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

Abstract 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 1–3 cm³ 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 1‐andnocyclopropane‐1‐carboxylic acid, a C₂H₄ precursor, inhibited the germination of pre‐soaked 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.