of a granular soil. We progressively complicate the interaction between roots and the mechanical obstacles by observing via time-lapse photography the root growth in arrays of fixed posts of different rigidities, or even in assembly of grains