Embryo and seed coat factors produce seed dormancy in capeweed (Arctotheca calendula)

Abstract

Capeweed [Arctotheca calendula (L.) Levyns.] is common in annual pastures of southern Australia. The physiological basis of seed dormancy in capeweed seeds was studied to determine the likely role of dormancy in the success of this species. Two dormancy mechanisms were identified: one embryo-based and the other imposed on the embryo by the seed coat. Embryo dormancy could be overcome by leaching, suggesting that this form of dormancy was caused by a water-soluble germination inhibitor, possibly localised in the cotyledons. Following an initial after-ripening period, embryo dormancy was rapidly relieved under conditions experienced on the soil surface during summer. Ungerminated embryos entered secondary dormancy in winter. Seed coat-imposed dormancy persisted longer than primary embryo dormancy, but was not associated with mechanical constriction of embryo growth, or with limited water or oxygen supply to the embryo.

Embryo and seed coat dormancy mechanisms interacted to maintain a high level of dormancy in these seeds at all times. Thus, a large proportion of seeds produced might enter a persistent seed bank, enabling re-establishment following years when seed production is limited by unfavourable seasonal conditions or cropping activities.