Extensibility of isolated cell walls: Measurement and changes during cell elongation

1. The technique of measuring the extensibility of isolated cell walls with an extensometer (Instron technique) has been modified so that compliance values which characterize the plastic and elastic extensibility are obtained (DP and DE, respectively). 2. DP and DE values are influenced by the conditions under which the measurements are made. DP is affected by the rate of extension, the applied force and the presence of the protoplast. DE is inversely proportional to the applied force but is independent of the presence of the protoplast. Because of retarded elastic behaviour and of hysteresis in the elastic extension-stress curves, measurement of DE during extension and relaxation gives different values. 3. Irreversible extension as determined in this procedure appears to be due primarily to strain-hardened plastic deformation with a minor component due to some form of viscoelastic flow. Auxin increases the extensibility by acting on the strain-hardening function. 4. Changes in DP and DE which occur during the course of auxin-induced cell elongation of Avena coleoptile sections have been determined. DP increases following addition of auxin, reaches a maximum after 90-120 min, and then remains constant for up to 24 hours. Sucrose has no effect on the change in DP. DE shows smaller but similar changes. 5. An increase in IAA concentration up to 5×10(-5) M produces similar increases in growth rate and DP. When IAA is raised to higher levels, DP remains constant while the growth rate drops. 6. These results are in agreement with the concept that auxin exerts its effect on cell elongation by regulating wall extensibility but indicate that the growth rate is also influenced by other factors such as the osmotic potential of the cells.