Interrelations between Carbon Dioxide and Ethylene on the Stimulation of Cocklebur Seed Germination

Interrelations between CO 2 and C 2 H 4 on promotion of seed germination were examined in more detail at 23°C with presoaked upper seeds of Xanthium pennsylvanicum Wallr. The germination-promoting effect of C 2 H 4 decreased gradually as its application time was delayed during a soaking period, whereas CO 2 was most promotive in application at 5 days of soaking, then its effect declined. CO 2 and C 2 H 4 were additive in earlier soaking periods and synergistic in later periods. Such changes in germination behavior in response to CO 2 and/or C 2 H 4 during a soaking period were closely associated with growth responsiveness of the axial tissues, but not of the cotyledonary ones. Growth responsiveness of axial tissues to CO 2 or C 2 H 4 disappeared finally during a soaking period, but their extinct responsiveness to any one of these gases was almost fully restored in the simultaneous presence of the other. The extinct responsiveness to CO 2 was partially recovered by a preexposure to C 2 H 4 . This suggests that in the later period of soaking, unlike the case in a very early period of soaking, the C 2 H 4 -sensitive phase for seed germination precedes the CO 2 -sensitive phase in which CO 2 potentiated axial growth. The restoration of CO 2 responsiveness in axial growth occurred not only after C 2 H 4 treatment but also after exposure to 8 or 33°C or after KCN treatment. Thus, secondarily dormant Xanthium seeds could germinate in response to CO 2 alone, when they were previously exposed for shortterms not only to C 2 H 4 but also 8°C, 33°C, or KCN.