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 (Valt) to cytochrome path flux (Vcyt), which had been very low during the secondary dormancy stage. Maximal values of Valt and Vcyt 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 Vcyt and Valt, although it did not significantly increase the Valt/Vcyt ratio. O2 enrichment was ineffective in stimulating germination of these seeds and in increasing the Valt/Vcyt ratio, since it stimulated the flux via the cytochrome path rather than the alternative one. It is concluded that both the large increase of Valt and the high Valt/Vcyt ratio are required for seed germination and that KCN, NaN3, O2 and C2H4 exert their actions on the germination by increasing either or both.