  Mechanical characterizations of plant roots are of primary importance in
geophysics and engineering science for implementing mechanical models for the
stability of root reinforced-soils, as well as in agronomy and soil science for
understanding the penetration of roots in soils and optimizing crop. Yet the
mechanical properties of plant roots depend on their water content, which can
drastically evolve with drying or flooding of the external soil. The present
work deals with the determination of the longitudinal Young's modulus of single
non-lignified plant roots, chick-peas (Cicer arietinum L.), tested in
compression along their root axis for different external environments: in
controlled conditions of natural drying in air or in a osmotic solution of
mannitol at the isotonic concentration where no water exchange occurs between
the root and the external solution. We submitted the chick-pea radicles to
successive mechanical compression cycles separated by rest periods to follow
the time evolution of the root mechanical properties in drying and non-drying
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 in these cases was observed to
scale as a decaying power-law with the root's cross section measured at
diffrent times of drying. We interpreted our results in the framework of the
mechanics of cellular foams.
