Evaluation of dormancy and germination responses to temperature in Carduus acanthoides and Anagallis arvensis using a screening system, and relationship with field-observed emergence patterns

Experiments on the facultative winter annuals Carduus acanthoides and Anagallis arvensis were performed: (i) to determine thermal conditions that induce or release dormancy, (ii) to investigate to what extent changes in dormancy level resulting from those thermal conditions explain the seasonal pattern of emergence of these species, and (iii) to estimate required thermal time and base temperature for the germination of non-dormant seeds. Carduus acanthoides required high temperatures followed by decreasing temperatures for dormancy release; however, low winter temperatures did not induce secondary dormancy as expected for a winter annual. To the contrary, low temperatures stimulated dormancy release in the long term. In A. arvensis, dormancy relief was enhanced by dry storage at 25 degrees C, and the response to low temperature was different depending on moisture conditions. Prolonged exposure to moist-chilling increased the dormancy level of the population, while dry storage at 4 degrees C relieved dormancy. For both species, changes in the thermal range permissive for germination as a result of dormancy modifications explained to a large extent the timing of the emergence periods observed In me field. In neither species did base temperature for germination change with the dormancy level of the population. Thermal time required for germination of C. acanthoides varied with dormancy, while for A. arvensis seeds it was constant.