Breaking of Secondary Dormancy in Cocklebur Seeds by Cyanide and Azide in Combination With Ethylene and Oxygen, and Their Effects on Cytochrome and Alternative Respiratory Pathways

Abstract

The mechanism by which O₂, C₂H₄, KCN and NaN₃ stimulate germination was examined using completely after-ripened, non-dormant upper seeds of Xanthium pensylvanicum Wallr. in which secondary dormancy was induced by presoaking. Although the presoaked seeds had lost the ability to respond to these chemicals applied singly, they were highly responsive to C₂H₄ alone, but not to O₂ alone, when pretreated with KCN or NaN₃. Transfer of seeds exposed to KCN or NaN₃ to C₂H₄ under O₂ enrichment was the best practice for breaking secondary dormancy. Both O₂ and C₂H₄ stimulated total respiration of untreated seeds by increasing the ratio of alternative path flux (V_alt) to cytochrome path flux (V_cyt), which had been very low during the secondary dormancy stage. Maximal values of V_alt and V_cyt were obtained when O₂and C₂H₄ were applied together. The respiration surge induced by O₂ or C₂H₄ was also produced when the seeds were pretreated with KCN or NaN₃, but it did not occur until KCN or NaN₃ treatment was continued beyond 32 h. This time corresponds to that required for complete restoration of C₂H₄ responsiveness. The respiration surge did not occur when the KCN solution was renewed, suggesting that the operation of the cytochrome system, even if restricted, was required to develop the alternative path. In order to observe the respiration-enhancing action of C₂H₄ in seeds pretreated with KCN or NaN₃, C₂H₄ had to be applied in combination with O₂. This combination led to maximal fluxes of both V_cyt and V_alt, although it did not significantly increase the V_alt/V_cyt ratio. O₂ enrichment was ineffective in stimulating germination of these seeds and in increasing the V_alt/V_cyt ratio, since it stimulated the flux via the cytochrome path rather than the alternative one. It is concluded that both the large increase of V_alt and the high V_alt/V_cyt ratio are required for seed germination and that KCN, NaN₃, O₂ and C₂H₄ exert their actions on the germination by increasing either or both.