Seed dormancy and germination of Senna marilandica and S. obtusifolia were compared in greenhouse and laboratory studies. About 90% of the S. obtusifolia seeds were green and had hard seed coat dormancy, whereas the other 10% were brown and nondormant. Seed-colour morphs did not occur in S. marilandica, and nearly 100% of the seeds had hard seed coat dormancy. Seeds of S. obtusifolia were significantly heavier than those of S. marilandica. Mechanical scarification was very effective in overcoming dormancy in seeds of both species. However, concentrated sulfuric acid, absolute ethanol and boiling water were less effective in breaking dormancy in seeds of S. marilandica than in those of S. obtusifolia. Further, incubating seeds at 30/15 to 40/25 degrees C and dry-heat treatments at 80-100 degrees C were ineffective in breaking dormancy in S. marilandica, but significantly increased germination percentages in S. obtusifolia. In neither species were simulated daily/seasonal temperature shifts effective in breaking dormancy. Scarified seeds of both species germinated over a wide range of temperatures in both light and darkness. Under near-natural temperature conditions, seeds of S. marilandica germinated in spring only, whereas those of S. obtusifolia emerged in late spring and throughout summer. Both species can form a long-lived seed bank. Dormancy break by high field temperatures in seeds of S. obtusifolia allows this species to germinate throughout the warm growing season and thus contributes to its success as a weed in arable crops.
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