  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).
