Transverse Viscoelastic Extension in Nitella

نویسنده

  • JEAN-PIERRE METRAUX
چکیده

Transverse vicoelstic extensbli was measued directdy in isolated wails of Nitella internode cell. Cell wails extended transversely exhibit a yield point which is approximately twice the yield point in the lontudinal direction. Walk from youn, growing cels are four to seven times more extenble t than transversely, whrle wall from mature, norowng cells are only two times more extensibe longtdinlbly. Although lon Iudinal extensibilit decres drast ith the decreae in the growth rate, lateral exten y is constant through development. There ls a discrepancy between the lateral growth rate and trnserse creep, since the lateral growth rate is not constnt. However, the degree of wai anisotropy observed is consistent with the view that the trasversely oriented celllose microwb act a a "reinforng filer" inNitella cell was. Isolated plant cell walls extend viscoelastically in response to an externally applied constant load (2). This means that an instantaneous elastic extension is superimposed on a time-dependent, plastic extension, the latter being roughly proportional to log time. The time-dependent component is termed creep (2) and the amount of deformation or strain resulting from creep under defined conditions is a measure of the viscoelastic extensibility of the wall. In the longitudinal direction (parallel to the cell axis) extensibility has been correlated with rates of elongation in vivo (1, 6, 14, 15). This correlation forms the basis for the view that wall extensibility is an important factor regulating elongation growth. The factors regulating lateral expansion in cylindrical cells are less well understood. In their studies on Nitella wall mechanical properties, Probine and Preston (14) examined the elasticity of Nitella walls in the transverse direction and found no significant correlation with the growth rate. No creep behavior was observed in the transverse direction, leaving the question of the role of viscoelastic extensibility unresolved (14). Kamiya et al. (7) detected a small amount of transverse plastic extension in mature, nongrowing Nitella cells by expanding them with mercury, but no attempt was made to correlate transverse extensibility with the growth rate. Since the diameter of Nitella cells may double during development (E. Loung, unpublished data) it follows that the wall is capable of extending laterally. In order to clarify the role of wall extensibility during transverse growth we have reinvestigated the creep properties ofNitella walls using a highly sensitive optical extensometer. I This research was supported by Grant BMS75-03391 from the National Science Foundation to L. T. MATERIALS AND METHODS Nitella axillaris Braun was cultured in the laboratory as previously described (8). The growth rate was calculated from the difference in length of individual cells over a 24-hr period, as measured with a ruler. Longitudinal extension was measured with a laser optical lever auxanometer as reported previously (8). For transverse extension measurements an internode cell was excised with a razor blade and the cytoplasm was delicately scraped out under a dissecting scope using a hair loop. A short segment, about 0.1 mm long, was cut from the middle of the cell wall cylinder. The i.d. of this narrow loop was about 0.25 to 0.4 mm, just wide enough to allow two Teflon-coated silver wires (0.127 mm in diameter each) to be passed through. The wires were bent twice at right angles using forceps and threaded through the loop, so that the region of the wire resting against the inner wall surface was straight (Fig. 1). One wire was hooked to the base of a perfusion chamber and the other was attached to a single pan balance. During extension the walls were perfused with 1 mm citrate-phosphate buffer (pH 6.5) at room temperature, and extension was recorded by the LOLA2 method. Strain rates were determined over a 20-min period, between 1 and 21 min after the initiation of extension. Control experiments with a loop of wire showed no perceptible creep behavior until the force was at least five times greater than the forces used in wall experiments. Experimental stresses and in vivo stresses were calculated in the following manner. Experimental stress (dynes/cm2) is equal to F/A, where F = applied force and A = cross-sectional area of the wall bearing the applied force. When F is applied longitudinally, the value for the cross-sectional area is approximated by the equation: AL = 2irrt (1) where 2frr = wall circumference and t = wall thickness (10). The cross-sectional area when F is applied transversely is AT = 2wt (2) where w = width of the wall loop and t = wall thickness. Wall thickness was estimated as 6 ,um for young cells and 8 ,um for old cells, based on measurements by Kamiya et al. (7) and on a developmental study of cell wall mass/unit area (E. Loung, unpublished data). The in vivo stresses due to turgor pressure for a cylindrical cell are given by the formulas:

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تاریخ انتشار 2005