Contrasted effects of CO₂ on the regulation of dormancy and germination in xanthium pennsylvanicum and setaria faberi seeds

The effects of CO₂ 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 CO₂ at a concentration of 30 mmol mol⁻¹ regardless of the sowing conditions. However, seeds which failed to germinate in the presence of CO₂, entered a secondary phase of dormancy under unfavourable germination conditions. If CO₂ 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 O₂ or a treatment of 0·1 mol m⁻³ ABA, development of secondary dormancy was accelerated. These contrasted effects of CO₂ were observed in ecological studies. Under natural field conditions germination of buried giant foxtail seeds responded positively to CO₂ during a period of release from primary dormancy from Feb. to May, but CO₂ accelerated secondary dormancy commencing in early Jun. In other words, in the presence of CO₂, both the environmental conditions and the germination states of the seeds clearly showed secondary dormancy-inducing effects. Thus, it seems that CO₂ has contrasted effects on regulation of dormancy and germination of seeds depending on the germination conditions.