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Wall stress relaxation and the action of expansins. Wall relaxation depends on
pH and expansin activity. The diagram below takes you step-by-step through the
procedure of measure wall relaxation and illustrates the effect of pH and
expansin.
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In the figure at left, A-D diagrams the
procedure for obtaining the stress relaxation spectrum of the wall; E-G
illustrates our specific results with cucumber hypocotyl walls.
(A) To measure the stress relaxation spectrum, an isolated wall specimen
is clamped in an extensometer and extended rapidly (stretch phase) until a
predetermined force is reached. Thereafter the specimen is held at
constant length during the subsequent relaxation phase. (B) Time course
for force applied to the wall during the stress and relaxation phases. (C)
Force from B is expressed on a log time scale, and (D) differentiated with
respect to log time to give an estimate of the stress relaxation spectrum.
(E) Native walls from the cucumber hypocotyl exhibit greater stress
relaxation when incubated in acidic buffer compared with neutral buffer.
(F) The pH-dependent relaxation of cucumber walls is nearly lost by brief
heat inactivation of wall proteins. (G) The heat-labile, pH-dependent
relaxation process is largely restored by pre-incubation of walls with a
protein extract containing expansin. |
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