A comparative study of seed dormancy and germination in an annual and a perennial species of Senna (Fabaceae)

Seed dormancy and germination of Senna marilandica and S. obtusifolia were compared in greenhouse and laboratory studies. About 90% of the S. obtusifo lia 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. mariland ica, and nearly 100% of the seeds had hard seed coat dormancy. Seeds of S. obtusifolia were significantly heavier than those of S. marilandica. Mechan ical scarification was very effective in overcoming dormancy in seeds of bo th species. However, concentrated sulfuric acid, absolute ethanol and boili ng water were less effective in breaking dormancy in seeds of S. marilandic a 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 germina tion percentages in S. obtusifolia. In neither species were simulated daily /seasonal temperature shifts effective in breaking dormancy. Scarified seed s of both species germinated over a wide range of temperatures in both ligh t and darkness. Under near-natural temperature conditions, seeds of S. mari landica germinated in spring only, whereas those of S. obtusifolia emerged in late spring and throughout summer. Both species can form a long-lived se ed bank. Dormancy break by high field temperatures in seeds of S. obtusifol ia allows this species to germinate throughout the warm growing season and thus contributes to its success as a weed in arable crops.