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

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Interrelations between CO2 and C2H4 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 C2H4 decreased gradually as its application time was delayed during a soaking period, whereas CO2 was most promotive in application at 5 days of soaking, then its effect declined. CO2 and C2H4 were additive in earlier soaking periods and synergistic in later periods. Such changes in germination behavior in response to CO2 and/or C2H4 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 CO2 or C2H4 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 CO2 was partially recovered by a preexposure to C2H4. This suggests that in the later period of soaking, unlike the case in a very early period of soaking, the C2H4-sensitive phase for seed germination precedes the CO2-sensitive phase in which CO2 potentiated axial growth. The restoration of CO2 responsiveness in axial growth occurred not only after C2H4 treatment but also after exposure to 8 or 33°C or after KCN treatment. Thus, secondarily dormant Xanthium seeds could germinate in response to CO2 alone, when they were previously exposed for shortterms not only to C2H4 but also 8°C, 33°C, or KCN.

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