Pre- and post-dispersal factors regulate germination patterns and structural characteristics of Scotch thistle (Onopordum acanthium) cypselas

Summary • Although Scotch thistle (Onopordum acanthium) is known to have cypselas whose dormancy is affected by maturation temperature, a detailed study of plant development, cypsela structure and germination responses after maturing under contrasting temperatures has not been done. • Plants were grown under high and low temperature regimes in glasshouses and under field conditions in 2000 and 2001. Each year, phenological and developmental characteristics of plants were monitored and cypselas were collected twice. Cypselas were germinated fresh and after 4 months after-ripening, and also examined for surface and internal characteristics by scanning electron microscopy. • Plants from the lower temperature regime were taller, had thicker shoots, larger leaves, larger capitula that appeared sooner, and more, slower-to-mature and larger cypselas with smoother surfaces, thicker coats and higher moisture contents. However, their germination percentages were lower. After after-ripening most cypselas germinated. • In both years, consistent and significant differences in germination patterns and structural characteristics between cypselas from the two temperature regimes indicated that both pre- and postdispersal factors were involved in regulating the germinability of O. acanthium cypselas.