Abstract: Bidens alba, an invasive annual species from the genus Bidens in Asteraceae, has heteromorphic achenes. However, the ecological significance of heteromorphic achenes is still unknown. The differences of morphology, germination and seedling growth of the heteromorphic achenes under drought stress were compared by laboratory observation and bioassay. The differences of germination strategies and the adaptation of the heteromorphic achenes of B. alba to drought were examined. The results showed that: (1) There were significant differences in morphology and germination strategies between central achenes and peripheral achenes of B. alba. Total length, body length, and thousandgrain weight of central achenes were higher than those of peripheral achenes, but the width was reverse, central achenes germinated slowly, and peripheral achenes germinated fast and evenly. (2) Compared with the control, at 5%-25% of PEG-6000, germination rate, germination potential, germination index and vigor index of central achenes were enhanced at lower concentrations but inhibited at higher concentrations, and the solutions mostly showed increasing inhibition on the four indices of peripheral achenes withincreasing concentrations (P<0.05). (3) Compared with the control, seedling root length of central achenes was significantly promoted by PEG-6000 solutions at 5% concentration but inhibited at 20%-25% concentrations (P<0.05). Seedling height and fresh mass of central achenes was significantly inhibited at 10%-25% and 15%-25% concentrations respectively (P<0.05). The seedling root length of peripheral achenes was significantly inhibited at 15%-25% concentrations, and their seedling height and fresh mass was significantly inhibited at 5%-25% concentrations (P<0.05). Our results suggest that there were significant differences in the morphology and germination strategies between central achenes and peripheral achenes of B. alba. The centralachenes had stronger adaptation to drought. Those characteristics were helpful for the colonization of central achenes in new habitats and for population continuation of peripheral achenes in original habitats.

Key words: ionic homeostasis, magnetic effect, NaCl stress.