GERMINATION ECOLOGY OF BIDENS-LAEVIS (ASTERACEAE) FROM A TIDAL FRESH-WATER WETLAND

Eight or more weeks of cold stratification (5-degrees-C) were required to break dormancy in achenes of Bidens laevis (L.) BSP from a tidal f reshwater wetland. When achenes began to come out of dormancy, they ge rminated to 50% first at 35/20-degrees-C, and then with additional str atification the minimum temperature for 50% germination decreased to 1 5/6-degrees-C. Germination of stratified achenes was inhibited by dark ness and was completely prevented by hypoxia (inundation in closed jar s). Seeds inundated in open jars and those in Petri dishes, however, g erminated to > 80%. Germination declined sharply with depth of burial (0, 1, and 5 cm). Moisture regime (drained, saturated, and inundated) also significantly affected germination, but afterripening condition ( cold greenhouse or 5-degrees-C) altered the effect. Interaction betwee n depth of burial and moisture regime was significant (P less-than-or- equal-to 0.05). At the end of the period of burial, nongerminated ache nes transferred to the soil surface were capable of germinating over a 19 mo period. Storage conditions (approximately 0, 20, 50, and 100% r elative humidity) of fresh achenes for 1 wk to 4 mo followed by at lea st 16 wk stratification at 5-degrees-C had no effect on total germinat ion. However, storage at approximately 0% RH for 4 mo reduced the rate of germination. Light and oxygen requirements are reflected in the re sponses to burial and inundation. These, in turn, provide a mechanism for maintenance of a persistent seed bank (> 1 yr) following winter af terripening of achenes in the field.