Involvement of ABA in Induction of Secondary Dormancy in Barley (Hordeum vulgare L.) Seeds

At harvest, barley seeds are dormant because their germination is difficult above 20C. Incubation of primary dormant seeds at 30C, a temperature at which they do not germinate, results in a loss of their ability to germinate at 20C. This phenomenon which corresponds to an induction of a secondary dormancy is already observed after a pre-treatment at 30C as short as 46 h, and is optimal after 2448 h. It is associated with maintenance of a high level of embryo ABA content during seed incubation at 30C, and after seed transfer at 20C, while ABA content decreases rapidly in embryos of primary dormant seeds placed directly at 20C. Induction of secondary dormancy also results in an increase in embryo responsiveness to ABA at 20C. Application of ABA during seed treatment at 30C has no significant additive effect on the further germination at 20C. In contrast, incubation of primary dormant seeds at 20C for 48 and 72 h in the presence of ABA inhibits further germination on water similarly to 2448 h incubation at 30C. However fluridone, an inhibitor of ABA synthesis, applied during incubation of the grains at 30C has only a slight effect on ABA content and secondary dormancy. Expression of genes involved in ABA metabolism (HvABA8OH-1, HvNCED1 and HvNCED2) was studied in relation to the expression of primary and secondary dormancies. The results presented suggest a specific role for HvNCED1 and HvNCED2 in regulation of ABA synthesis in secondary seed dormancy.