Contrasted Effects of CO2 on the Regulation of Dormancy and Germination in Xanthium pennsylvanicum and Setaria faberi Seeds

Abstract The effects of CO 2 on dormancy and germination were examined using seeds of cocklebur ( Xanthium pennsylvanicum Wallr.) and giant foxtail ( Setaria faberi Herrm.). The rate of germination of the giant foxtail seeds as well as cocklebur was promoted by exogenously applied CO 2 at a concentration of 30 mmol mol -1 regardless of the sowing conditions. However, seeds which failed to germinate in the presence of CO 2 , entered a secondary phase of dormancy under unfavourable germination conditions. If CO 2 was applied to seeds under conditions such as water stress imposed with a 200 mol m -3 mannitol solution, a hypoxic atmosphere of 100 mmol mol -1 O 2 or a treatment of 0·1 mol m -3 ABA, development of secondary dormancy was accelerated. These contrasted effects of CO 2 were observed in ecological studies. Under natural field conditions germination of buried giant foxtail seeds responded positively to CO 2 during a period of release from primary dormancy from Feb. to May, but CO 2 accelerated secondary dormancy commencing in early Jun. In other words, in the presence of CO 2 , both the environmental conditions and the germination states of the seeds clearly showed secondary dormancy-inducing effects. Thus, it seems that CO 2 has contrasted effects on regulation of dormancy and germination of seeds depending on the germination conditions.