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 degrees 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 degrees 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 degrees C, 33 degrees C, or KCN.