Loss of stability, pH, and the anisotropic extensibility of Chara cell walls

We investigated the effects of acid conditions on the extensibility of isolated wall segments from growing Chara corallina cells, providing the first detailed multi-azimuthal description of the anisotropic elastic modulus of the walls. The values of anisotropic modulus were obtained by loading a tensile force on wall ribbons excised from the cell walls along twelve different azimuths, and measuring the resulting elongation of the ribbons. Our study differs from previous studies in which mechanical loading of the wall materials was performed under creep conditions. We used ramp-loading conditions which meet the requirements for Loss of Stability. The results show that whereas a linear relationship between wall extension and log time is typical for creep-based experiments, it is not seen under ramp-loading conditions. To clarify the relative values of the wall moduli, the complete all-around anisotropic modulus is presented in polar coordinates, with the value of longitudinal modulus normalized to one unit. Acid pH enhances the extensibility of the wall materials, especially when medium pH <= 4, indicating that it is necessary to eliminate the prolonged plastic extension in order to obtain meaningful modulus values of wall materials immersed in acid environments.