The sustained and warped helicoidal pattern of a xylan-cellulose composite: the stony endocarp model

Abstract

The study was devoted to the microstructure of the thick walled cells of the endocarp of prune (Prunus domestica L.), cherry (Prunus cerasus L.), walnut (Juglons regia L.). The tissue is formed of closely associated cells showing a homogeneous development characterized by an intense constructive activity of ordered walls with a typically twisted pattern (cholesteric-like). The arced layers are produced in tens, each corresponding to a 180° full rotation of the molecules (axis of rotation oriented radially) and their succession gives rise to a basic regular and monotonous periodicity. On the other hand, observation of the tissue revealed the large capacity of the helicoidal morphogenesis to adjust itself under the influence of two topological contingent constraints: (1) the spherical shape (and derivated shapes) of the cell and (2) the numerous pit canals which maintain the symplastic transport and produce a recess during the construction of the wall. Spherical shape (closed surfaces) and recess both introduce additional internal strains which are relieved by deviations of the molecular array in the basic pattern (moire and knotty aspects). Special attention was given to the defects integrated in the spherical twist. The defects emerging in the angled stacks of microfibrils (disclinations, distortions) were a diagnostic feature of an actual liquid crystal behaviour under mechanical constraints. The abundance of such defects, of cusps and spiral motions strengthened the hypothesis that a transient fluid phase, later on consolidated and stiffened, operates during the cellulose ordering. The saddle-like figures developed in the complex polylobed situation of walnut were particularly demonstrative. The fractionation of the secondary wall yielded the glucidic matrix in the same ratio as cellulose. The bulk of this embedding matrix was composed of acidic xylans more or less tightly bound to the microfibrils. The coat of negatively charged polysaccharides visualized by the binding of cationic gold to wall strips might be expected to act as a surfactant generating an electrostatic repulsion between microfibrils. This could be a cooperative mechanism for the self-positioning (aligment in sheets and progressive rotation) of the composite.