THE EFFECTS OF SELECTION FOR REDUCED DORMANCY IN CHARLOCK (SINAPIS ARVENSIS) J. R. WTCOMBE* and W. J. WHITflNGTON University of Nottingham School of Agriculture, Sutton Bonington, Lou ghborough, Leicestersh ire Received 24.ix.71 1. INTRODUCTION DORMANCY of seeds is a character of frequent occurrence amongst annual weed species of temperate and colder regions. It enables a species to over- come the hostile environment of winter and allows the seeds to germinate in more favourable environmental conditions. Dormancy is determined by many different physiological mechanisms (Wareing, 1969), and in charlock, Edwards (1968) showed that growth inhibiting substance is produced at low oxygen concentration in the interior of the seed. Purified extracts from the testa of charlock have also been shown to have inhibitory effects on germina- tion (Witcombe, Hillman and Whittirigton, 1969). That dormancy is under genetical control has been shown by many workers. For example, in subterranean clover (Trifolium subterraneum L.), Morley (1958) showed that it was at least partly dependent on the genotype of the embryo as distinct from the testa. Results from the analysis of an F2 generation and a diallel cross showed that dormancy was highly heritable. Interspecific crosses in the genus Papaver have shown that dormancy was heritable and that the influence of the maternal parent was not predominant. Instead the behaviour of the hybrid seeds was often unlike the germination of the seeds of either parents (Harper and McNaughton, 1960). Further evidence of maternal and paternal effects with various levels of interaction can be found in work reported by Simmonds (1964) on potato, and by Battle and Whittington (1971) on sugar beet. Environmental factors may modify germination behaviour and in parti- cular the dormancy of charlock seeds may be broken by gibberellic acid (Edwards, 1968). The present investigation examines the genetic control of dormancy in charlock and in particular demonstrates that dormancy and germination characteristics can be modified by selection. 2. METHODS AND MATERIALS Charlock seeds were obtained from a natural population growing in Nottinghamshire. Germination tests were carried out, except where otherwise stated, at 25° C. in continuous darkness in plastic petri dishes on moist filter paper. Germination was defined throughout the experiment as the emergence of the radicle through the testa. In certain of the selection experiments, gibberellic acid (GA3) was used to break dormancy and the specific effects of GA on * School of Plant Biology, University College of North Wales, Memorial Buildings, Bangor, Caernarvonshire. 02 87