Physical Nature of Irreversible Deformation of Plant Cells

Abstract

Summary. Etiolated mung bean hypocotvl segments were incubated in 0.25 M mannitol solutions with indoleacetic acid. They were then deformed mechanically with a longitudinal tensile force at a constant strain rate. The mnagnitudes of the mechanical forces were comparable to those of the hydrostatic forces existing in normally growing, tissues. Each segment was repeatedly deformed and returned to zero force. The total deformation was increased at each cycle.The irreversible and elastic changes in length and diameter were measured for each deformation and the changes in surface area and volume calculated. In addition the applied stress and the iwork of irreversible and of elastic deformation were determined as functions of deformation.It was found that irreversible elongation, irreverslible change in surface area and total change in surface area all were linear functions of total imposed elongation. However, very little change in volume occurred during the deformations.The work of irreversible deformation was found to be independent of temperature between 80 and 250. It was also virtually independent of rate of deformation measured over a 5-fold range of deformation rates. From these results it is concluded that the irreversible deformation of mung bean hvpocotyl tissue occurs by plastic deformation rather than by viscous flow. Thus, the :rreversible deformation occurred as a result of breaking cross-links of a cross-linked polymer system.It is now well established that elongation of plant stem, hypocotyl and coleoptile tissues occurs as a result of the hydrostatic pressure within the cell together with the physical yield of the cell wall (4,6,8,9,10,12). Normally, the rate of wall deformation is a function of the rates of the metabolic processes that act on the cell wall and of the turgor pressure existing within the cell. The turgor pressure is, in turn, a function of the osmotic pressure of the tissue and the water conduictivity of the system (7).The effects of whatever metabolic processes are necessary to maintain cell wall deformability mav be studied by measuring the changes that occur in the physical properties of the walls as a result of this metabolic action. In order to separate the metabolic effects and the deformation process the turgor pressure can be reduced. This will allow the metabolic processe-s that act on the twall to continue without concurrent wall yield. After incubation for an appropriate time the physical properties can be measured under known applied forces (hydrostatic or meChanical forces) (4,8,9). For the present studies the hydrostatic pressure was reduced by incubation in approximately isotonic mannitol solutions.

Growth Physics in Nitella: a Method for Continuous in Vivo Analysis of Extensibility Based on a Micro-manometer Technique for Turgor Pressure

“…WNith previous growtlh in auxin the decrease is roughly 25 %. Lockhart (23) found onix small viscous properties in mung beani. A 5 X increase in rate of stretch gave only a 50 % inicrease in the Nwork of deformation.…”

The Mechanical Behavior of Isolated Avena Coleoptile Walls Subjected to Constant Stress

“…There is considerable evidence that auxin controls the rate of cell elongation, at least in part, by altering the ability of walls to be physically extended (5,12,18), but the manner in which this is accomplished is still in doubt. One block to our understanding is our lack of knowledge concerning the nature of the wallstretching process.…”

Metabolic and Physical Control of Cell Elongation Rate

“…1, top) has to be abandoned because (a) the steady growth of many tissues increases with turgor only above a threshold value of turgor (1,10,18) and (b) in Nitella the immediate response of rate to a shift in turgor is far more than proportional to the change in turgor (7). Both objections are removed if the effective driving force is regarded as only that part of cell turgor which exceeds some yielding threshold of the wall, also measured as a pressure:…”