Cell Wall Texture Along The Growth Gradient OF The Mung Bean Hypocotyl: Ordered Assembly and Dissipative Processes

A step-by-step analysis of wall characteristics was performed along a defined growth gradient in order to follow the dynamics of changes in wall organization according to the state of elongation of the cell. Specimens were taken from the elongating zone of 3-day-old mung bean (Vigna radiata L. Wilczek) hypocotyls grown in constant culture conditions. A technical procedure was developed in order to perform a complete series of observations along the growth gradient of individual hypocotyls and at the same time to locate the levels in relation with the growth state. Weight and thickness measurements were performed in parallel with observations of changes in wall texture using ultrastructural cytochemistry (periodic acid/thiocarbohydrazide/silver proteinate test for polysaccharides associated with dimethylsulphoxide and methylamine extractions). Ultrastructural observations were made on cortical cells, mainly on epidermal cells. For a cell that runs through the complete growth process the preliminary phase is a massive elaboration of wall subunits. The wall thus built is highly ordered (twisted plywood). Both thickness and order are maximal when the cell reaches the exponential phase of extension. An unstable balance exists between ordered assembly (with new layer deposition) at the inner wall, and dispersion of subunits (resulting from extension) at the outer wall. Growth takes place at the expense of thickness and the pre-established order. The balance is rapidly shifted and the dispersion of the subunits progressively predominates. Wall construction being no longer continued, elongation slows down and stops. The wall acts as a dissipative structure changing from order to a subsequent irreversible disorder. In terms of growth capacity, the more ordered the wall is, the more extendible is the cell. The cessation of growth occurs when order has been completely lost. The results indicate that apparently conflicting concepts proposed in the literature, for the organization of the wall during cell elongation, are not mutually exclusive in the tissues studied.