No abstract

Stratification of common ragweed (Ambrosia artemisiifolia) seeds at 4 C was most successful for breaking dormancy, whereas -5 C was least effective and 10 C was intermediate. Germination in the light exceeded that in the dark at all stratification and germination temperatures. The optimum temperatures for germination in the light were 10/20, 15/25, and 20/30. Maximum germination in the dark occurred at 20/30 C for seeds stratified at 4 and 10 C but the optimum temperatures for seeds stratified at -5 C were 10/20, 15/25, and 20/30. Seeds stratified at -5 and 10 C germinated best after 15 weeks of stratification, whereas 12 weeks of stratification at 4 C resulted in maximum germination. Secondary dormancy was induced in seeds which did not germinate in the dark. This was affected by stratification temperature and duration and germination temperature. The ecological significance of these germination characteristics is discussed.

Ambrosia artemisiifolia (ragweed) is a dominant species in the first year of old field succession but rarely persists for more than two years. Ragweed and Raphanus raphanistrum (wild radish), also an early invader, failed to become re‐established in plots cleared of second stage perennial vegetation (dominated by Aster pilosus), despite the large number of seeds of these primary invaders present in the soil. Edaphic experiments revealed that this pattern of succession was not due to mineral or physical properties of the soil. Field soil from the second stage of succession inhibited the growth and germination of ragweed and wild radish while soil from the first stage had no effect. Inhibitory volatile materials from ragweed were not detected. However, root exudate of ragweed, and shoot extracts of ragweed and aster inhibited the germination and growth of early invaders of abandoned fields. These results indicate that the vegetational change from the first to the second stage of succession may be mediated at least partially by an allelopathic response. Chromatography and bioassay techniques revealed the inhibitory compounds to be phenolic acids, including caffeic and chlorogenic acids.

ACKNOWLEDGMENTSThe author wishes to express his sincere thanks to Dr. E. L.

Stratification of common ragweed (Ambrosia artemisiifolia) seeds at 4 C was most successful for breaking dormancy, whereas ‐5 C was least effective and 10 C was intermediate. Germination in the light exceeded that in the dark at all stratification and germination temperatures. The optimum temperatures for germination in the light were 10/20, 15/25, and 20/30. Maximum germination in the dark occurred at 20/30 C for seeds stratified at 4 and 10 C but the optimum temperatures for seeds stratified at ‐5 C were 10/20, 15/25, and 20/30. Seeds stratified at ‐5 and 10 C germinated best after 15 weeks of stratification, whereas 12 weeks of stratification at 4 C resulted in maximum germination. Secondary dormancy was induced in seeds which did not germinate in the dark. This was affected by stratification temperature and duration and germination temperature. The ecological significance of these germination characteristics is discussed.