Micromechanics and anatomical changes during early ontogeny of two lianescent Aristolochia species

The mechanical properties of young stems of Aristolochia macrophylla Lam. a nd Aristolochia brasiliensis Mart. et Zucc. were studied during elongation growth and primary differentiation. Data for the modulus of elasticity, for the viscoelastic behaviour caused by longitudinal tension and for the shea r modulus resulting from torsion around a longitudinal axis were related to the underlying structural changes by quantitative analysis of stem anatomy , tissue distribution, ultrastructure, and cell wall biochemistry. The orie ntation of cellulose microfibrils was determined by light microscopy and sm all-angle X-ray diffraction, and the lignin content was determined by thiog lycolic acid derivatization and spectroscopic quantification. It was demons trated that the increase in stability during early development is due to th e complementary effects of increase in cell wall material, lignification, a nd cellulose microfibril alignment. A detailed micromechanical model, consi dering internal prestresses, is proposed to explain the characteristic biph asic stress-strain behaviour as well as the strain-hardening observed.