Dormancy in Cereal Seeds

Abstract

A wide spectrum of respiratory inhibitors has been found to stimulate the breaking of dormancy in barley. These include carbon monoxide, cyanide, azide, hydrogen sulphide, sodium sulphide, hydroxylamine, diethyldithiocarbamate (DIECA), fluoride, iodoacetate, malonate, monofluoroacetate, and 2,4-dinitrophenol (DNP). In rice, only the first six of these have been shown to be effective. Apart from CO, all the above inhibitors were tested on winter oats, but in this material only cyanide, azide, and hydroxylamine were found to increase the germination of dormant seeds. All the terminal-oxidase inhibitors except CO were tested on perennial ryegrass, but in this case only cyanide was found to break dormancy. As compared with air, an atmosphere of 96 per cent oxygen applied to barley during the first 24 h after the seeds have been set to germinate stimulates the breaking of dormancy. When applied at later stages, this high oxygen tension inhibits the germination of dormant seeds although it has no effect on nondormant seeds. Paradoxically, the stimulatory effects of respiratory inhibitors applied during the initial stages of germination are related to their ability to inhibit oxygen uptake. Thus cyanide, azide, malonate, and monofluoroacetate, while stimulating the breaking of dormancy in barley, also inhibit oxygen uptake. In rice, cyanide and azide had similar effects, but fluoride, which had no effect on dormancy, also had no effect on the oxygen uptake of dormant seeds. These results are compatible with the hypothesis that some oxidation reaction is necessary for germination. This oxidation is not part of the normal respiratory pathway, and does not proceed satisfactorily in dormant seeds. It may be stimulated, however, by increasing the oxygen tension or by reducing normal respiratory competition with respiratory inhibitors.