CO2 stimulated C2H4 production in the axial and cotyledonary tissues of cocklebur (Xanthium pensylvanicum Wallr.) seeds regardless of their dormant states. Axial tissues were more responsive to CO2 than cotyledonary ones, and the tissues of upper seeds were more responsive than those of lower ones. Responsiveness of upper seed tissues to CO2 increased in the following order: primarily dormant (PD), non-dormant (ND) and secondarily dormant (SD) and also with increasing the period of presoaking. The CO2 stimulation of C2H4 production occurred not only in air but also under hypoxic conditions. The pronounced C2H4 production at hypoxia in the axes of PD seeds, as compared with those of ND ones, was a result of more active CO2 evolution in PD axes than in ND ones. A lag period of 2-3 h was required for the CO2 enhancement of C2H4 production to occur, and the CO2-enhanced C2H4 production was completely inhibited by aminoethoxyvinylglycine. It was thus suggested that CO2 may promote C2H4 production by stimulating the biogenesis of 1-aminocyclopropane-1-carboxylic acid. Also, the seed-coat of cocklebur seeds produced C2H4 and its C2H4 production was enhanced by CO2. It thus became obvious that the enhancement of C2H4 production by CO2 is independent of the growth stimulation of seed tissues by CO2, since the seed-coat itself has no growth potential.

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At 23°G, both GjH^. and GO2 stimulated the germination of freshly itnbibed upper cocklebur {Xattthiut7i petmsylvanicum Wallr,) seeds, but G2H4, unlike GOj, 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 °G, the degree of inhibition increasing with the duration of pre-soaking. Maximum inhibition occurred at 1-3 cm^ m"^ G2H4 when seeds were pre-soaked for 1 week; further increases of G2H4 concentration and pre-soaking period decreased the inhibitory effect. G2H4 was synergistic with GO2 when G2H4 promoted germination, whereas it was antagonistic when inhibitory. Such a transition of the C2H4 action occurred at ca. 27 °G. Also 1-aminocyclopropane-l-carboxylic acid, a G2H4 precursor, inhibited the germination of presoaked seeds at 33 °G, although it promoted the germination at 23 °G. When pre-soaked seeds were prepared for germination by chilling at 8 °G for 3 d, the inhibitory effect of G2H4 on the subsequent germination was manifested even at 23 °C. The reversal of the G2H4 action from promotion to inhibition in cocklebur seed germination is discussed in relation to the engagement of two respiratory pathways in the imbibed seeds.

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