Plant root growth against a mechanical obstacle: The early growth response of a maize root facing an axial resistance is consistent with the Lockhart model

Plant root growth is dramatically reduced in compacted soils, affecting
the growth of the whole plant. Through a model experiment coupling force
and kinematics measurements, we probed the force-growth relationship of a
primary root contacting a stiff resisting obstacle, that mimics the
strongest soil impedance variation encountered by a growing root. The
growth of maize roots just emerging from a corseting agarose gel and
contacting a force sensor (acting as an obstacle) was monitored by
time-lapse imaging simultaneously to the force.The evolution of the
velocity field along the root was obtained from kinematics analysis of the
root texture with a PIV derived-technique. A triangular fit was introduced
to retrieve the elemental elongation rate or strain rate. A parameter-free
model based on the Lockhart law quantitatively predicts how the force at
the obstacle modifies several features of the growth distribution (length
of the growth zone, maximal elemental elongation rate, velocity) during
the first 10 minutes. These results suggest a strong similarity of the
early growth responses elicited either by a directional stress (contact)
or by an isotropic perturbation (hyperosmotic bath).