Dark-mediated dormancy release in stratified Lolium rigidum seeds is associated with higher activities of cell wall-modifying enzymes and an apparent increase in gibberellin sensitivity

Dormancy release in freshly matured, imbibed annual ryegrass (Lolium rigidum) seeds is inhibited by light and involves a decrease in seed sensitivity to abscisic acid. Other processes involved in dormancy release in the dark were investigated by measuring seed storage compound mobilisation and the activity of cell wall-degrading enzymes. Activities of endo-beta-mannanase and total peroxidase were higher in dark-stratified compared to light-stratified seeds, indicating that weakening of the structures constraining the embryo was accelerated in the dark. A dramatic degradation of storage proteins in light-stratified seeds, accompanied by induction of a high molecular mass protease, suggests that maintenance of storage(like) proteins is also important in dark-mediated dormancy release. alpha-Amylase activity was induced in dark-stratified seeds at least 48 h prior to radicle emergence upon transfer to conditions permitting germination, or in light-stratified seeds supplied with exogenous gibberellin A(4). This suggests that (a) alpha-amylase is involved in stimulation of germination of non-dormant L. rigidum seeds, and (b) dark-stratified seeds have an increased sensitivity to gibberellins which permits the rapid induction of alpha-amylase activity upon exposure to germination conditions. Overall, it appears that a number of processes, although possibly minor in themselves, occur in concert during dark-stratification to contribute to dormancy release. (C) 2010 Elsevier GmbH. All rights reserved.