RHEOLOGICAL ANALYSIS OF VISCOELASTIC CELL-WALL CHANGES IN MAIZE COLEOPTILES AS AFFECTED BY AUXIN AND OSMOTIC-STRESS

The effects of auxin and osmotic stress on elongation growth of maize (Zea mays L.) coleoptile segments are accompanied by characteristic ch anges in the extensibility of the growth-limiting cell walls. At full turgor auxin causes growth by an increase in wall extensibility (wall loosening). Growth can be stopped by an osmotically produced step-down in turgor of 0.45 MPa. Under these conditions auxin causes the accumu lation of a potential for future wall extension which is released afte r restoration of full turgor, Turgor reduction causes a reversible dec rease in wall extensibility (wall stiffening) both in the presence and absence of auxin. These changes in vivo are correlated with correspon ding changes in the theological properties of the cell walls in vitro which can be traced back to specific modifications in the shape of the hysteretic stress-strain relationship. The longitudinally load-bearin g walls of the coleoptile demonstrate almost perfect viscoelasticity a s documented by a nearly closed hysteresis loop. Auxin-mediated wall l oosening causes an increase of loop width and thus affects primarily t he amount of hysteresis in the isolated wall. In contrast, turgor redu ction by osmotic stress reduces loop length and thus affects primarily the amount of viscoelastic wall extensibility. Pretreatment of segmen ts with anoxia and H2O2 modify the hysteresis loop in agreement with t he conclusion that the wall-stiffening reaction visualized under osmot ic stress in vivo is an O-2-dependent process in which O-2 can be subs tituted by H2O2 Cycloheximide specifically inhibits auxin-mediated wal l loosening without affecting wall stiffening, and this is mirrored in specific changes of the hysteresis loop. Corroborating a previous in vivo study (Hohl et al. 1995, Physiol. Plant. 94: 491-498) these resul ts show that cell wall stiffening in vivo can also be demonstrated by theological measurements with the isolated cell wall and that this pro cess can be separated from cell wall loosening by specific changes in the shape of the hysteresis loop.