Evolution of HCN from both rice (Oryza sativa) and cocklebur (Xanthium pennsylvanicum) seeds increased during a pre-germination period and preceded the evolution of (C2H4). These two species were adopted as the representatives of starchy and fatty seeds, respectively. Ethylene promotes seed germination of many species. However, HCN evolution declined abruptly when the radicles emerged and before the peak in C2H4 evolution. Moreover, both rice and soybean (Glycine max) seeds showed some activity of beta-cyanoalanine synthase (CAS, EC 4.4.1.9) even in the unimbibed dry state. The activities of CAS in the lower seed of cocklebur and in soybean seeds increased of the radicle. However, the CAS of rice seeds, with high activity in the dry state, exhibited a bimodal change, gradually decreasing until radicle emergence had occurred, but then increaing It is thus likely that HCN evolution during initial imbibition may be derived from cyanogenic reserves and controlled by both pre-existing and subsequently-developing CAS. The exogenous application of C2H4 stimulated the activities of CAS in both rice and upper cocklebur seeds and reduced their cyanogen contents. Therefore, the decline of HCN evolution after germination seems to be due to the increased activities of CAS by endogenously produced C2H4.
