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At 23°C, both C2H4 and CO2 stimulated the germination of freshly imbibed upper cocklebur (Xanthium pennsylvanicum Wallr.) seeds, but C2H4, unlike CO2, changed to an inhibitor of germination under some soaking conditions. However, when seeds were pre-soaked for more than several hours at 23°C prior to treatment, C2H4 strongly inhibited their germination at 33°C, the degree of inhibition increasing with the duration of pre-soaking. Maximum inhibition occurred at 1-3cm3 m-3 C2H4 when seeds were pre-soaked for 1 week; further increases of C2H4 concentration and pre-soaking period decreased the inhibitory effect. C2H4 was synergistic with CO2 when C2H4 promoted germination, whereas it was antagonistic when inhibitory. Such a transition of the C2H4 action occurred at ca. 27°C. Also l-aminocyclopropane-l-carboxylic acid, a C2H4 precursor, inhibited the germination of presoaked seeds at 33°C, although it promoted the germination at 23°C. When pre-soaked seeds were prepared for germination by chilling at 8°C for 3 d, the inhibitory effect of C2H4 on the subsequent germination was manifested even at 23°C. The reversal of the C2H4 action from promotion to inhibition in cocklebur seed germination is discussed in relation to the engagement of two respiratory pathways in the imbibed seeds. [ABSTRACT FROM AUTHOR]