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-germinationperiod were examined in relation to effects of the two gaseson germination. At 33?C, cocklebur seed germination was greatlystimulated. This high temperature-stimulated germination wasseverely inhibited by C₂H₄, but not by CO₂, although both gasesstimulated germination at 23?C. C₂H₄ promoted seed respirationat 23?C, but its promotive effect decreases with increasingtemperature and disappeared at about 35?C, while CO₂ stimulatedrespiration regardless of temperature. CO₂ augmented the operationof 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 augmentedthe operation of both paths, particularly of the AP, at 23?C,where it promoted germination. However, at 33?C where germinationis suppressed by C₂H₄, C₂H₄ preferentially stimulated respirationvia the AP, thus leading to an extremely high ratio of AP toCP. The inhibitory effect of C₂H₄ on germination at 33?C disappearedcompletely in enriched O₂, under which conditions CP is knownto be augmented. At 23?C, CO₂ and C₂H₄ acted independently incontrolling seed respiration, but they were antagonistic at33?C. The independent action appeared when the AP flux was verylow relative to the CP flux, while the antagonism appeared whenthe AP flux had risen. This differential action of the two gasesat 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 actionsof both CO₂ and C₂H₄ are fundamentally manifested through themodification of respiratory systems. However, the germination-inhibitingeffect of C₂H₄ at 33 ?C was not removed by inhibitors of AP,and there was little difference in the adenylate compounds betweenthe C₂H₄-treated and non-treated seeds at 33?C. Therefore, thephysiological action of C₂H₄ can not be explained only in termsof regulation of the respiratory system. (Received January 24, 1986; Accepted November 17, 1986)