Viscoelastic versus plastic cell wall extensibility in growing seedling organs: a contribution to avoid some misconceptions

In a recent publication (Kutschera, 1996), it was reported that the cell walls of growing rye coleoptiles exhibit irreversible (plastic) extensibility in a rheological extension test, Basically similar measurements with cell walls of maize coleoptiles had previously shown that the apparent plastic extensibility determined in this material is in reality due to the slowly reversible (viscoelastic) extensibility of the walls. A recent reinvestigation of this discrepancy showed that rye coleoptile walls also behave as a perfectly viscoelastic material if precautions are taken to prevent measuring artefacts, Similar results were obtained with cell walls from the growing zone of various other seedling organs (maize mesocotyl, maize root, cucumber hypocotyl). It is concluded that plastic extensibility has not yet been convincingly demonstrated by rheological tests that determine the intrinsic material properties of cell walls, Reported changes in mechanical material properties of cell walls produced by growth-controlling factors such as auxin or light may generally be attributed to changes in viscoelasticity which are not directly related to the chemo-rheological processes controlling wall extension of growing cells.