Acclimations to wind loads and/or contact stimuli? A biomechanical study of peltate leaves of Pilea peperomioides.

Plants are exposed to various environmental stresses. To mechano-stimulation, such as wind and touch, leaves immediately respond by bending and twisting or acclimate over a longer time period by thigmomorphogenetic changes of mechanical and geometrical properties. We selected the peltate leaves of Pilea peperomioides for a comparative analysis of mechano-induced effects on morphology, anatomy and biomechanics of petiole and transition zone. The plants were cultivated for six weeks in a phytochamber divided into four treatment groups: control (no stimulus), touch stimulus (brushing every 30 s), wind stimulus (constant air flow of 4.6 ms -1), and a combination of touch and wind stimuli. Comparing the treatment groups, neither the petiole nor the transition zone show significant thigmomorphogenetic acclimations. However, comparing the petiole and the transition zone, the elastic modulus (E), the torsional modulus (G), the E/G ratio and the axial rigidity (EA) differed significantly, whereas no significant difference was found for the torsional rigidity (GK). The twist-to-bend ratios (EI/GK) of all petioles ranged between 4.33 and 5.99, and of all transition zones between 0.67 and 0.78. Based on the twist-to-bend ratios, we hypothesise that bending loads are accommodated by the petiole, while torsional loads are shared between the transition zone and petiole.