Cessation of cell elongation in rye coleoptiles is accompanied by a loss of cell-wall plasticity

The mechanism by which endogenous cessation of coleoptile elongation after emergence of the primary leaf is brought about was investigated in rye seedlings {Secale cereale L.) that were either grown in darkness or irradiated with continuous white light. In 3-d-old etiolated (growing) coleoptiles a turgor pressure of 0.59 MPa was measured. In 6-d-old coleoptiles, which had ceased to elongate, cell turgor was 0.51 MPa and thus only 13% lower than in the rapidly growing organ. Hence, the driving force for growth (turgor) is largely maintained. Cell-wall plasticity (£„,) and elasticity (£al) were determined with a constantload extensiometer both in vivo (turgid coleoptile segments) and in vitro (frozen-thawed samples). Cessation of coleoptile elongation was correlated with a 95% reduction in £pl, whereas £., was only slightly affected. Extension kinetics were measured with living and frozen-thawed segments cut from growing and non-growing coleoptiles. The corresponding stress-strain (load-extension) curves indicate that the cell wall of the growing coleoptile behaves like an elastic-plastic material whereas that of the nongrowing organ shows the behaviour of an elastic solid. These data demonstate that £pl represents a true plastic (irreversible) deformation of the cell wall. It is concluded that cessation of coleoptile growth after emergence of the primary leaf is attributable to a loss of cell-wall plasticity. Hence, a mechanical stiffening of the cell wall and not a loss of turgor pressure may be responsible for the deceleration of cell elongation in the rye coleoptile.