Reversal of ethylene action on cocklebur seed germination in relation to duration of pre-treatment soaking and temperature.

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]