Physical Nature of Irreversible Deformation of Plant Cells

Abstract

Etiolated mung bean hypocotyl segments were incubated in 0.25 m mannitolsolutions with indoleacetic acid. They were then deformed mechanicallywith a longitudinal tensile force at a constant strain rate. Themagnitudes of the mechanical forces were comparable to those of thehydrostatic forces existing in normally growing tissues. Each segmentwas repeatedly deformed and returned to zero force. The total deformationwas increased at each cycle.The irreversible and elastic changesin length and diameter were measured for each deformation and thechanges in surface area and volume calculated. In addition the appliedstress and the work of irreversible and of elastic deformation weredetermined as functions of deformation.It was found that irreversibleelongation, irreversible change in surface area and total changein surface area all were linear functions of total imposed elongation.However, very little change in volume occurred during the deformations.Thework of irreversible deformation was found to be independent of temperaturebetween 8 degrees and 25 degrees . It was also virtually independentof rate of deformation measured over a 5-fold range of deformationrates.From these results it is concluded that the irreversible deformationof mung bean hypocotyl tissue occurs by plastic deformation ratherthan by viscous flow. Thus, the irreversible deformation occurredas a result of breaking cross-links of a cross-linked polymer system.