Possible involvement of volatile compounds in the after-ripening of cocklebur seeds

The mechanism of emergence from primary dormancy, the process of after-ripening, in cocklebur (Xanthium pennsylvanicum) seeds was examined in relation to the involvement of volatile compounds and to the relative humidity (RH) in which the seeds were stored. The after-ripening of these seeds proceeds only at water contents between 7 and 14% which are conditioned under RHs of 33% to 53% and are identified with water-binding region II. After-ripening of cocklebur seeds occurred even in water-binding region I. imposed by 12% RH. when exposed to HCN gas during the storage period. Exposure of dormant seeds to acetaldehyde (ethanal) retarded after-ripening. even in water-binding region II. thus decreasing germinability. This decrease of germinability by ethanal was found also in the after-ripened seeds, suggesting that ethanal accelerates seed deterioration rather than retarding the after-ripening. The contents of ethanal. ethanal and HCN were high only in the dormant seeds held at 12% RH. Regardless of RH. a possible conversion of ethanal to ethanol. perhaps via alcohol dehydrogenase. was far larger in dormant than in non-dormant seeds. In contrast, the reverse conversion of ethanol to ethanal was more profound in non-dormant seeds. Pre-exposure of both types of seeds to HCN reduced the contents of both ethanal and ethanol at 12% RH. The contents of various adenylales including ATP in seed tissues were higher in dormant seeds stored at 12% RH than in non-dormant seeds after-ripened at 44% RH. It is suggested that emergence of cocklebur seeds from primary dormancy by HCN treatment at 12% RH may result from the reduction in the contents of ethanal via an unknown mechanism incurring the consumption of ATP. This implies involvement of volatile compound metabolism at the water-binding region II in the after-ripening process of cocklebur seeds.