Biphasic mechanical behaviour of plant tissues

The mechanical properties of young stems of Aristolochia macrophylla and Aristolochia brasiliensis have been studied. The characteristic phenomenon of a biphasic stress–strain behaviour in tension, together with pronounced viscoelasticity and strain hardening, was observed. The changes of the mechanical properties during the plants development were quantified together with the underlying structural changes in stem anatomy, cellulose microfibril orientation and lignin content. The modulus of elasticity rises in Ar. macrophylla during the development from about 10 MPa of young shoots to 3000 MPa of fully differentiated stems and in Ar. brasiliensis from 10 to 400 MPa. It was demonstrated that this increase during early development is due to the complementary effects of increase in cell wall material, lignification, and cellulose microfibril orientation. The biphasic stress–strain behaviour is conserved throughout development as a general characteristic. A detailed micromechanical model, considering internal prestresses, is proposed.