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 a fter emergence of the primary leaf is brought about was investigated i n rye seedlings (Secale cereale L.) that were either grown in darkness or irradiated with continuous white light. In 3-d-old etiolated (grow ing) coleoptiles a turgor pressure of 0.59 MPa was measured, In 6-d-ol d 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 pla sticity (E(pl)) and elasticity (E(el)) were determined with a constant -load extensiometer both in vivo (turgid coleoptile segments) and in v itro (frozen-thawed samples). Cessation of coleoptile elongation was c orrelated with a 95% reduction in E(pl), whereas E(el) was only slight ly affected, Extension kinetics were measured with living and frozen-t hawed segments cut from growing and non-growing coleoptiles. The corre sponding stress-strain (load-extension) curves indicate that the cell wall of the growing coleoptile behaves like an elastic-plastic materia l whereas that of the non-growing organ shows the behaviour of an elas tic solid, These data demonstate that E(pl) represents a true plastic (irreversible) deformation of the cell wall. It is concluded that cess ation of coleoptile growth after emergence of the primary leaf is attr ibutable to a loss of cell-wall plasticity, Hence, a mechanical stiffe ning of the cell wall and not a loss of turgor pressure may be respons ible for the deceleration of cell elongation in the rye coleoptile.