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 (E(pl)) and elasticity (E(el)) were determined with a constant-load extensiometer both in vivo (turgid coleoptile segments) and in vitro (frozen-thawed samples). Cessation of coleoptile elongation was correlated with a 95% reduction in E(pl), whereas E(el) 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 non-growing organ shows the behaviour of an elastic solid, These data demonstate that E(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.