A Biological and Robophysical Investigation of Root Circumnutation through Heterogeneous Substrates

Circumnutation, the circular motion exhibited by the tip of a growing root, occurs in a diversity of plants, but its function is not understood. To investigate hypotheses about substrate penetration benefits of such motion, we constructed a simple robotic model of root growth. A robotic arm, instrumented with a load cell, was outfitted with a rotating, compliant end effector (a hot glue gun stick spun by a stepper motor). Existing work observed force reduction effects from circumnutation in homogeneous granular material [Dottore et al. 2016]. We tested the hypothesis that circumnutation aids in a root’s ability to penetrate heterogeneous substrates, e.g. hard obstacles, by plunging the robotic root into a lattice of rough cylinders. Systematic variation of initial root positions revealed that non-rotating roots were significantly more likely to become pinned to obstacles and unable to progress further (75% blocked) compared to a rotating root (10% blocked). Further, the rotating root required less mechanical work (~40%) to penetrate the lattice, suggesting root circumnutation benefits the plant via greater penetration capability with reduced energetic expense.