EFFECTS OF LIGHT AND TEMPERATURE ON GERMINATION OF HETEROMORPHIC ACHENES OF BIDENS-ODORATA (ASTERACEAE)

Bidens odorata Cav. (Asteraceae) is an annual weed with heteromorphic achenes that differ in size, weight, and shape within each capitulum. To investigate if the variability of the achene size was correlated with differences in the germination response, achenes were classified in three categories based on size (4, 5-6, and 7 mm), and laboratory experiments were conducted with freshly harvested and dry-stored seeds under several light and temperature conditions. For all conditions tested with fresh seeds, 7-mm achenes germinated faster and with a higher final germination percentage than 4-mm achenes. Germination was inhibited by darkness and far-red light conditions. Experiments conducted with different times of exposure to white light (1 min, 10 min, and 12 h) showed that 4-mm achenes required longer time of exposure to light to maximize their percent germination. However, 7-mm achenes reached similar total percent germination without regard to the period of illumination. Dry storage increased the percent germination significantly in the shorter achenes and altered the light requirement for germination differentially among the morphs. The results suggest a physiological basis of achene heteromorphism that might be related to colonization of different microenvironments.