Rapid Actuation in Plants: Hydraulics or Mechanics?

The Venus flytrap carnivorous plant rapidly snaps in about 100 ms using a snap-buckling instability of its shell-shaped trap. While this dynamic is well understood at the macroscopic level, the internal motor used by the plant to trigger this closure and cross the instability threshold remains unknown. Here we investigate the physical origin of this "active" actuation at both the macroscopic and microscopic levels. We first unveil the active dynamics by disentangling the buckling instability in cut or clamped traps. We show that the time scale of this dynamics is too fast to result from water transport through the trap thickness, as deduced from cell pressure probe measurements and macroscopic swelling experiments. We then study the mechanical properties of the epidermal tissue before and after triggering using nanoindentation coupled with casting methods and numerical modeling. Our work shows that rapid movements in plants can occur through a rapid change in cell wall stiffness, in a tissue pre-stressed by the internal hydraulic pressure.