Physical extensibility of maize coleoptile cell walls: apparent plastic extensibility is due to elastic hysteresis

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Abstract
Segments of maize (Zea mays L.) coleoptiles demonstrate plastic cell-wall extensibility (Epl) as operationally defined by the amount of irreversible strain elicited by stretching living or frozen-thawed tissue under constant load in an extensiometer (creep test). Changes of Epl are correlated with auxin- and abscisic-acid-dependent growth responses and have therefore been causally related to hormone-controlled cell-wall loosening. Auxin induces an increase of Epl specifically in the outer epidermal wall of maize coleoptiles which is considered as the growth-limiting wall of the organ. However, detailed kinetic measurements of load-induced extension of frozen-thawed coleoptile segments necessitates a revision of the view that Epl represents a true plastic (irreversible) wall deformation. Segments demonstrate no significant irreversible extension when completely unloaded between loading cycles. Moreover, Epl can be demonstrated repeatedly if the same segment is subjected to repeated loading cycles in the extensiometer. It is shown that these phenomena result from the hysteresis behaviour of the cell wall. Stress-strain curves for loading and unloading form a closed hysteresis loop, the width of which represents Epl at a particular load. Auxin-treatment of segments leads to a deformation of the hysteresis loop, thereby giving rise to an increase of Epl. These results show that the creep test estimates the viscoelastic (retarded elastic) properties rather than the plastic properties of the wall.