ETHYLENE PRODUCTION IN PEA AND COCKLEBUR SEEDS OF DIFFERING VIGOR

Relationships between seed vigour and ethylene (C2H4) production were studied using C2H4-responsive fatty cocklebur seeds (Xanthium pennsylvanicum Wallr.) and C2H4-insensitive starchy pea seeds (Pisum sativum L. cv. Alaska), which had been harvested in different years and subjected to different storage conditions. In both species, the seeds with the highest vigour evolved the largest amounts of C2H4 during a period of water imbibition. The reduction of C2H4 production in cocklebur seeds occurred concomitantly with the reduction in the growth potentials of both axial and cotyledonary tissues. Similarly, the activity of ACC-C2H4 conversion increased with soaking, and was greater in seeds of high vigour compared with those of low vigour. However, the change in ACC content in pea seeds differed from that in cocklebur seeds. That is, pea seeds with high vigour accumulated less ACC during an imbibition period than those with low vigour. From these results it was suggested that the inferior C2H4 production by low vigour pea seeds is mainly attributable to low ACC-C2H4 conversion, whereas that by low vigour cocklebur seeds is due to the shortage of ACC supply in addition to the reduced ACC-C2H4 conversion. However, germination of deteriorated cocklebur seeds was not restored by exposure to ACC or C2H4, suggesting that the loss of seed vigour reduces the responsiveness of seeds to C2H4 as well as C2H4 production.