Interrelation between ethylene and carbon dioxide in relation to respiration and adenylate content in the pre-germination period of cocklebur seeds

Effects of C₂H₄ and CO₂ on respiration of pre-soaked upper cocklebur (Xanthium pennsylvanicum Wallr.) seeds during a pre-germination period were examined in relation to effects of the two gases on germination. At 33°C, cocklebur seed germination was greatly stimulated. This high temperature-stimulated germination was severely inhibited by C₂H₄, but not by CO₂, although both gases stimulated germination at 23°C. C₂H₄ promoted seed respiration at 23°C, but its promotive effect decreases with increasing temperature and disappeared at about 35°C, while CO₂ stimulated respiration regardless of temperature. CO₂ augmented the operation of the CN-sensitive, cytochrome path (CP) regardless of temperature, resulting in an increase in the ratio of the CP flux to a CN-resistant, alternative path (AP) flux. On the other hand, C₂H₄ augmented the operation of both paths, particularly of the AP, at 23°C, where it promoted germination. However, at 33°C where germination is suppressed by C₂H₄, C₂H₄ preferentially stimulated respiration via the AP, thus leading to an extremely high ratio of AP to CP. The inhibitory effect of C₂H₄ on germination at 33°C disappeared completely in enriched O₂, under which conditions CP is known to be augmented. At 23°C, CO₂ and C₂H₄ acted independently in controlling seed respiration, but they were antagonistic at 33°C. The independent action appeared when the AP flux was very low relative to the CP flux, while the antagonism appeared when the AP flux had risen. This differential action of the two gases at different temperatures was also observed in the ATP level, adenylate pool size and energy charge of the axial tissues. These results suggest that the germination-controlling actions of both CO₂ and C₂H₄ are fundamentally manifested through the modification of respiratory systems. However, the germination-inhibiting effect of C₂H₄ at 33 °C was not removed by inhibitors of AP, and there was little difference in the adenylate compounds between the C₂H₄-treated and non-treated seeds at 33°C. Therefore, the physiological action of C₂H₄ can not be explained only in terms of regulation of the respiratory system.