The biomechanics of path of least resistance of roots in heterogeneous substrates

Rooting depth is critical for plants to acquire water and nutrient efficiently. However, when progressing deeper into the soil, a growing root must overcome physical obstacles such as stones and zones with different mechanical impedance (like hard pans and aggregates) which results in tortuous trajectories and a reduced ability to reach deeper soil horizons. We have developed different model systems which consists of roots growing in artificial substrates made of a customized arrays of stiff or deformable obstacles which the root can either bypass or penetrate based on the resistance of the obstacle. High-throughput imaging systems were used to capture time lapse data and image analysis techniques were used to track root responses to obstacles. In the presence of rigid obstacles, only a limited number of growth responses were observed with a transition from vertical to oblique trajectories observed as a function of size and distance between physical obstacles. When obstacles were deformable the likelihood of penetration could be predicted from factors such as the incidence angle, the length of the root that can bend freely, and the degree to which previous obstacles compress and anchor its base. Overall, our results showed that primary root growth in heterogeneous substrates is largely deterministic and can be predicted from the maximum curvature a root can bend, the spatial arrangements of obstacles and the mechanical stress anchoring the base of the root. Keywords: root, soil, mechanical impedance, heterogeneity, biomechanics