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 ce ll walls of growing rye coleoptiles exhibit irreversible (plastic) ext ensibility in a rheological extension test, Basically similar measurem ents with cell walls of maize coleoptiles had previously shown that th e apparent plastic extensibility determined in this material is in rea lity due to the slowly reversible (viscoelastic) extensibility of the walls. A recent reinvestigation of this discrepancy showed that rye co leoptile walls also behave as a perfectly viscoelastic material if pre cautions are taken to prevent measuring artefacts, Similar results wer e obtained with cell walls from the growing zone of various other seed ling organs (maize mesocotyl, maize root, cucumber hypocotyl). It is c oncluded that plastic extensibility has not yet been convincingly demo nstrated by rheological tests that determine the intrinsic material pr operties of cell walls, Reported changes in mechanical material proper ties of cell walls produced by growth-controlling factors such as auxi n or light may generally be attributed to changes in viscoelasticity w hich are not directly related to the chemo-rheological processes contr olling wall extension of growing cells.