Biomechanical study of the effect of a controlled bending on tomato stem elongation: global mechanical analysis

An experiment was designed to apply a controlled bending to a tomato stem a nd simultaneously to measure its effect on stem elongation. Stem elongation was measured over 2 d until steady and equal rates were obtained for the c ontrol and the treated plants. Thereafter, the basal part of the stem was s ubmitted to a transient controlled bending at constant displacement rate us ing a motorized dynamo; meter. After load removal, stem elongation was agai n measured for 2 d. The tested plants were mature (height visible internode s) and only the basal part of the stem, which had already finished elongati on, was loaded (hypocotyl and the first three internodes). A few minutes af ter the application of bending, elongation stopped completely for 60 min. T hereafter it took 120-1000 min to recover a rate of elongation similar to t he control. The growth response was exclusively due to the bending of the b asal part of the stem. It was shown that the side mechanical perturbations on the roots and on the stem tissues interacting directly with the clamp we re not significantly involved on the elongation response. These results giv e evidence for mechanical perception and plant signalling from the basal st em to the upper elongating zone. However, none of the variables characteriz ing the global mechanical state of the bent part of the stem (i.e. the maxi mal force, bending moment, inclination, mean curvature of the stem, stored mechanical energy) could quantitatively explain the variability of the grow th response. A more local mechanical analysis is therefore needed to elucid ate how the mechanical stimulus is perceived.